D.P. Stern_2002_A Millenium of Geomagnetism

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A MILLENNIUM OF GEOMAGNETISM

David P. SternLaboratory for Extraterrestrial Physics NASA Goddard Space Flight Center Greenbelt, Maryland, USA

Received 26 January 2001; revised 28 December 2001; accepted 9 September 2002; published 23 November 2002.

[1] The history of geomagnetism began around the year1000 with the discovery in China of the magnetic com-pass. Methodical studies of the Earth’s field started in1600 with William Gilbert’s De Magnete [Gilbert, 1600]and continued with the work of (among others) EdmondHalley, Charles Augustin de Coulomb, Carl FriedrichGauss, and Edward Sabine. The discovery of electro-magnetism by Hans Christian Oersted and Andre-Marie Ampere led Michael Faraday to the notion of fluiddynamos, and the observation of sunspot magnetism byGeorge Ellery Hale led Sir Joseph Larmor in 1919 to theidea that such dynamos could sustain themselves natu-

rally in convecting conducting fluids. From that camemodern dynamo theory, of both the solar and terrestrialmagnetic fields. Paleomagnetic studies revealed that the

Earth’s dipole had undergone reversals in the distantpast, and these became the critical evidence in establish-ing plate tectonics. Finally, the recent availability of scientific spacecraft has demonstrated the intricacy of the Earth’s distant magnetic field, as well as the exis-tence of magnetic fields associated with other planetsand with satellites in our solar system. I NDEX T ERMS : 1714

History of Geophysics: Geomagnetism and paleomagnetism; 1739

History of Geophysics: Solar/planetary relationships; 1599 Geomag-

netism and Paleomagnetism: General or miscellaneous; K EYWORDS :

geomagnetism; history; lodestone; magnetic survey; geomagnetic dy-

namo; solar magnetism

Citation: Stern, D., A millennium of geomagnetism, Rev. Geophys.,

40(3), 1007, doi:10.1029/2000RG000097, 2002.

1. INTRODUCTION

[2] This brief history follows two earlier ones on mag-netospheric physics [Stern, 1989, 1996] and is directed atthree audiences: (1) geophysicists seeking to comple-ment their professional expertise with its underlying

history, (2) students of geology and geophysics, inter-ested in the origins and underpinnings of their disci-plines, and (3) historians of science, looking for infor-mation about the evolution of the discipline of geomagnetism. It is meant to be a starting point ratherthan an exhaustive review, and those who seek moredetails may seek them in the many references. Thearticle supplements a recent web site (home page http:// www.phy6.org/earthmag/demagint.htm, by D. Stern) which covers the same subject but at a more popularlevel. A chronology of the events covered here is given in Appendix A. For a list of “who was who” in the history

of geomagnetism, please see the web site (home pagehttp://www.phy6.org/earthmag/authors.htm, by D.Stern), which gives the names of the scientists who arecited in this article, including their first names. All toooften, first names are reduced to initials (for example, inreference lists), and when no other record exists, thescientists’ full names are lost from history.

[3] History is an integral part of scientific knowledge.It not only provides the framework of its field, explain-

ing how its concepts arose and developed, but italso provides an insight into the actual path of progress, something scientific articles and texts gener-ally omit. Geomagnetism is of particular interest, be-cause it may well claim to be the oldest discipline ingeophysics. In its long history, several threads can be

distinguished, now and then intersecting but largelyindependent. It is this pattern which the article ismeant to highlight.

2. EARLY DISCOVERIES

[4] The year 2000 was an important anniversary ingeophysics. It marked 400 years since William Gilbertpublished in London his book De Magnete (Latin for“On the Magnet”) [Gilbert, 1600; see also Barraclough,2000; Schroder , 2000; Chapman, 1944; Watson, 1944],

proposing among other things that the Earth itself was agiant magnet, thus explaining the strange directivity of the compass needle (Figure 1). See also historical ap-pendix given by Chapman and Bartels [1940], chapter 26“Historical Notes,” pp. 898–937. It may also be, veryapproximately, the 1000th anniversary of the discoveryof the magnetic compass. The ancient Greeks knewabout “lodestones” (or loadstones), rare natural mag-nets, with the power to attract iron [ Mitchell, 1946]. One

This paper is not subject to U.S. copyright. Reviews of Geophysics, 40, 3 / September 2002

Published in 2002 by the American Geophysical Union. 1007, doi:10.1029/2000RG000097

● B-1 ●



site where such stones were found was near the city of Magnesia in Asia Minor (now Turkey), and from that(perhaps) came the term “magnetism.” However, it may

have been an unknown Chinese scholar around the year1000 [ Mitchell, 1932; Knapp, 1962] who first placed alodestone on a “boat” floating in a bowl of water andobserved that wherever and whenever the experiment was performed, the boat always rotated to face south(though some claims date this Chinese discovery asmuch as 1000 years earlier). The magnetic force merelyrotated the needle, it did not pull it bodily southward, orin any other direction. Apparently, the Chinese alsoknew steel needles could be permanently magnetized,for Shon Kua (1030 –1093) wrote “fortune tellers rub thepoint of a needle with the stone of a magnet in order to

make it properly indicate the south.” According to An- drade [1958; see also Smith, 1992], the earliest European

mention of magnetizing iron by having it “touched by alodestone” dates to about 1200.

[5] The discovery spread from China to Europe, and

it would be hard to imagine the great sea voyages of Vasco Da Gama, Christopher Columbus, and FerdinandMagellan without its help. The magnetic compass alsofound another use: small folding personal sundials, time-pieces that needed to be aligned northward to workproperly. But the phenomenon itself was a mystery. According to some reports, helmsmen on British ships were forbidden from eating garlic (on pain of flogging),because of the irrational belief (dating back to Pliny)that the pungent fumes destroyed the magnetic powerand thus could disable the compass.

[6] This brief history tries to tell the story of the

compass needle and its more sophisticated successors,from those early days to the present age. It avoids

Figure 1. Front page of the 1628 edition of De Magnete [Gilbert, 1600].

B-2 ● Stern: A MILLENNIUM OF GEOMAGNETISM 40, 3 / REVIEWS OF GEOPHYSICS



mathematics and fine details, and also glosses over thedetails of paleomagnetism, which is outside this writer ’sexpertise; instead, it seeks a synoptic overview, a broadoutline of the subject, referring readers interested inmore detailed or technical aspects to articles and books;more general expositions may be found in the books by

Aczel [2001], Vershuur [1993], and Livingston [1996]. Theauthor has also set up on his World Wide Web domaina related set of web pages, with Spanish, German, andFrench translations, at http://www.phy6.org/earthmag/demagint.htm.

[7] We know from Alexander Neckam (1157–1217), amonk at St. Albans, that by the year 1187, magneticneedles were being mounted on pivots, free to rotatetoward any horizontal direction like modern compassneedles [ Mitchell, 1932]. That design is also mentionedin a 1269 letter by the Frenchman Petrus Peregrinus(Pierre Pelerin de Maricourt), who conducted somesimple experiments on magnetism.

[8] Wrought iron is “magnetically soft” and loses itsmagnetism when removed from the lodestone, but high-carbon steel can remain permanently magnetized. Thetraditional way to make a compass was to fashion a flatsteel needle and balance it horizontally on a pivot, thenmagnetize it by stroking with a lodestone. But a strangething was observed: after the needle became magnetic,its north pointing end always slanted down, as if it hadgained weight. Its tip had to be snipped off (or a coun-terweight attached) to maintain the balance. The slant of a balanced needle after it was magnetized was noted byGeorg Hartmann in 1544 [Chapman and Bartels, 1940,

section 26.5]. His observation was recorded in a letterfound in the Konigsberg archives, only discovered in1831.

[9] A British compass maker named Robert Normantried to track down the reason for this behavior. By 1581he had found it [ Mitchell, 1939; historical appendix inChapman and Bartels, 1940]: north of the equator, theforce on the north pointing end of the needle was not atall horizontal, but slanted downward into the Earth, atan angle now known as the “dip” or “inclination.” Hedemonstrated this in an ingenious experiment (Figure 2) with a needle threading a small ball of cork, which was

carefully whittled down until, in a goblet of water, itneither sank to the bottom nor floated to the surface.Only after that did Norman magnetize the needle, ob-serving that a magnetized needle free to rotate in anydirection pointed down as well as northward.

[10] The other discovery of those early years was thateven the horizontal part of the force was not directedexactly northward, but usually varied by a few degreesfrom true north (i.e., celestial north, derived from themotion of the Sun and stars), an angle now known as thedeclination [ Mitchell, 1937]. Compass builders rotatedtheir dials to compensate for the discrepancy, so that a

compass used in (say) the Baltic was calibrated differ-ently from one used in the Mediterranean.

3. WILLIAM GILBERT

[11

] This was the scene when William Gilbert (1544?–1603) developed his interest in magnetism. A distin-guished doctor, president of the Royal College of Phy-sicians [ Langdon-Brown, 1944], Gilbert set out around1581 to find all he could about magnetism, from books,experiments, and observations. In the process he discov-ered or confirmed all the main properties of permanentmagnetism, the way poles rearranged themselves when amagnet was broken (previously noted by Petrus Pereg-rinus), the way magnetism was induced in iron placednext to a magnet, how a heated bar of iron lost itsmagnetism (though the attraction itself could cross aflame), and how a hot bar of steel, pounded by a black-

smith as it cooled while aligned in a north-south direc-tion, became weakly magnetized, capturing the prevail-ing field of the Earth (Figure 3).

[12] Gilbert also studied other types of attraction,especially the attraction of straws and other light objectsto certain materials, after these were lightly rubbed withcloth or fur. One such substance was amber, fossilizedpine pitch, called “electron” by the Greeks, who alreadyknew of its attractive properties. Gilbert thereforenamed this the “electrick force” and studied its differ-ences from magnetic attraction, for example, its suscep-tibility to humidity. He did not note that two distinct

types of electric charge existed, and also puzzled overthe attraction between droplets of water, which tended

Figure 2. Robert Norman’s experiment ( De Magnete [Gil-

bert, 1600, book V, chapter 9]).

40, 3 / REVIEWS OF GEOPHYSICS Stern: A MILLENNIUM OF GEOMAGNETISM ● B-3



to fuse together, a process now attributed to surfacetension.

[13] Gilbert guessed that the reason the compass nee-dle pointed north was that the Earth itself was a giantmagnet. He supported his hunch by a simple experiment.He constructed a scale model of a magnetic Earth, amagnetized sphere he named the “terrella” (littleEarth), and by sliding a small compass around its surface(or placing it in different positions next to a compass),reproduced not only the observed northward pointingproperties, but also the dip angle (Figure 4).

[14] He also believed that the close proximity of mag-

netic north and true north was no accident, that theEarth’s rotation around its axis stemmed from its mag-netism. In Gilbert’s age, his claim that the Earth was notthe immovable center of the universe, and his strongsupport, voiced in his book, of the theory of Copernicus, were, in some circles at least, articles of heresy. EdwardWright, in introducing the book, tried to blunt thecharge by writing

Nor do those things which are adduced from the sacredscriptures seem to be specially adverse to the doctrine of the mobility of the Earth; nor does it seem to have beenthe intention of Moses or of the Prophets to promulgateany mathematical or physical niceties, but to adapt

themselves to the understanding of the common peopleand their manner of speech, just as nurses are accus-

tomed to adapt themselves to infants, and not to go intoevery unnecessary detail.

[15] Still, some copies of De Magnete had the offend-ing pages torn out. Galileo, who highly praised the book,obtained his copy as a gift from “a peripatetick philos-opher of great fame, as I believe, to free his library of itscontagion.” Today, De Magnete stands as a benchmarkon the boundary between medieval scholarship, mystical,citing claims of accepted authorities without botheringto check them, and the modern observational approach.It rewards the reader with delightful nuggets, quite dif-ferent from the impersonal prose of today’s journals.

Commenting on attempts to use magnetism for gener-ating perpetual motion, such as the one made PetrusPeregrinus, Gilbert wrote: “Oh that the gods would atlength bring to a miserable end such fictitious, crazy,deformed labours, with which the minds of the studiousare blinded!”

[16] If magnetism and rotation went together, why dida small “declination” angle (Gilbert called it “ variation”)exist between true north and magnetic north? Gilbertingeniously proposed that if the Earth were a perfectsphere, the two directions would indeed be the same.However, the Earth’s shape is not quite spherical: con-

tinents rise up, while the oceans are nestled in deepgashes. Since Gilbert attributed magnetic attraction to

Figure 3. A blacksmith pounding a cooling bar of steel while it is aligned in the north-south (septentrio-auster) direction. The bar captures some of the Earth ’s magnetism ( De Magnete [Gilbert, 1600, book III,chapter 12]).




the mass of the Earth (but not, apparently, to the waterfilling the oceans) he proposed that the elevated por-tions added to the pull, while the gashes decreased it.

[17] He therefore suggested that the compass needle

near the eastern and western edges of the AtlanticOcean would be deflected toward the nearby continents,as was indeed observed at that time. Just to make sure,though, Gilbert also verified the effect experimentally,using a terrella that had “a certain part corroded toresemble the Atlantic or great Ocean.” He found thatthe needle was deflected away from the gash (but sincethe depth of the ocean basins is less than 1/1000 of theEarth’s radius, this was probably not a valid modeling).Edmond Halley (see section 4) later found the oppositeeffect occurred near Brazil.

[18] Because continents and oceans do not shift overthe historical timescale, Gilbert confidently predicted

that magnetic declination would not change: “Unlessthere should be a great dissolution of a continent and asubsidence of the land such as there was in the region Atlantis of which Plato and the ancients tell, the varia-tion will continue perpetually immutable.”

4. EDMOND HALLEY

[19] In 1601 Gilbert was appointed physician toQueen Elizabeth I, but he died not long afterward, in1603, of the bubonic plague, endemic in London and

probably a professional hazard to doctors. He was there-fore no longer around when his prediction of “perpetual

immutability” was demonstrated to be false, one gener-ation later. In 1634, Henry Gellibrand (1597–1636)showed [ Malin and Bullard, 1981] that the magneticdeclination observed near London had undergone asystematic shift. Subsequent observations confirmedsuch variations and also showed them to be worldwideand without any clear-cut pattern. If the Earth was

permanently magnetized (and in the 1600s no othermagnetization was known), how could its magnetism vary?

[20] The only solution left, proposed ingeniously byEdmond Halley [ Bullard, 1956; Evans, 1988; Chapman,1941, 1943a, 1943b; Bauer , 1896, 1913; Clark, 2000] wasthat the interior of the Earth consisted of concentricspherical shells, each magnetized differently, and thatsome rotated differently from others. (The 1943a articleby S. Chapman includes poems by Halley honoring New-ton and the inventor of the compass, both on page 231.The 1943b article is the same as 1943a, but without the

second poem.) Halley was so proud of his theory that when at age 80 he had his portrait painted, he appearedin it next to a diagram of his spherical shells. He thoughthe could locate four distinct magnetic poles, belongingto two different layers. The modern theory of the Earth’sfield actually suggests that the solid inner core of theEarth might rotate at a slightly different rate [ Buffett andGlatzmaier , 2000], but this is a completely different pro-cess and permanent magnetism is not involved.

[21] The name of Halley (1656 –1742) is nowadaysmost commonly associated with that of a periodic comet whose return he predicted. However, he was also one of

the main pillars of the British scientific communityaround 1700: the second Astronomer Royal, an activemember of the “Royal Society,” a man without whosehelp and encouragement Newton’s Principia might nothave been published. He was as well the leader of theearliest global magnetic survey.

[22] Feeling the need for more accurate magneticcharts of the Atlantic Ocean, their Lordships of theBritish Admiralty lent Halley a small sailing ship, the52-foot Paramore (or Paramour ), and instructed him tocarry out a magnetic survey of the Atlantic Ocean and itsbordering lands [Thrower , 1981]. Perhaps consideringthis task an insuf ficient justification of the expedition,

they also gave him a second one: “to stand soe farr intothe South, till you discover the Coast of the TerraIncognita, supposed to lye between Magelan’s Streightsand the Cape of Good Hope” [ Bullard, 1956].

[23] The Paramore set out in October 1698, but it wastroubled by both leaks and by a personal conflict be-tween Halley and the navy of ficer in charge of the ship,Lieutenant Harrison. Harrison countermanded Halley’sorders and spoke insultingly of him before the crew,until Halley had the man arrested and turned the shipback to England, where a court of inquiry upheld himand gave him sole command of the ship. It turned out

that Harrison had published a small book, Idea Longi-tudinis, proposing a way of helping navigation at sea. The

Figure 4. Inclination of a compass needle near the surface of the terrella depends on its position: vertical at the poles (on themagnetic axis), horizontal on the equator. The “Orb of Virtue”is Gilbert’s term for “sphere of influence.” ( De Magnete [Gil-

bert, 1600, book V, chapter 2]).




book was presented to the Admiralty and the RoyalSociety but received an unfavorable report, and Halley was among its reviewers.

[24] The Paramore set out again in September 1699and soon encountered a severe storm, in which the cabinboy was swept overboard, never to be found. By 1February 1700, the ship reached southern latitude

5240. Some highly unusual wildlife was seen and later, when Halley’s magnetic map appeared, it showed in thatregion two rather unusual creatures, with the legend“The sea in these parts abounds with two sorts of Ani-malls of a Middle Species between a Bird and a Fish,having necks like Swans and Swimming with their wholeBodyes always under water only putting up their longNecks for Air.”

[25] But there was no “Terra Incognita.” Threestrange islands appeared on the horizon, and Halleysketched them in his logbook, but next day he realizedthey were nothing but enormous floating icebergs. Then

a dense fog descended, and dodging the ice became adangerous game:

Between 11 and 12 this day we were in iminant danger of loosing our Shipp among the Ice, for the fogg was all themorning so thick, that we could not See a furlong aboutus, when on a Sudden a Mountain of Ice began to appearout of the Fogg, about 3 points on our Lee bow . Seabeing smooth and the Gale Fresh wee got Clear: God bepraised. This danger made my men reflect on the haz-zards wee run, in being alone without a Consort, and of the inevitable loss of us all in case we Staved our Shipp

which might easily happen amongst these mountainsof Ice in the Foggs, which are so thick and frequentthere.

[26] After this encounter, the ship continued toTristan da Cunha, St. Helena, Brazil, Barbados, Ber-muda, Newfoundland, and finally, at the end of August,back to England. From Halley’s survey the first magneticmap of the Atlantic was compiled with contour linesconnecting points of equal declination, the first knownuse of contour lines: for the next century, such lines wereknown as “Halleyan lines.” By incorporating observa-tions made by others, Halley in 1702 extended his chart,and it was reprinted and revised many times.

5. CHARLES AUGUSTIN DE COULOMB

[27] It is only natural that when scientists develop anymethod of measurements, they push it to its limits.Careful observation of the position of the tip of a longcompass needle by George Graham (1675–1751), a Lon-don clockmaker and instrument builder, showed in 1722that the direction of the magnetic force in Londonunderwent a 24-hour cycle, a diurnal variation. In 1741he and Anders Celsius in Uppsala, Sweden also ob-served simultaneous perturbations due to the polar au-rora [Chapman and Bartels, 1940, section 26.9; Beckman,

2000].[28] The 24-hour “diurnal” magnetic variation was

barely observable, and in 1773 the Paris Academy of Sciences offered a prize for finding “the best manner of constructing magnetic needles, of suspending them, of

making sure that they are in the true magnetic meridian,and finally, of accounting for their regular diurnal vari-ations.” The offer was renewed in 1775, and was claimedin 1777 by a French military engineer, Charles AugustinCoulomb [Gillmor , 1971; Shamos, 1959, p. 59].

[29] Coulomb’s instrument, known as the “torsion bal-ance,” served as model for magnetic instruments overnearly two centuries. A magnetic needle was suspendedfrom a long thin twistable wire, long enough and thinenough that even a small torque produced a notabletwist. That twist, furthermore, could be accurately mea-sured by attaching a small mirror just above the needle,and observing the shifts of a spot of light reflected from

it (Figure 5). Indeed, the instrument was so sensitive,that Coulomb not only had to place it inside a glassenclosure to shield it from stray air currents, but alsofound that static electric charges sometimes interfered with his magnetic observations. As it turned out, thissensitive instrument allowed much more to be measuredthan the magnetic diurnal variation.

[30] The twisting moment (torque) of the suspension wire is proportional to the angle of twist, and can bemeasured, for instance, by suspending from its end anonmagnetic bar and timing its back-and-forth oscilla-tion around the equilibrium position. If next a long

magnetic needle is suspended, and one end is placednear a magnetic pole of the same kind, the end of the

Figure 5. Coulomb’s torsion balance, after Shamos [1959].The calibrated knob on top, from which the magnet is sus-pended, can be twisted.




needle will be repelled to a new position. However, theknob from which the wire is suspended can now betwisted to restore the needle to its previous direction.Now the extra torque (deduced from the extra twist of the knob) can tell the strength of the repulsion.

[31] By methods like this Coulomb showed that themagnetic repulsion between magnetic poles, and also

their attraction, varied inversely with the square of thedistance. How much like gravity, indeed! Replacing thesuspended magnet with a small straw covered with wax,carrying a pith ball at its end (counterbalanced by someobject at the other end of the straw), Coulomb repeatedthe experiment with electrical forces. He charged a sim-ilar pith ball on an insulating stand inside the enclosure,made the two balls share their charge by touching eachother, and again measured the force against the twist of a wire. It turned out, again, so much like gravity!, thatthe strength of electric forces also decreased like theinverse square of the distance.

[32] Finally, in 1796, Henry Cavendish used a similartorsion balance (an experiment possibly proposed by theReverend John Michell) to measure the gravitationalattraction between massive spheres, a much weakerforce and much more dif ficult to measure. This demon-strated that Newton’s law of planetary attraction, inversely proportional to the square of the distance, heldin the laboratory, as well. For a while all nature seemedin harmony, three fundamental forces, all obeying theinverse squares law, of which only gravity differed, byalways attracting and never repelling.

6. HANS CHRISTIAN OERSTED AND ANDRE-

MARIE AMPERE

[33] This nice symmetry was upset in 1820 by anunexpected connection between magnetism and electric-ity. This is not the place to tell how Luigi Galvani and Alessandro Volta introduced an entirely new way of generating electricity, not as a static charge, generatedby rubbing, but as a continuous flow of electric charge,an electrical current generated by a chemical process[see, e.g., Segre, 1984; Vershuur , 1993]. Volta’s “ voltaiccell” and “ voltaic pile” were the ancestors of today’s “dry

cells” and car batteries.[34] Hans Christian Oersted (1777–1851) was born in

Rudkoebing in southern Denmark, a town so small itlacked a school [ Dibner , 1962]. Nevertheless, Hans andhis younger brother Anders found willing teachersamong local citizens and acquired enough education tobe accepted in 1793 by the University of Copenhagen. Anders studied law and later became quite famous in hisfield; Hans took up medicine and science, but his inter-ests must have been much broader, for in 1797 he won agold medal for an essay on “Limits of Poetry and Prose.” After some travels he joined the university in 1806 and

became a regular professor in 1817. Two years later hestruck a friendship with a poor boy of 14 who had just

arrived in the city, this was Hans Christian Andersen,later a writer of folk tales. Their friendship continued forthe rest of Oersted’s life.

[35] Oersted’s interest centered on electricity andchemistry and on what then was still a novelty, theelectric battery. In the spring of 1820 he arranged at hishome a lecture on electricity and magnetism, before a

group of friends and/or students.[36] Accounts differ as to what exactly happened, but

they all agree that Oersted’s equipment included a mag-netic compass, as well as an electric battery and a thinmetal wire, and that one demonstration involved heatingthe wire by an electric current from the battery. Mostprobably it was only by accident that the wire passedover the compass or near it, although Oersted laterhinted that he had put it there deliberately, acting on along-standing suspicion that a link existed between elec-tricity and magnetism. Accidentally or not, whenever the wire was connected to the battery and a current flowed,

the magnetic needle moved, and whenever the currentceased, it returned to its old position. No one elsenoticed and Oersted said nothing, but in the months thatfollowed he conducted many experiments, unsuccess-fully trying to understand what had happened.

[37] Oersted would have been less puzzled if the wirehad attracted the needle, the way a magnet would do,but no, the needle tried to turn at right angles to theelectric current! And when the connections to the bat-tery were exchanged so that the current flowed in theopposite direction, the compass needle followed suit andalso reversed.

[38

] There was surely a message here, but try as he would, Oersted could not decode it. Still, it was no meanfeat: here was the first clear-cut evidence connectingelectricity and magnetism, and by 21 July Oersted an-nounced it to the world in a four-page report, written inLatin just like Gilbert’s book [Shamos, 1959, pp. 121–127]. Perhaps Oersted intended to provide a text from which translations could be made, for Latin certainly wasno longer a universal language of scholars, as it was inGilbert’s time. The experiment was easy to repeat, andthe best scientific minds of Europe turned at once toexploit and explore this new “electromagnetism”.

[39] A report of Oersted’s discovery (and of its con-

firmation by de la Rive in Geneva) reached Paris onMonday, 11 September 1820, and was discussed at ameeting where among others Andre-Marie Ampere(1777–1836) was present. In a tremendous feat of imag-ination and insight, Ampere solved much of the riddle within one week, and soon afterward, in a series of elegant experiments, confirmed a completely new viewof magnetism [Williams, 1989; Segre, 1984].

[40] The fundamental interaction in magnetism, Am-pere said in essence, had nothing to do with the attrac-tion or repulsion of magnetic poles. Instead, the basicingredient of magnetism was the electric current. Mag-

netism, Ampere implied, would have existed even if there were no permanent magnets, because its basic




feature was the force between electric currents. Whentwo currents flowed in the same direction along parallel wires, the wires attracted, and when the currents flowedin opposing directions, they repelled each other.

[41] Ampere then showed that an electric currentcirculating around a wire loop acted like a short magnet. A current flowing in a coil of 1000 turns produced

magnetism 1000 times stronger, because the magneticforces of all its loops added up, and two coils with thesame axis attracted or repelled, depending on whethertheir flows were parallel or opposed. The magnetism of iron, Ampere implied, may have come about becauseiron atoms contained small circulating currents, whichcould be lined up so that they reinforced each other [seealso Livingston, 1996]. Inserting an iron core in a cur-rent-carrying coil increased its magnetic pull, by liningup the magnetic domains of the iron, but iron was in no way essential.

7. THE LODESTONE

[42] The study of magnetism began with the lode-stone, and at this point one can finally look back and ask, what created those strange minerals in the first place?

[43] Lodestones are an iron-rich ore, magnetite. Somesuch ores can support magnetism, but only in the pres-ence of an active source, a permanent magnet or a coil with current circulating in it. Their magnetism, like that

of soft iron, is “soft” and disappears again when thesource is removed. Peter Wasilewski, who currentlystudies these matters at NASA, found that a certaintransformation to a fine-grained structure was needed,occurring only when the ore underwent certain geolog-ical changes, under high temperature and pressure[Wasilewski, 1977; Wasilewski and Kletetschka, 1999].

[44] Even magnetically “hard” substances, however,become magnetic only if exposed to a suf ficiently strongmagnetic field. The exposure may last no more than abrief instant, like the instant when a magnetic tape ordisk rapidly passes in front of the recording head, but thefield strength must exceed a certain minimum. Exposing

a “latent lodestone” to the weak field of the Earth, evenfor millions of years, will not accomplish the same result.

[45] Wasilewski believes (as had been proposed be-fore) that the magnetization was produced by the strongelectric current which briefl y flows when lightning strikesan outcrop of suitable rock. He exposed latent lode-stones to natural lightning at the Langmuir Laboratoryfor Atmospheric Research in New Mexico, built atop apeak where lightning often strikes, and found they be-came magnetized.

[46] Interestingly, William Gilbert had a clue to thisprocess but (not surprisingly) missed its significance. In

De Magnete (book III, chapter 12) he cited the followingpassage from a book published in Italy:

A druggist of Mantua showed me a piece of ironentirely changed into a magnet, drawing anotherpiece of iron in such a way that it could be comparedto a loadstone. Now this piece of iron, when it had fora long time held up a brick ornament on the top of thetower of St. Augustine in Rimini, had been at lengthbent by the force of the winds, and remained so for a

period of ten years. When the monks wished to bendit back to its former shape, and had handed it over toa blacksmith, a surgeon named Maestro Giulio Cae-sare discovered that it was like a magnet and attractediron.

[47] In hindsight, we would guess that the churchtower had been hit by lightning, which had magnetizedthe iron (and possibly also bent it). Gilbert, however,attributed it to long-term exposure to the Earth’s mag-netism, “by the turning of its extremities towards thepoles for so long a time.”

8. CARL FRIEDRICH GAUSS AND ALEXANDER

VON HUMBOLDT

[48] The work of Oersted and Ampere drew to thestudy of magnetism one of the sharpest minds of Eu-rope, that of Carl Friedrich Gauss (1777–1855). Gauss was a professor of mathematics at the German universityof Gottingen and rarely traveled away from home, but in1828 he attended a conference in Berlin, and stayedthere as house guest of Alexander von Humboldt [ Dun-

nington, 1955]. Humboldt (1769 –1859) was a naturalist

who had earlier explored the jungles of South Americaand later the author of Kosmos, a fi ve-volume encyclo-paedic compendium of the natural sciences, whichgreatly helped spread public interest in science. Hum-boldt was also quite interested in magnetism [ Malin and

Barraclough, 1991].[49] During this visit, Humboldt showed Gauss his

collection of magnetic instruments and encouraged himto apply his talents to magnetism. That Gauss did, to-gether with his young assistant Wilhelm Weber (1804 –1891), contributing greatly to the understanding of theEarth’s magnetic field [Garland, 1979].

[50] In their magnetism lab, Gauss and Weber con-

structed the first magnetic telegraph, using it until light-ning knocked down the wire. They devised a new sus-pension for observatory magnets (big magnets, slow torespond, later replaced by more nimble instruments). In1832, Gauss and Weber also devised a clever method of using an auxiliary magnet to measure not only the direc-tion of the Earth’s magnetic force, but also its intensity.Today, undergraduate physics students use the Gaussmethod for measuring the strength of the Earth’s field asa standard lab experiment, not realizing its historicalsignificance. Actually, it made possible for the first timea global network of magnetic observatories, because now

every instrument could be calibrated locally, indepen-dently of any others.




[51] But perhaps the most lasting contribution was theuse of a precise mathematical method to represent theglobal magnetic field of the Earth and to combine ob-servations at many locations. That was spherical har-monic analysis, previously used for analyzing gravita-tional fields in celestial mechanics. According to SidneyChapman, it was introduced to geomagnetism by the

French mathematician Simeon Denis Poisson (1781–1840) [Chapman, 1964, p. 4].

[52] The attraction of the Earth’s gravity, like that of a single magnetic pole, diminishes with distance R like1/ R2. The force is three-dimensional, but because of itssimple structure, a single mathematical expression,known as the harmonic potential, can describe it all. If the Earth repelled instead of attracted, the only change would be a sign reversal. As long as all magnetic forcescould be ascribed to a collection of magnetic poles,combinations of such harmonic potentials could describethem as accurately as was desired (this holds in any

simply connected current-free region).[53] With two equal and opposite magnetic poles, one

attracts like 1/ R2 and the other repels like 1/ R2. Faraway, the distances from the poles are about equal, andthe attraction of one tends to cancel the repulsion of theother; but a small difference remains, depending ondirection and going down like 1/ R3: that is the “dipole”field, the main component of the magnetic field of theEarth, and also of a bar magnet. Combining two equalbut slightly separated dipoles that almost cancel eachother gives a “4-pole,” whose field weakens with distancelike 1/ R4, and so on: With enough terms, such a “spher-

ical harmonic analysis” can describe the magnetic fieldto any desired accuracy. Incidentally, to fully describethe field, two distinct expansions are needed: one (themain one, in inverse powers of R) for fields originatinginside the Earth, and another (with positive powers) forthose originating on the outside. The method used byGauss also showed that at least 99% of the field origi-nated inside the Earth.

[54] The process somewhat resembles the turning of asquare piece of wood into a round wheel. First, cut off the four corners. This leaves eight obtuse corners, and bycutting off these in a suitable way, one is left with 16corners, each making an angle only slightly smaller than

180. Continue the process long enough and your wheelgets as round as you may wish. Today’s models of theEarth’s magnetic field, derived largely from satellitedata, carry those expansions to hundreds of terms [e.g.,Olsen et al., 2000].

9. EXPLORATIONS AND SURVEYS

[55] It took the ingenuity of Gauss and Weber todevise the tools for conducting a global survey of themagnetic field of the Earth. It took crafty politics and the

collaboration of many individuals to establish the worldwide network of observatories which actually conducted

such a survey. Gauss and Weber started it in 1834 bysetting up the “Gottingen Magnetic Union” (GottingenMagnetischer Verein), an international network of ob-servatories. However, while these observatories coveredEurope quite well, most of the world was left uncovered.

[56] Alexander von Humboldt then stepped in andasked for the support of the British Royal Society [ Ca-

wood, 1979]. The participation of the British Empire wasessential, because it alone had outposts around theglobe, and its involvement was helped by the curious factthat due to a kinship of ruling families, Gottingen too was under British rule until 1837. Humboldt also got theRussian Czar to support a chain of observatories acrossSiberia and even one in Sitka, Alaska, which operatedfrom 1842 to 1864.

[57] Humboldt had earlier noted large-scale magneticdisturbances (possibly already observed by George Gra-ham) and gave them the name they still bear, “magneticstorms.” The worldwide net of magnetic observatories

confirmed that such “storms” were worldwide phenom-ena. The change in the magnetic attraction of the Earth was quite small, rarely exceeding 1% of the total value,but its pattern across the world was remarkably similar,suggesting a large-scale phenomenon.

[58] The British participation was greatly helped bythree dedicated explorers: Rear Admiral John Ross(1777–1856), his nephew James Clark Ross(1800 –1862), and an artillery of ficer named EdwardSabine (1788 –1883). For many years, the British navyhad searched for a “Northwest Passage” to the PacificOcean, an east-west sea lane among the icy islands of

northern Canada, but all its efforts were blocked bypolar ice. The Rosses, with Sabine conducting magneticobservations, tried to get through in 1818 and failed. An1819 expedition, under Edward William Parry, reached abit further before ice stopped it, but Sabine had sailed with it, and his observations convinced him he hadactually passed north of the magnetic pole.

[59] The Rosses were denied any further support bythe British admiralty, but in 1829 they found an inde-pendent backer: Felix Booth, owner of Booth’s distillery(Booth’s gin is still being made). They bought a 150-tonpaddle steamer, the Victory, and with favorable weathersailed it to the eastern shore of a long peninsula jutting

northward from North America. They named it in honorof their sponsor “Boothia Felix,” or “happy land of Booth,” which became “Boothia Peninsula” on today’smaps [Serson, 1981; Barraclough and Malin, 1981].

[60] At Boothia the Victory stuck in ice for the winter.In the following summer it managed to move only a fewmiles, and after the third winter the explorers aban-doned it, moved on by sledge, and were fortunate to bepicked up by a whaling ship. They had not managed totraverse the Northwest Passage.

[61] They did, however, discover the north magneticpole [Good, 1991]. After a series of careful magnetic

measurements, James Clark Ross became convincedthat the pole was no more than 100 miles west of the




spot where the Victory was wedged. In the summer of 1831, aided by local Eskimos (as desolate as the area was, it was inhabited), he set out toward that spot. On 1June, his measurements indicated that he was very close: A horizontal magnetic needle, suspended on a silkstring, showed no preference for any direction, and thedip needle pointed within 1 minute of arc of the vertical.

Ross spent the entire day making measurements, built astone cairn to mark the spot, raised the British flag andclaimed the land for the British crown. He was lucky tofind the magnetic pole so far south: Because of thegradual change in the Earth’s magnetic field, the polechanges its position, and in 1831 it was near the end of a long southward excursion [ Dawson and Newitt, 1982,Figure 1]. It is now at the edge of the Arctic Ocean, farnorth of where it was then. The south magnetic pole, in Antarctica, was reached by Douglas Mawson in 1909;more recently it has moved offshore.

[62] When Gauss and Weber began organizing a

global magnetic survey, the Rosses and Sabine (later SirEdward Sabine) became their leading lobbyists and sup-porters, and soon were the de facto leaders of a “mag-netic crusade.” By 1841, much of the worldwide network was actually in operation. The British had set up stationsin Greenwich, Dublin, Toronto, St. Helena, Cape of Good Hope, and “Van Diemen’s Land” (Tasmania), theEast India Company (also British) added four more inIndia and Singapore, Russia established 10 stations in itsown territory and one in Beijing, while other observato-ries were set up elsewhere, for a total of 53. The first worldwide survey had begun, a flood of magnetic data

began arriving, and the first worldwide magnetic chartscould be drawn.[63] The new tool of spherical harmonic analysis pro-

vided the first quantitative description of the Earth’smagnetic field, both its direction and strength. Sincethen, magnetic surveys have been carried out repeatedly[Good, 1985, 1988, 1994] across land, aboard specialnonmagnetic ships, and most recently, by satellites whose orbits sweep above the entire Earth every day.These included surveys by Vanguard 3 in 1959 [ Heppner ,1963], by Magsat in 1980 [ Langel and Estes, 1985], and bya Danish satellite aptly named “Oersted,” launched in1999 [Olsen et al., 2000], as well as by some Soviet

satellites. Many scholars, most recently, Jeremy Blox-ham and Andrew Jackson, also collected earlier compassobservations and tried to derive from them models of the field before the era of Gauss, as well as of its slow“secular” variation [ Bloxham, 1992; Bloxham and Gub-

bins, 1989].[64] The analysis has shown, as expected, that the

two-pole (“dipole”) part of the field greatly exceeded allothers. In other words, the two-pole terrella has alwaysbeen a good approximation to the actual field, although,if one wants to be a stickler for accuracy, the magneticcenter, the location of the dipole which fits observations

best, is offset a few hundred kilometers from the Earth’scenter. The surprising feature is that since the time of

Gauss, the strength of this “dipole” component hassteadily declined by about 5% per century, the declineaccelerating somewhat after 1970.

[65] Even though the measured strength of the mag-netic field is decreasing, it would be more accurate to saythat it is becoming more complex, rather than weaker. As the dipole part of the field is weakening, the more

complex parts (four-pole, etc.) are gaining strength. Inother words, as the strength of the field associated withthe largest spatial scale (dipole) decreases, that associ-ated with smaller scales increases.

[66] If all the Earth’s magnetism comes from its liquidcore (whose radius, about half the Earth’s, is knownfrom seismology), the nondipole components of the fieldare much stronger there, because they diminish muchfaster with distance R, like higher powers of R. Theorysuggests that the total magnetic energy cannot easilychange as quickly as the observed changes in the field.Calculations indeed confirm that the sum total of that

energy has changed very little, just that the complicatedparts have grown at the expense of the dipole, and sincethose decrease faster with distance, the field at thesurface has weakened, too.

[67] So, if we wait 1500–2000 years, is it possible thatthe dipole part will become very small, while the overallstructure of the field becomes complex, with perhapsmore than one pair of magnetic poles? And could it bethat 2000 years after that, the dipole part will havecontinued to grow in the opposite direction, until it againdominated the global structure of the magnetic field, the way it does now, but with opposite polarity? The geo-

magnetic record suggests this is not likely to happen, andthat the trend will probably change. However, it also tellsus (section 15) that the above scenario cannot be ruledout, and that such reversals have apparently happenedmany times during the Earth’s magnetic past.

10. MICHAEL FARADAY’S LINES OF FORCE (FIELD

LINES)

[68] Gauss avoided speculating on the source of theEarth’s magnetism. Just measuring it accurately waschallenging enough, and in the process, proving that it

originated inside the Earth, not outside it. The first stepstoward understanding that source were made by aslightly younger contemporary, Michael Faraday [Wil-

liams, 1965; Segre, 1984].[69] Faraday (1791–1867) was the son of an English

blacksmith, apprenticed by his parents to a bookbinder.Faraday took advantage of his job to read books broughtto the bindery, among them a volume of the Encyclo-paedia Britannica with an article about electricity. Sci-ence aroused his interest, and as a young man he at-tended free scientific lectures by Humphry Davy,Britain’s leading chemist [ Knight, 2000], taking careful

notes of what he heard. When Davy dismissed his assis-tant for brawling and advertised for a replacement, Fara-




day applied, sending his notes as proof of his interest,and ended up with the job.

[70] Starting as little more than Davy’s servant, Fara-

day rose to make a name for himself in the study of chemistry and magnetism, discovering (for instance) thebasic laws of electrolysis. What he lacked in formalschooling and mathematical analysis he made up inintuition and insight. He had a remarkable talent of expressing complicated scientific ideas in simple words,and was noted for his lucid lectures and writings.

[71] One notable contribution was a simple way of visualizing the magnetic force. When Gilbert exploredthe “orb of virtue” around his terrella, the region wheremagnetic forces could be sensed, he found that at everypoint in space, the force on his magnetic needle (includ-

ing the downward dip) had a certain direction. Faradaymade visible the overall pattern of those directions, byconnecting them with continuous lines.

[72] Those “lines of force” (today we call them fieldlines) spread out from the south pole of the terrella, archaround its middle, and converge again near the northpole (Figure 6). This not only describes the pattern of the directions of the magnetic force. It also tells aboutthe strength of that force: Where the lines crowd to-gether, the force is strong; where they are spaced widelyapart, it is weak.

[73] (Please note that purists sometimes name theends of a bar magnet “north seeking” and “south seek-

ing” rather than “north” and “south,” because if theEarth’s field were due to a bar magnet near its center,the north seeking end of that bar would be closer to thesouth magnetic pole, and vice versa.)

[74] Magnetic field lines have a strange history. Theystarted out as no more than a visualization aid, like linesof latitude and longitude, which have no real existencebut merely help visualize positions on the globe. Fara-day, however, felt [ Baggott, 1991] that space in whichmagnetic forces could exist was itself modified, and suchspace later became known as a “magnetic field.” Fara-day’s younger contemporary, James Clerk Maxwell, a

good friend and outstanding physicist, but unlike Fara-day, also a skilled mathematician, extended that notion

and showed that an “electromagnetic field” could sup-port a wave motion which spread at the velocity of lightand had the observed properties of light. Light, radio waves, X rays, microwaves all belong to the same family,making the concept of electromagnetic fields one of thefoundations of modern science and engineering.

[75] More was to come in the 20th century. Much of

space above the Earth’s atmosphere (and indeed,throughout the universe) contains plasmas, gases hotenough to contain free electrons and positive “ions,”atoms stripped of some electrons. The Sun consists of plasma, space around the Earth is filled with plasma, andin such plasmas, only very occasionally do free electronsrecombine with positive ions, usually not for long. This isquite unlike the familiar environment near the surface of the Earth, where free ions and electrons are rare andneutral atoms and molecules are the rule. In most of space, plasmas are so rarefied that collisions are rare,and ions and electrons spiral around magnetic field lines,

guided by them the way a bead is guided by a wire on which it is strung (e.g., Cowling [1957] or any text onplasma physics).

[76] Magnetic field lines in space thus assume yetanother role: defining the direction in which ions andelectrons travel most easily, as do electric currents andheat carried by them. Plasma waves also flow differentlyalong magnetic field lines and across them. In a rarefiedplasma, magnetic field lines are like the grain in a pieceof wood, which outlines the “easy” direction in which the wood readily splits. All these concepts sprang from the visual and intuitive imagination of Faraday, from a sim-

ple idea which grew into an extremely useful and versa-tile one.

11. FARADAY’S DISK DYNAMO

[77] Oersted and Ampere had shown that electriccurrents were the primary source of magnetism; forexample, a current following a wire, wrapped around aniron core, turned it into a magnet. However, reversingthe process, winding a conducting wire around a magnet,did not produce any electric current. Instead, Faradaydiscovered that electric currents were generated in a

closed conducting circuit only if that circuit sensed achanging magnetic field. The change could come from variations in the strength of the magnetic source, or itcould arise from relative motion between the source andthe conductor.

[78] This led to machines (once called “dynamos,”though today “generators” is the more common term) in which conductors are whirled around and around,through the fields of magnets, producing electric cur-rents. To serve a useful purpose (e.g., charge a storagebattery), those currents, of course, had to be led outsidethe rotating part. That is usually done by metal rings

attached to the rotating shafts (but electrically insulatedfrom them), connected to the rotating wires while at the

Figure 6. Magnetic field lines of a magnetic dipole at thecenter of the Earth. North is on top, where field lines enter theEarth.




same time touching sliding contacts mounted on thenonrotating part. From the sliding contacts other wireslead the current to wherever it is needed.

[79] Not every motion qualifies, because there is alsothe matter of energy. As Ampere had shown, a wirecarrying an electric current through the magnetic fieldencounters a force. Only when the force opposes themotion, so that one has to invest energy to overcome it,does a current flow, and not surprisingly, the energyinvested exactly equals the energy needed to drive the

current. Like a careful accountant, nature always bal-ances her ledger books.[80] Using these principles, Faraday devised the sim-

plest of all dynamos, the so-called “Faraday disk.” Imag-ine the space between the poles of a magnet (Figure 7), with straight magnetic field lines connecting one pole tothe other. In that space, a metal disk rotates on a shaftparallel to the field lines. The shaft and disk form themoving part of the circuit: The magnet and the wires thatcomplete the circuit are stationary, and the wires areelectrically linked to the disk by two sliding contacts, oneon the axle, one near the periphery.

[81] Because of the symmetric arrangement of the

disk and magnetic field, the current also tends to flowsymmetrically (a symmetry upset by the placement of thesliding contacts and external circuit), from the axle to therim, or vice versa, depending on the direction of rotationand on the magnetic polarities of the two magnet faces.Suppose it flows from the axle to the rim: The magneticforce on an electric current is perpendicular to both thecurrent and the magnetic field lines, so with appropriaterotation of the disk, it resists the rotation. By overcom-ing that resistance, we invest energy, which reappears asthe energy needed to drive the electric current, creatinga dynamo.

[82] (Inject current from some outside battery, rotatein the opposite direction so that the motion is helped by

the magnetic force rather than opposed by it, and yougain energy. Instead of a dynamo, you have a motor.Many types of dynamos can similarly become motorsrotating in the opposite direction. In some electric cars,the motor becomes a dynamo when the car coasts down-hill, braking its motion and at the same time returningenergy to the storage battery.)

[83] While Faraday’s disk dynamo illustrates the prin-ciple of dynamo action, it is not a practical generator of

electricity, unless one requires huge currents driven bytiny voltages. To carry such currents, very massive con-ductors would be needed, and the copper wiring used inconventional machinery would be completely inade-quate. But in a large volume filled with conductingmaterial, for instance, the liquid core of the Earth,believed to be largely molten iron, or on the Sun, suchflows are quite possible, and one notes that instead of asolid wheel, a rotating fluid eddy would serve just as well.That idea ultimately became the foundation of moderndynamo theory, briefl y described in section 14, whichseeks to explain how the magnetic fields of the Sun andthe planets, including the Earth, are generated.

[84] The basic “dynamo conditions,” a magnetic fieldand a closed conducting path, part of which movesacross the magnetic field while another part stays fi xed,also occur under other conditions. One of them, Faradayreasoned, was the flow of the river Thames throughLondon [Williams, 1965, p. 206 –208]. The magnetic field was provided by the Earth, the moving conductor wasthe flowing water, and the circuit was closed by a non-moving wire strung across Waterloo Bridge, dipping intothe water at both banks (Figure 8). The idea was sound,but the voltage generated was too small to be detected,in the presence of voltages due to chemical interactions

of metals.[85] Faraday had also experimented with electric cur-

Figure 7. Faraday’s disk dynamo, spinning between twomagnetic poles.

Figure 8. Faraday’s experiment with a “fluid dynamo” at

Waterloo Bridge.




rents flowing through glass containers from which mostair had been removed. In those rarefied gases he ob-served glows somewhat similar to the polar aurora or“northern lights.” He therefore speculated that the Gulf Stream, a broad flow in the Atlantic Ocean along theU.S. eastern seaboard, may similarly generate a voltage,and if its circuit were closed through the high atmo-

sphere, it might generate there the light of the polaraurora. His speculation was completely wrong, since theactual atmosphere is too poor a conductor of electricityto complete the circuit from the ocean to the upperatmosphere; but otherwise the mechanism was in prin-ciple possible.

12. SUNSPOTS

[86] The story of the Earth’s magnetism is stronglytied to that of solar research, in several ways. Large

magnetic storms, and observations of northern lights farsouth from their usual locations (i.e., of the “auroraborealis,” now commonly known as “polar aurora”), were found to be associated with solar phenomena. Andnot only did the Sun have a magnetic field, but concen-trated sources of that field, dark sunspots, were visibleon its surface, quite unlike the sources of the Earth’sfield which are buried deep in the Earth’s core. This ledto valuable insights into how the geomagnetic field mightbe generated.

[87] Sunspots were first reported in 1609, indepen-dently by Galileo, Scheiner, and Fabricius [ Newton,

1958;Phillips

, 1992], all of whom used the newly in- vented telescope. Sporadic reports of earlier observa-tions also exist, because the unaided eye can see largesunspots when thick haze dims the Sun near the horizon.Unfortunately, sunspots practically disappeared for a70-year period starting around 1645 (“the Maunder min-imum”). During those years the interest of astronomers wandered elsewhere, and when spots again became fre-quent, they were not investigated systematically.

[88] The 11-year sunspot cycle was discovered acci-dentally by Heinrich Schwabe (1789 –1875), a Germanpharmacist and amateur astronomer living in the town of Dessau [ Meadows, 1970; Newton, 1958]. Schwabe was

looking for a yet unknown planet of the Sun, movinginside the orbit of Mercury. Such a planet (given byother searchers the tentative name of Vulcan) would behard to spot (except during a total solar eclipse) becauseits position in the sky would always be close to that of theSun, where daylight would obscure it. Schwabe hoped toobserve it as a dark spot moving across the face of theSun, and day after day, year after year, whenever the sky was clear, he observed the Sun and looked for it.

[89] To properly conduct such a search, Schwabe alsohad to identify and track sunspots, to make sure none was mistaken for a new planet. He did so from 1826

onward, and by 1843 he noted a cyclical rise and fall intheir number, as well as in the number of days when no

spots were observed. He then published a table of his yearly totals, but until 1851 it attracted little notice,except from Rudolf Wolf, noted below. Then Alexander von Humboldt republished it (extended to 1850) in thethird volume of his “Kosmos,” and suddenly sunspotsand their cyclic behavior became a hot scientific topic.

[90] Rudolf Wolf (1816–1893) of Berne (later of Zu-

rich) collected earlier observations, tracing sunspot cy-cles before Schwabe’s time. He introduced the “Zurichsunspot number,” an empirical criterion for the numberof spots, taking into account the fact they usually oc-curred in tight groups. The length of the sunspot cycleturned out to vary, but the average value was near 11 years. Sir Edward Sabine (1852) found an associationbetween the sunspot cycle and the occurrence of largemagnetic storms, and Richard Carrington (1826 –1875),also in England, studied the rotation of sunspots aroundthe Sun, noting that their period and other propertiesdepended on latitude.

[91] In September 1859, Carrington (as well as R.Hodgson, another British observer) saw by chance abright outburst of light in a group of large sunspots,lasting about 5 minutes [ Meadows, 1970; Newton, 1958].This was followed 17 hours later by a very powerfulmagnetic storm, strongly suggesting a connection, al-though Carrington cautiously commented “One swallowdoes not make a summer.”

[92] But what were the sunspots themselves? We nowbelieve that they appear darker because they are slightlycooler than the regions that surround them. Galileospeculated that they might be clouds floating in the Sun’s

atmosphere, blocking some of its light. Their most sig-nificant feature, however, was discovered only in 1908,by George Ellery Hale (1868 –1938), leader among U.S.astronomers, founder of great observatories, and de-signer of novel instruments [Wright, 1966]. One of hisinventions (in 1892) was the spectroheliograph, alsodevised independently in France by Henri AlexandreDeslandres (1853–1948), a spectrograph adapted to scanthe Sun in a single spectral color, producing a photo-graphic image. Whereas in white light the Sun presented(apart from its spots) a bland appearance, the spectro-heliograph (e.g., tuned to the red H line of hydrogen)isolated light from higher layers in the Sun’s atmosphere

and revealed many new features. These included mot-tling of the surfaces, prominences arching high above theSun (turning to dark linear features when passing infront of the Sun) and bright areas near sunspots. Halealso found that “solar flares,” such as the one observedby Carrington and Hodgson, were much more frequentlyseen in H light, and big ones indeed often precededmagnetic storms.

[93] In 1896, Pieter Zeeman discovered the “Zeemaneffect” by which the characteristic colors (“spectrallines”) of elements, when emitted by a gas located in astrong magnetic field, often split into two or more com-

ponents of slightly different wavelength, with their sep-aration depending on the intensity of the field. Using the




Zeeman effect, Hale in 1908 showed that sunspots werein fact strongly magnetic, with a typical field intensity of 1500 G (0.15 T). The spots generally appeared in pairs of opposite polarity, suggesting that field lines emergedfrom the Sun at one of the pair and reentered at theother. One spot was usually ahead of the other in thedirection in which the Sun rotated, and the magnetic

polarity of the “leading” spot north of the equator, inany solar cycle, was always the same, and was opposite tothe leading polarity south of the equator. In the follow-ing solar cycle, both these polarities were always reversed, suggesting that the sunspot cycle was a magneticphenomenon, with an average period near 22 years.

[94] Hale’s method was greatly refined by HoraceBabcock [ Babcock, 1960, 1963; Phillips, 1992; Eddy,1978], Robert Leighton, and others. Using the polariza-tion of Zeeman lines to construct a “solar magneto-graph,” they greatly increased the sensitivity of Hale’smethod, to the point where not only sunspot fields could

be observed, but also a general dipole field of the Sun, of the order of 5 gauss. The existence of such a field hadbeen suspected from a feature of the solar corona, theSun’s outer atmosphere, previously seen only duringtotal eclipses of the Sun. At the poles the corona dis-played rays or “plumes” in a pattern which remindedobservers of magnetic field lines of a dipole, like theones above the poles in Figure 6. The magnetographalso showed that this field reversed each 11-year solarcycle, typically 3 years after sunspot minimum.

13. THE DYNAMO PROCESS ON THE SUN

[95] The Sun is a giant ball of gas, hot enough toconduct electricity (i.e., a plasma), much hotter thananything that exhibits permanent magnetism. Sunspotmagnetism therefore had to come from electric currents,and in 1919 Sir Joseph Larmor proposed that suchcurrents could be produced by a self-sustaining fluiddynamo [ Larmor , 1919a, 1919b, 1919c, 1929]. A Faradaydisk can serve as a model of such a dynamo, if itsmagnetic field is created by an electromagnet, poweredby the output current of the same disk (other types of generator also can be so designed). Energy must be

supplied by the outside force which rotates the disk, andthe presence of iron is not essential: If the electromagnethas an iron-free core, the magnetic field produced in itmay be much weaker, but it would still exist.

[96] However, what comes first in a dynamo, the mag-netic field needed for producing the current, or theelectric current needed to produce the field? Thissounds a bit like asking, What came first, chicken or egg? Actually, if conditions are appropriate, even a very weakinitial field is amplified exponentially, rising until (ne-glecting friction, viscosity, acceleration, etc.) the oppos-ing magnetic force on the current creates a torque

matching the applied one.[97] Larmor suggested that a similar situation might

arise in an electrically conducting fluid, in which suitableflows are produced, e.g., circulation of the fluid due toheat convection, that can play the role of a rotating disk.He was quite tentative about it, just one brief paragraph,followed by two alternative suggestions (and his fol-low-up note of 1927 did not get any closer). The ideasurvived in part because nothing else seemed to lead

anywhere, but implementing it (finding such flows in aconducting medium) proved quite dif ficult. In a conven-tional dynamo, the electric current is channeled by wires wrapped in insulators, in a way forcing it to generate therequired magnetic field. In a continuous conductingfluid, no such channeling exists.

[98] Attempts to model fluid dynamos were furtherdiscouraged by the “antidynamo” theorem of Thomas G.Cowling (1906 –1990), who proved that the fluid dynamoproblem had no axially symmetric solutions [Cowling,1933, 1985]. That eliminated easy and simple solutions,and some researchers began to wonder whether solu-

tions existed at all.[99] A useful concept in the theory of plasmas and of

conducting fluids is the “freezing” of magnetic fieldlines, developed by Wale n [1946] and by Alfve n [1950;

Alfve n and Fa lthammar , 1963], although the idea itself may be older. It can be shown that in a fluid withextremely high electrical conductivity, such field lines are“frozen” into the plasma, in the sense that two particles which share the same field line at any time, usuallycontinue doing so later on, even when the flow hasseparated them and has deformed their magnetic field.In fluids whose conductivity is merely large, not infinite,

Cowling showed that field lines slowly slipped from their“frozen” positions. The criterion for “extremely high”also involved the dimensions of the flow, and was there-fore more likely to be fulfilled by large-scale flows, onthe Sun and in the Earth’s core, rather than in thelaboratory.

[100] For instance, the “solar wind” which flows moreor less radially outward from the Sun is a hot plasma andmay be viewed as such a conductor. The region at whichthis wind originates is permeated by magnetic fields,originating in the global solar dipole and in sunspotregions. As some solar wind ions move out radially to theEarth’s orbit and beyond, others on the same field lines

but deeper in the Sun’s atmosphere stay behind. By the“field line preservation” theorem, however, they con-tinue to share the same field line.

[101] Out near the Earth, the field carried by the firstgroup forms the local interplanetary magnetic field(IMF). The particles left behind on the Sun, on the otherhand, have meanwhile rotated with the Sun to othermeridians. Solar wind particles on the same field line willtherefore be on different meridians than the “roots” of that line on the Sun. The longer these particles havetraveled through space, the more have their “roots”rotated and therefore the greater the difference in solar

longitude from those roots, suggesting that the magneticfield lines curve to form a spiral, which ultimately be-




comes wrapped tighter and tighter. This curvature hasindeed been observed by spacecraft, at the Earth ’s orbitand throughout the solar system, suggesting that thesolar wind “remembers” its solar roots for days and evenmonths after leaving the Sun.

[102] The stretching of magnetic field lines in generalimplies an amplification of the magnetic field. The in-tensity of the IMF, for instance, decreases with distance

R from the Sun like R

1

(for the azimuthal component,in the direction of the rotation) or like R2 (for theother components), whereas that of the undistorted di-pole decreases much faster, like R3. If the Sun’s field were a pure dipole, its intensity at the Earth’s orbit would be much weaker than what is actually observed.

[103] On the Sun, such stretching may be expectedfrom the observed uneven (“differential”) rotation of the photosphere, the visible surface layer of the Sun. Therotation period, as seen from Earth (whose motionaround the Sun adds to it about 2 days) can be measuredfrom the apparent motion of sunspots, and its value indays, as function of the latitude in degrees, is shown inTable 1 [ Newton, 1958].

[104] The difference in periods presumably reflectsheat-driven flows in the Sun. If the Sun had started off with a dipole field, the differential rotation would havegradually wrapped field lines around it (Figure 9), inopposite directions north and south of the equator.

[105] The original idea was that this happened not toofar below the visible surface (today we are less sure[ Parker , 2000]). As field lines become draped around theSun, they become denser, meaning the intensity B of the wrapped field would grow, and theory predicted that agrowing “magnetic pressure” (proportional to B2) would

develop in the tube-like region those lines occupy. Thatpressure pushes plasma out of the tube, reducing itsdensity and causing parts of the tube to float up andbreak the surface, becoming visible as sunspot pairs.

Note that since the field’s direction is reversed in oppo-site hemispheres, the spots leading the pair in the direc-tion of the rotation have opposite polarities north andsouth of the equator.

[106] A subtle problem needs to be addressed, how-ever [ Elsasser , 1955, 1956a, 1956b]. In a spherical geom-etry, like that of the Sun, magnetic fields can be divided

into two classes, toroidal and poloidal fields. If the fieldis axially symmetric, poloidal field lines lie in meridionalplanes (like those of the dipole field), while toroidal fieldlines form circles around the axis of symmetry. However,nonsymmetric modes also exist in each class; for in-stance, the main field due to the Earth’s core and ob-served near its surface is poloidal. In principle, everymagnetic field can be resolved into toroidal and poloidalparts. If for some reason the dynamo process creatingthe field stopped, electric resistance would cause a grad-ual decay of the field, and it can be shown that each classdecays independently of the other.

[107] The scenario envisioned above began with adipole field, which is purely poloidal. The stretching of the field line due to uneven rotation adds and amplifiesa toroidal field, proportionally to the “seed” poloidalfield. However, unless that “seed field” itself gets rein-forced, one can foresee that after some time, electricalresistance will cause the currents that produce it todecay, and without the seed, the entire dynamo processcomes to a halt.

[108] Resistive decay might take many millions of years, but what is actually observed is different and muchfaster, the reversal of the poloidal field (and hence also

of the toroidal field it produces) every 11-year solarcycle. This suggests some sort of feedback, and EugeneParker suggested [ Parker , 1955, 1979, 2000] that it arosefrom cyclonic motion, like the one seen in hurricane,namely, the interaction between the rising motion of plasma above sunspots and the Sun’s rotation caused theplasma to rotate around a vertical axis as it rose. Fieldlines embedded in the plasma were also twisted, fromthe toroidal wraparound direction to the meridionaldirection, which returned part of their field back to thepoloidal component.

[109] The above ideas of the solar cycle were devel-oped by Babcock [1961] in the 1960s. More recent work

[ Parker , 2000] suggests that the Sun’s magnetism mayextend to a substantial depth, but many unsolved ques-tions remain, including the causes of the uneven rota-tion.

14. THE EARTH’S DYNAMO

[110] If a fluid dynamo exists inside the Earth, it has tobe in its core, with about half the radius of the Earth.The reason is that the process requires a conductingfluid, and the propagation of earthquake waves has

suggested the core is liquid, with a solid inner core in itsmiddle [ Brush, 1980]. Its high density fits a liquid metal,

Table 1. Rotation Period of the Sun as Function of Latitude

Latitude, deg Period, days

0 26.910 27.120 27.6

30 28.440 29.6

Figure 9. Schematic view of the way the faster rotation of theSun’s equator might wrap magnetic field lines around the Sun.




generally believed to be iron (possibly, with other ele-ments dissolved in it), a relatively abundant element.The iron has sunk to the center of the Earth because itis heavy, the same reason that molten iron sinks to thebottom of a blast furnace.

[111] For a while, around 1950, another idea was pro-posed, namely, that every spinning massive object devel-

oped an intrinsic magnetic field, proportional to its an-gular momentum. According to that view, the fieldasymmetries observed on the Earth’s surface were dueto secondary processes, such as the circulation of liquidiron in the core. While Gilbert suggested that the Earthrotated because it was magnetic, this explanation arguedthe opposite: The Earth was magnetic because it rotated.

[112] Arthur Schuster (1851–1934) was the first topropose the idea [Schuster , 1912; Warwick, 1971], and itsmain supporter in the 20th century was Patrick M.Blackett (1897–1974), a British physicist who wasawarded the 1948 Nobel prize for his work on cosmic

rays [ Blackett, 1947]. By then it was known that in elec-trons and protons, spin and magnetic moment wererelated: could not the same hold for matter at large?However, the observation that the field did not weakenin deep mines ( Runcorn [1948]; includes appendix bySydney Chapman) was evidence against the idea, andBlackett finally ruled it out by an experiment with aspinning gold sphere [ Blackett, 1952; Massey, 1974].

[113] Meanwhile other physicists, initially, Walter El-sasser at Columbia University tried to find mathematicalsolutions to the dynamo problem [ Elsasser , 1946, 1947,1956b]. The method devised by Gauss to represent the

Earth’s internal field outside the region of electricalcurrents can be generalized to describe any toroidal andpoloidal fields, and this generalization should in princi-ple be applicable to represent the flows and fields in theEarth’s core. Unfortunately, because of Cowling’s anti-dynamo theorem, no axially symmetric solutions wereexpected, leading researchers to assume a most generalclass of fields and motions. This produced complicatedrelations, which led to even more complicated ones, withno closure in sight.

[114] For a while some scientists wondered whetherany solutions existed at all. Then in 1964, StanislawBraginsky [ Braginsky, 1964a, 1964b] in Russia surprised

the world by producing an entire class of solutions which were almost symmetric: a symmetric field, plus a smalladdition. Because the addition was small, the procedureconverged.

[115] All these were solutions of the “kinematic dy-namo problem,” in which the flow pattern of the con-ducting fluid could be freely specified. A realistic modelof the dynamo also requires a mechanism and a sourceof energy driving the flow. Bullard [1949] pointed outthat a modest amount of heat generation by radioactivity would suf fice. Braginsky suggested that the solidificationof molten iron and its deposition on the inner core could

also supply the heat which produced the flows and thusdrive the dynamo effect.

[116] Considerable progress has taken place sincethen [ Levy, 1976; Inglis, 1981; Jacobs, 1987; Buffett, 2000;

Roberts and Glatzmaier , 2000]. The problem of feedbackfrom toroidal to poloidal modes was addressed by the“alpha-mode” dynamo of Steenbeck et al. [1966] of theInstitute of Astrophysics in Potsdam, Germany. Theyshowed that fluid turbulence whose statistical properties

had no mirror symmetry could lead to a mean electro-motive force, driving electric currents whose magnitude was times the magnetic intensity, in the direction of the underlying average field. This may be viewed as ageneralization of Parker’s idea for explaining the solarcycle. An interesting “dynamo experiment” in the labo-ratory was reported by Lowes and Wilkinson [1963], whospun two iron cylinders (each representing an eddy)inside a container of mercury, at right angles to eachother. When the angular velocity exceeded a certain value, the magnetic field jumped to a new higher pla-teau, and that was viewed as the initiation of dynamo

action.[117] One interesting development has been the math-

ematical simulation of the dynamo process, using high-speed computers [Glatzmaier and Roberts, 1997; Glatz-

maier et al., 1999; Coe et al., 2000]. The simulationassumes a heat source in the core and heat loss throughthe core-mantle interface. Dynamo action is generallyobtained, but the complexity of the field and the fre-quency of main field reversals (discussed in the nextsection) depend greatly on the distribution of the heatloss, whether it was uniform across the interface, highestat the pole, equator, or in between, etc. The simulations

also found occasional “excursions” in which the fieldseemed to head toward a reversal but then reasserted itsoriginal polarity, a class of events also deduced from thepaleomagnetic record. Interestingly, a simulation run with a much smaller inner core produced no reversals atall.

15. DIPOLE REVERSALS AND PLATE TECTONICS

[118] When lava flows out of a volcano and hardens,the result is often a black rock known as basalt. Basalt isslightly magnetic, and as it solidifies and cools, it be-

comes magnetized in the direction of the prevailingmagnetic field. The process is somewhat similar to thecapture of the prevailing magnetic field by a cooling barof steel, noted by Gilbert (Figure 3).

[119] Volcanoes are often active for long periods, andby comparing the magnetization of their lava flows fromdifferent times (especially if such flows can be dated bysome method), one can learn about changes in thedirection of the local magnetic field. Bernard Brunhes(1867–1910) found ancient lava flows in France whosemagnetization appeared to be reversed [ Brunhes, 1906].Other examples were then found, and the Japanese

geophysicist Motonori Matuyama (1884–1958) exam-ined the evidence and suggested that the magnetic sig-




natures were evidence of actual reversals [ Matuyama,1929]. Matuyama proposed that long periods existed, inthe past history of the Earth in which the polarity of themagnetic poles was the opposite of what it is now.

[120] Unfortunately, the situation was complicated bythe work of Louis Neel (1904 –2000), later awarded the1970 Nobel prize, and of John Graham, who showed that

some materials exhibited spontaneous self reversal of their magnetization. Only in the early 1950s did JanHospers, starting with a study of Icelandic basalts, con- vince many in the geophysics community that most rocks with reversed magnetism were not self reversed but wererelics of epochs when the Earth’s had reversed magneticpolarity [ Hospers, 1951; Cox et al., 1967; Cox, 1973;

Frankel, 1987].[121] Meanwhile, in the early years of the 20th cen-

tury, a German meteorologist, geophysicist, and polarexplorer named Alfred Wegener (1880 –1930) proposeda radical new idea [ Hallam, 1973; Georgi, 1962; Kious

and Tilling, 1996; Oreskes, 1999]. He first formulated it in1912, expanded it in 1914 while recovering from woundsreceived in World War I, and published it in 1915 in aGerman book titled The Origin of Continents and Oceans

[Wegener , 1929].[122] It was long ago noted that elevations on the

Earth were not smoothly distributed, but tended to clus-ter at two levels [ McGill, 1982, Figure 1], the continentsand the ocean floors, with a relative scarcity of in-between depths. Like others before him, Wegener notedthat the coastlines of some continents, especially theones bordering the Atlantic Ocean, would fit together

remarkably well (as was shown by Alex du Toit (1878 –1948) and later by Carey and Bullard (1907–1980), theedges of the continental shelf fit even better [ LeGrand,1988, pp. 204 –205]). He then collected evidence showingthat the fit between matching edges of continents ex-tended to geological formations and even to fossils of animals and plants.

[123] His explanation was the theory of “continentaldrift.” Geologists had previously proposed that conti-nents were slabs of lighter rock floating on top of densermaterial below, the way ice slabs floated in the ArcticOcean. Wegener went one step further and proposedthat such slabs could slowly move or “drift.”

[124] His theory was vigorously attacked, especially bySir Harold Jeffreys (1891–1989), Britain’s leading theo-retical geophysicist. Jeffreys argued that the layers on which continents floated were too viscous to permit suchdrifts, that their resistance to such motions would be toohigh. Wegener’s theory had little support among geo-physicists, and he even found it dif ficult to secure auniversity position in Germany (he ended up as profes-sor in Graz, Austria). Wegener died on a polar expedi-tion in 1930, at age 50, caught by bad weather whilereturning by dogsled from a supply mission to a weatherstation on top of the Greenland ice cap.

[125] A few diehards kept Wegener’s ideas alive [e.g., du Toit, 1937], and by the 1950s, when the geophysics

community finally accepted the reality of magnetic reversals, a variant of it emerged, the theory of polar wandering. By that theory, promoted by Keith Runcorn(1922–1995) and Thomas Gold, the crust of the Earthslowly slid around the interior, somewhat like one of Halley’s layers, so that areas now in the polar zone oncehad temperate climate, and vice versa [ Runcorn, 1955].

This explained phenomena such as coal seams in thepolar islands of Spitzbergen (Svalbard) and evidence of glaciation in countries such as South Africa and India, as well as variations in the magnetization of crustal lava.

[126] The theory reached its peak in the early monthsof 1955, when it was debated in a seminar in Cambridgeand then put to a vote by the participants: Only one voice was raised against it (E. Deutsch, private commu-nication, 1985). Soon, however, trouble arose in inter-preting magnetic data: Observations from the same pe-riod but from different continents disagreed in assigningthe positions of ancient magnetic poles.

[127] The resolution of all these controversies camefrom the seafloor [ Bullard, 1975; Kious and Tilling, 1996].Oceanographic surveys had shown a “mid-Atlanticridge,” a raised ridge running roughly north-south, ap-proximately halfway between the continents to its eastand its west. The ridge was clearly volcanic, as shown bya concentration of earthquake epicenters along it and by volcanic islands located on it, such as the Azores. Othersuch linear features were found in the Pacific and Indianoceans, and all were linked in a worldwide pattern.

[128] Two prominent geophysicists, Robert Dietz(1914 –1995), who was at the time with the Naval Elec-

tronics Laboratory Center in San Diego, and Harry Hess(1906 –1969) of Princeton University, proposed that theseafloor might be spreading out from the ridges [ Dietz,1961] (this started the term “seafloor spreading,” thoughthe idea began with Hess [ Frankel, 1980]). In their view,new seafloor was formed by lava emitted from theridges, while old seafloor descended again to lower lev-els at oceanic trenches, deep slots in the ocean floor,generally parallel to chains of islands. (A biography of Robert Dietz is available from Scripps Institution of Oceanography, at their website http://scilib.ucsd.edu/sio/archives/siohstry/dietz-biog.html.)

[129] The convincing evidence, though, came from

magnetic surveys. In the late 1950s, electromagneticmagnetometers (developed in World War II) becameavailable to geophysicists. Unlike the suspended-needleinstruments, they could easily operate aboard ships, air-planes, or even (a bit later) aboard satellites. Some wereproton precession instruments, related to the nuclearmagnetic resonance (NMR) method now extensivelyused in chemistry and for medical imaging. Others werefluxgate magnetometers, using substances which enteredmagnetic saturation at a stable and well-defined field value. These instruments were accurate enough to mapthe magnetism caused by local materials of the Earth’s

crust, by deriving the deviations from the smooth globalfield (“anomalies”). On land, where such instruments




were used in the search for oil, the observed magneticanomalies showed no consistent patterns. Above theocean floor, on the other hand, the patterns were wellordered, often in long stripes of opposite magnetic po-larity.

[130] In 1962, Lawrence Morley [ Morley, 1986; Lear ,1967] proposed an explanation, which independently

also occurred to Fred Vine and Drummond Matthews[Vine and Matthews, 1963]. Their idea was that the sea-floor indeed spread out from the mid-ocean ridges, toboth sides, at about an inch a year. As the lava oozed out(a process later photographed from research subma-rines) it hardened, acquired the prevailing magnetiza-tion, and was then gradually carried away to either sideof the ridge.

[131] Now and then, however, the main dipole reversed its direction, and the magnetic imprint reverseddirection too. Each magnetic stripe therefore consistedof lava that cooled during an epoch of a certain magnetic

polarity (wide stripes from long epochs, narrow stripesfrom short ones) and, of course, all stripes tended to beparallel to the central ridge. The process was somewhatanalogous to that of a tape recorder, with the seafloornear the mid-oceanic ridges behaving like a pair of magnetic tapes, slowly unrolling in opposite directions while faithfully recording the Earth’s magnetic field atthe time they emerged.

[132] For a long time, Morley’s contribution was notacknowledged, because the referees of Nature and Jour-

nal of Geophysical Research to which he submitted hisarticle and) rejected it as too speculative. One referee

wrote: “His idea is an interesting one—I suppose— but itseems more appropriate over martinis, say [rather] thanin the Journal of Geophysical Research” [ Morley, 2001;see also Cox, 1973, p. 224]. It was finally cited in anon-scientific journal [ Lear , 1967]. Even Vine and Mat-thews met resistance, until Jim Heirtzler [ Heirtzler , 1968;

Heirtzler et al., 1966, 1968] produced a two-dimensionalmap of the striping of the Reykjanes Ridge near Iceland, where navigational radio aids allowed accurate determi-nation of position. The map (Figure 10) graphicallyillustrated the remarkable symmetry of the striping.

[133] But where did the seafloor go at the edge of thecontinents? In some places, for example, off Japan, it

indeed descended into deep oceanic trenches. Else- where, however (for instance, off the eastern seaboardof the United States) nothing remarkable seemed totake place. From this came the idea that just as thehigher parts of the Earth’s crust formed separate conti-nents, so the underlying layer, the lithosphere, was divided into “plates” which constantly moved. Plate mate-rial was continually created at the oceanic ridges, then asthe plates moved, the continents sitting on top of them were carried along, and at the trenches the plates de-scended again.

[134] This idea, later elaborated, became the theory of

“plate tectonics” (“tectonics” means “building up,” inthis case, of the Earth’s crust). Wegener had the right

idea, but nature carried it out differently from what hethought. The continents were not plowing through thelithosphere, the way ships plow through the ocean; in-stead, they sat on top of conveyer belts that carried themalong. Some plates also rotated, and some regionsslipped alongside others (“transform faults”), but theoverall size of the Earth, of course, did not change.

(Note that this section is a very abbreviated and cursorysummary of a broad subject on which an extensive liter-ature exists, only in part related to geomagnetism. See,for example, Frankel [1982], Legrand [1988], and Glen[1982].)

16. MAGNETIC STORMS AND RING CURRENTS

[135] The method used by Gauss and his followers toanalyze the observed magnetic field showed that at least99% of it originated inside the Earth. But external

sources also existed: Magnetic storms (typically changingthe field by 1% or less) were worldwide phenomena andseemed to indicate that a powerful electric current wasestablished around the Earth’s equator, flowing at anundetermined distance for many hours or even days. Arthur Schuster in 1911 gave it the name we still use, thering current [Smith, 1963].

[136] The polar aurora (sometimes known as auroraborealis or northern lights) also seemed to be linked tothe magnetic field. Its dancing and glowing ribbons (usu-ally a shade of green, but sometimes red) tend to main-tain a constant distance from the magnetic pole of about

2000 –3000 km (i.e., a constant “magnetic latitude”);near the magnetic pole itself they are rarely seen. Ingreat magnetic storms they temporarily descend to lower(magnetic) latitudes and become visible at centers of population, more distant from the poles. At such loca-tions the aurora is a rare phenomenon, but it is not so forresidents of central Alaska or Canada. Auroral ribbonsoften consist of many parallel rays (Figure 11), con-stantly fading or brightening, each aligned with the di-rection of local magnetic field lines. As found in 1741 byHiorter and Celsius in Sweden [ Beckman, 2000], brightdisplays are also accompanied by local disturbances inthe Earth’s magnetic field, often several times stronger

than those of worldwide magnetic storms.[137] What caused all this? Kristian Birkeland (1867–

1917) in Norway thought (correctly) that the glow camefrom fast electrons hitting the high atmosphere, andtried to simulate the phenomenon on a small scale [seeStern, 1989]. Inside a glass vacuum chamber [ Brundtland,1998] he sent an electron beam toward a magnetizedsphere representing the Earth, which like Gilbert henamed terrella (Figure 12). He found that such beamstended to follow magnetic field lines, which guided themto the poles of a magnet or a terrella inside the chamber.The renowned French mathematician Henri Poincare

(1854 –1912) then calculated that, at least with straightconverging field lines, such electrons would in fact spiral




along those lines and would be reflected from regions of more intense magnetic field, closer to the cone’s apex.

[138] Auroral outbursts were known to be associated with solar activity, and Birkeland guessed that they orig-inated in beams of electrons emitted from the Sun.However, neither he nor his younger associate, themathematician Carl Stormer (1874 –1957), could explain why the aurora avoided the magnetic poles themselves,

preferring instead to occur in large ring-shaped regionsaround them [Sto rmer , 1955].

[139] Birkeland also observed magnetic disturbancesassociated with auroras, using a network of four groundstations, and concluded that they arose from electriccurrents flowing in the high atmosphere, along auroralarcs, linked at their ends to distant space, where thecircuit somehow closed. The auroras and their associ-ated currents occasionally intensified for an hour or so,and Birkeland termed such events polar magnetic

storms. Unlike regular magnetic storms, these were lo-calized and could not be observed at lower latitudes,

Figure 10. Map of magnetic striping of the seafloor near the Reykjanes ridge [ Heirtzler , 1968].




where most magnetic observatories (as well as centers of population) were located.

[140] The correlation between magnetic storms andactivity on the Sun indeed suggested that something wasbeing emitted by the Sun toward Earth. But not beamsof electrons, as Birkeland had suggested, for it could beshown that electrostatic repulsion would disperse suchbeams long before they reached Earth. Instead, SydneyChapman (1882–1970) and Vincent Ferraro(1907–1974) proposed in 1930 that the Sun emitted hugeclouds of plasma, containing equal numbers of positive

charges and electrons. Being electrically neutral, such acloud could travel without dispersing.

[141] However, it could not penetrate the Earth’smagnetic field. To do so, its ions and electrons wouldhave to attach themselves to terrestrial field lines andshare them with terrestrial plasma, e.g., that of theionosphere. By the theorem of field line preservation orthe “freezing” of field lines (see section 13 on the solardynamo), particles which initially did not share field linescannot suddenly start doing so.

[142] Field lines linked to the Earth’s poles wouldtherefore remain confined inside a “Chapman-Ferraro

cavity” around which the cloud would wrap itself. Thesudden confinement was expected to compress the

Earth’s field lines, and explained a small abrupt jump inmagnetic intensity (“sudden commencement”) observedon the surface at the start of many magnetic storms.Chapman and Ferraro speculated that somehow thecavity also generated the ring current, but how thishappened they did not know.

[143] The ring current presumably consisted of

charged particles trapped in the Earth’s magnetic field.Stormer traced some of their motions by numericalcalculations (in 1908 these had to be done by hand), buthis work concentrated on particles with extremely highenergies, not like those of the aurora. S. Fred Singer,however, showed [Singer , 1957] that low-energy parti-cles, of either sign, could also do the job, provided theirnumbers were large enough. Such particles spiraledaround field lines while sliding along them, but thesliding motion would stop and reverse (as Poincare hadshown) whenever the particle approached regions of more intense magnetic field. In the field of the Earth, the

magnetic intensity increased as one approached the“feet” of the field line in the atmosphere, since these were the points (on any given field line) closest to thecenter of the Earth. Many trapped particles would there-fore be reflected before reaching the dense atmosphere,and could stay trapped for a long time, bouncing backand forth across the equator (Figure 13).

[144] Singer noted a slow secondary mechanism that would meanwhile move trapped particles from theirguiding field lines, attaching them to neighboring onesand gradually transporting them all the way around theEarth [Stern, 1989, Figure 4]. Electrons would be trans-

ported in one direction, ions in the opposite one, and ineither case, the electric current they carried around theEarth was in the direction required by the ring current.

17. THE MAGNETOSPHERE

[145] (Relatively few references will be cited fromhere on, because more than 350 are given by Stern

[1996], also available on the World Wide Web (http:// www.phy6.org/Education/bh2_1.html). See also Stern

and Ness [1982]).[146] Sputnik 1, launched on 4 October 1957, was the

first artificial satellite of the Earth. It was followed byseveral thousand others, many of them carrying scientificinstruments. Scientific satellites soon confirmed someearlier guesses, disproved or modified others, and addedunexpected features to the picture.

[147] The satellites did find belts of trapped ions andelectrons, moving in the way proposed by Singer, butthey were a permanent feature, not a temporary onepresent only during magnetic storms [e.g., Stern, 1996;Stern and Ness, 1982]. The first U.S. satellites, Explorers1 and 3 built by James Van Allen and his team at theUniversity of Iowa, were launched in early 1958 and

discovered an intense belt of trapped fast protons, abovethe magnetic equator. This “inner radiation belt” turned

Figure 11. The polar aurora; woodcut by Fridtjof Nansen.The lines defining the “curtains” of the auroral arcs followmagnetic field lines.




out to be a secondary product of the cosmic radiation, which is a population of extremely energetic protonsbelieved to fill our galaxy. Cosmic rays provide a rather weak source, but the trapping is quite stable, allowingthe protons to accumulate over a long time.

[148] The inner belt, peaking at distances between 1.3and 2 R

E(Earth radii) from the Earth’s center, was

dense enough to constitute a radiation hazard (astro-nauts can cross it safely, but only if they do not linger),

however, it was not extensive enough to carry the ring

current. That was done by the outer radiation belt (2– 8 R

E), found to contain protons and electrons of moderate

energies but in much greater numbers. Magnetic storms

greatly increased its population, and with it also the

intensity of the ring current, though some of the outer

belt and of the ring current existed at all times.

[149] The Sun indeed emitted plasma which flowed

out radially at great speed, but again, it did so at all

times, not just during magnetic storms as Chapman and

Ferraro had assumed. That was the solar wind, predictedin 1958 by Eugene Parker, after he tried to calculate the

equilibrium structure of the Sun’s corona. The calcula-

tion indicated that an extremely hot plasma like that of Sun’s corona could not be held in gravitational equilib-

rium, the way the Earth’s atmosphere was. Instead,

Parker’s calculation suggested that the Sun continually

sloughed off plasma in a supersonic stream, expandingradially in all directions. Though Parker’s prediction wasat first controversial, spacecraft instruments soon con-firmed it. In 1961, its flow was measured by a “Faradaycup” placed aboard Explorer 10 by Herb Bridge and

Bruno Rossi (1905–1993), and more extensively by thespace probe Mariner 2 in 1962.

Figure 12. Kristian Birkeland with one of his terrella experiments.

Figure 13. Schematic view of the motion of an ion or elec-

tron trapped in the Earth’s magnetic field. Not to scale: actu-ally the orbit is much narrower near the Earth.




[150] By then Explorer 12 had crossed a well-definedboundary on the sun-facing side of the Chapman-Fer-raro cavity (renamed by Tom Gold “magnetosphere”),suggesting that the cavity also existed at all times. Theboundary was named “magnetopause.” The “suddencommencement” jumps of the magnetic intensity, at theonset of many magnetic storms, turned out to mark thearrival of fast plasma clouds, which plowed through the

ordinary solar wind and created shock fronts ahead of themselves.[151] One feature which no one had expected before

the satellite era was the long magnetic tail behind theEarth. On the sunward side the solar wind compressedthe magnetospheric cavity, creating an abrupt boundaryat an average distance of about 10.5 R

E, though some

interplanetary clouds, associated with magnetic storms,could push it to distances of 6 R

Eand even less. On the

night side, on the other hand, the Earth’s field lines werestretched out in two great bundles, each linked to one of the polar caps (Figure 14). Later spacecraft would ob-serve these bundles at distances of more than 3 times

that of the Moon, 200 R E from the Earth and beyond.[152] Wedged between the two bundles, the two “tail

lobes,” was the tail’s plasma sheet, a layer of hot plasma which turned out to be the main source of auroralelectrons. The magnetic field of the plasma sheet is weak, with hairpin-shaped magnetic field lines, and thatturned it into a somewhat unstable region, the origin of many of the magnetic disturbances observed in spaceand on the ground. The reason auroras are rare near themagnetic poles, but occur fairly regularly along largerings surrounding each pole, is that auroral electrons(and their associated electric currents) are guided along

magnetic field lines. Magnetic field lines from the “au-roral oval” lead back to the plasma sheet, whereas those

from the magnetic poles themselves (and the regionsnear them) end up in the twin bundles of the tail lobes, which contain rather little plasma.

[153] Birkeland’s observations were also validated,again with some modifications. What he called polarmagnetic storms are now known as magnetic substorms,so named by Chapman, who at first viewed them as

component parts of magnetic storms, before it was real-ized that they occurred at other times as well. Theyrepresent violent changes in the plasma sheet whichenergize its ions and electrons, hurl them earthward, andin that way contribute to the outer ring current. Mag-netic storms, following the arrival of a fast cloud, create very powerful disturbances of this kind and seem to bethe main agent replenishing the ring current, though thedetails still need to be worked out.

[154] The electric current system, which Birkelandclaimed accompanied the aurora, was also observed,though its structure was somewhat different from what

Birkeland had proposed. It forms an intricate patternfirst traced by Alfred Zmuda and James Armstrong,using a “piggyback” (free ride) magnetometer aboardthe Navy satellite Triad. Milo Schield, Alex Dessler, andJohn Freemen, who theoretically predicted such cur-rents in 1969, named them “Birkeland currents,” and thename is still used. Birkeland currents provide the energyfor most auroral arcs, but auroral electrons are thenenergized only close to Earth, getting their main push inthe lowest 1–1.5 R

Eof their guiding field lines.

18. MAGNETIC RECONNECTION

[155] In such a brief description it is hard to do justiceto the extensive field of magnetospheric physics. Likemost physical processes, those of the magnetosphere,too, must usually be paid for in energy, the universalcurrency of physics. Except for the inner belt, the sourceof that energy seems to be the solar wind. If the sepa-ration between interplanetary and terrestrial field lines were strictly enforced, it would be rather dif ficult totransmit energy from one region to the other, but thereexists a loophole. Magnetic field lines can change theirlinkage and “reconnect” in new ways, if the plasma in

which they are embedded flows through an “ x-type neu-tral point” (or “neutral line”) at which the field intensitydrops to zero and field lines cross in the pattern of theletter x (Figure 15).

[156] James Dungey in England proposed [ Dungey,1961; Stern, 1986] that two such points were formed onthe magnetopause, the boundary of the magnetosphere.One occurred on the sunward side, where interplanetarymagnetic field lines and terrestrial lines formed an x-shaped neutral point (or a continuous line of suchpoints), at which they split up and reconnected. If theinterplanetary field points exactly south, the plasma

flows along the thick arrows in Figure 15: the solar windplasma arriving from the left and the magnetospheric

Figure 14. Magnetic field lines and the bow shock (not a

field line) in the Earth’s magnetosphere, with some namedfeatures (not to scale).




plasma coming from the right. The northern half of theterrestrial line then connects to the northern half of theinterplanetary line, a similar process joins the southernhalves, and the plasmas attached to the newly recon-nected lines then flow outward, up or down, in Figure 15.

[157] This, Dungey assumed, created “open” fieldlines, starting in the polar regions of the Earth andextending to interplanetary space, and along such lines,energy and plasma easily flowed from the solar wind tothe magnetosphere. The process ended (in Dungey’soriginal theory) at a second neutral point in the distanttail, where reconnection reunited the two terrestrial

halves and the two interplanetary halves, after which thetwo plasmas were again separated.

[158] If the interplanetary field lines pointed notsouthward but just slanted toward the south, reconnec-tion is a bit more complicated, introducing a bend ineach open field line, at which the direction of the ter-restrial line gradually changes over to that of the inter-planetary one. If the field line slants northward, thebending is more severe. The northern half of the terres-trial field line must link up with the interplanetary half which comes from the south, and vice versa. If thenorthward slant is steep, reconnection is expected to

become increasingly dif ficult. The interplanetary fieldlines just have the “ wrong direction” for linking up.

[159] The actual process is probably much more com-plicated, but one of its predictions was amply confirmed.It was found [ Fairfield and Cahill, 1966] that the mostimportant factor promoting magnetic substorms, largeBirkeland currents, and other active phenomena was theslant of the interplanetary magnetic field. With southward slant activity was likely, with northward slant it was

inhibited, and other factors, such as the velocity anddensity of the solar wind, were much less important.

[160] Magnetic reconnection may also occur in theplasma sheet, constituting there an important element of the substorm process, but details will have to wait formore thorough studies of the magnetosphere, using si-multaneous data from a much larger number of satellitesthan is available now.

[161] Reconnection, acceleration, and energization of plasma in the magnetosphere all involve a complicatedinterplay between magnetic and electric fields. Staticmagnetic fields can trap and steer electrons and ions

without changing their energy. Since the fields do nothave to supply energy, they can maintain the trappingindefinitely without requiring any additional energy in-put.

[162] On the other hand, an electric field, a region of electric forces, is not only able to energize particles, itcan also help them move from one field line to another,for example, enter regions of magnetic trapping or es-cape from them. Electric fields therefore have a centralrole, both in bringing fresh particles into the ring cur-rent, as happens during magnetic storms, and in addingenergy to those particles.

[163

] In addition, electric forces parallel to magneticfield lines are also instrumental in accelerating (nega-tive) auroral electrons downward. By its nature, any“parallel electric field” which does so, will also grabpositive oxygen ions from the ionosphere and acceleratethem upward, toward the magnetic equator, where many join the ring current; such “ion beams” were discoveredin 1977 by the U.S. Air Force satellite S3-3. Further-more, electric fields are also essential to the energyexchange in substorms and to reconnection. The detailsof such processes (to the extent they are understood)are, however, completely beyond the scope of this brief and nonmathematical overview.

19. PLANETARY MAGNETOSPHERES

[164] The internal magnetic field of the Earth resultsfrom the interplay of some very definite features: theexistence of a liquid, electrically conducting core, therotation of the Earth, and the presence of energy sourcesin the core, which cause the fluid to circulate. It thuscame as a surprise that other planets of the solar system, very different from the Earth, also had their own mag-netic fields [ Bagenal, 1992].

[165] The first planetary magnetic field investigated,and the biggest one by far, was that of Jupiter. In early

Figure 15. Dungey’s view of plasma flow (thick arrows) andmagnetic field lines (thin ones) at an x-type neutral point. If this point is at the front-side “nose” of the magnetosphere inFigure 14, then the lines on the left (southward) are interplan-etary, on the right (northward) terrestrial, and those that exiton top and bottom are “open.”




1955, two young radio astronomers started working witha cross-shaped antenna array of the Carnegie Institu-tion’s Department of Terrestrial Magnetism. The arraycould select signals from a narrow range of directions,and Ken Franklin and Bernie Burke calibrated it using aknown source, the Crab Nebula, then began surveying

the surrounding sky.[166] They found another conspicuous radio source

[ Franklin, 1959], but unlike the Crab, its position shiftedas days passed. Standing next to the array one night,Bernie noted a star overhead and asked Ken “ what isthat bright thing up there?” It was Jupiter, and that was where the signal was coming from. In publishing theirresult, the astronomers speculated “the cause of thisradiation is not known but is likely to be due to electricaldisturbances in Jupiter’s atmosphere.”

[167] In 1959, when the Earth’s radiation belts werealready known, Frank Drake observed Jupiter’s emis-

sions and concluded from the relative intensities in arange of wavelengths that their radio waves were prob-ably produced by electrons trapped in a strong magneticfield. Then in 1973 the space probe Pioneer 10 passed byJupiter and found there, sure enough, an enormousplanetary magnetic field and a very intense radiationbelt.

[168] The strength of the source of Jupiter’s dipolefield, its dipole moment, is some 20,000 times that of theEarth. Like the Earth’s dipole, its axis is inclined byabout 10 to the rotation axis, but its polarity is in theopposite direction of the Earth’s (until the next reversal,at least). What produces that field is still unclear. Jupi-

ter’s core may well consist of hydrogen, compressed bythe huge weight of the planet’s outer layers to where itbecomes a metal and conducts electricity [ Nellis, 2000].The strange radio signals observed by Franklin andBurke came from Jupiter’s radiation belt, the most in-tense one in the solar system, so intense that after justone pass through it, Pioneer 10 suffered some (minor)radiation damage. Along with its radiation belt, Jupiteralso has intense auroras, observed from the Earth andfrom orbiting telescopes.

[169] The Jovian magnetosphere is very different fromthe Earth’s. It is much bigger, extending to 50 –100 R J

(Jupiter radii; 1 R J is about 10 R E), and unlike theEarth’s, its plasma seems to corotate with the planet up

to the dayside boundary with the solar wind. It containsnot just protons and electrons, but also ions of sulfur andsodium, probably emitted from the moon Io (see below)and it carries a very extensive ring current. That currentis concentrated near the equator, creating a magneto-sphere much more flattened than the Earth’s, so much

that it has been referred to as the Jovian “magnetodisc.”[170] Jupiter’s magnetic field has some interesting in-

teractions with the planet’s larger moons, which arebigger than ours and whose absorption creates distinctdips in the radial distribution of plasma. Io, the inner-most large moon, is a bizarre world heated internally byits tides, with active volcanoes and a thin atmosphere. Itsionosphere and/or body conduct electricity, and the rel-ative motion between it and Jupiter’s magnetospherecreates a dynamo circuit, producing currents of a fewmillion amperes which flow between Io and Jupiter’sionosphere.

[171

] This remarkable phenomenon was anticipatedby Goldreich and Lynden-Bell [1969] and by Piddington

and Drake [1968], who suggested it as an explanation forthe curious effect of Io’s position in its orbit on deca-meter wave radio emissions from Jupiter’s magneto-sphere. The theory of Goldreich and Lynden-Bell wasfinally confirmed in 1993 by the observation of infraredemission from the foot points in Jupiter’s ionosphere of the “Io flux tube,” of field lines which threaded Io[Connerney et al., 1993]. The space probe Voyager 1passed close to the Io flux tube on 5 March 1979, andobserved the magnetic field of its currents [ Acun a et al.,1981].

[172] All four giant planets, Jupiter, Saturn, Uranusand Neptune, were visited by Voyager 2. (The first two were also visited by Pioneers 10 and 11 and by Voyager1, and the probe Ulysses flew by Jupiter, while the probeGalileo is currently in orbit around it.) In all four thedipole moment, the strength of the bar magnet which, if placed at the center, gave a comparable field, was muchgreater than that of the Earth (Table 2).

[173] Voyager 2 unexpectedly found the magnetic axesof Uranus and Neptune to be inclined by about 60 and45 (respectively) to their rotation axes. The shape andproperties of a planetary magnetosphere depends on the

angle between the flow of the solar wind (i.e., the direc-tion from the Sun) and the magnetic axis, and for those

Table 2. Magnetic Fields of Earth and of the Giant Planets [from Bagenal , 1992]

Planet Radius,

km

Spin Period, hours

Magnetic Moment,

M Earth

Mean Equatorial

Field, gauss Dipole Tilt and

Sense, deg

Solar Wind

Density, cm 3

Distance to“ Nose,”

Planetary Radii

Earth 6378 24 1 0.31 11.3 10 11

Jupiter 71,400 9.9 20,000 4.28

9.6

0.4 50–100Saturn 60,300 10.7 600 0.22 0 0.1 16–22Uranus 25,600 17.2 50 0.23 59 0.03 18Neptune 24,800 16 25 0.14 47 0.005 23–26




two planets, that angle varies rapidly as the planet ro-tates. As a result, their magnetospheres undergo wild variations during each rotation, although each managesto contain some trapped particles. The origin of all thosefields is unknown. Saturn is big enough to producemetallic hydrogen in its core, and interestingly, its mag-netic and rotational axes are the same within observa-

tional accuracy. The magnetic fields of Uranus and Nep-tune might be generated in relatively poorly conducting“icy” interiors, perhaps explaining their large dipole tiltand complex field geometry.

[174] The planet Venus was visited by Mariner 10 in1974, which continued from there to Mercury. Venus was found to be unmagnetized. The solar wind is onlystopped by its upper atmosphere, the Venus ionosphere,creating a completely different type of magnetosphere,more like a comet tail. On the other hand, tiny Mercury,an airless rock only moderately bigger than our Moon,rotating very slowly, surprised observers by being mag-

netized. Its magnetic field is weak and probably does notextend far enough to trap many particles, but as thespacecraft passed through its nightside tail, it observed asudden spasm in which particles were apparently ener-gized. NASA has scheduled the Messenger mission to fl yto Mercury and orbit it, and the European Space Agencyis planning a Mercury mission as well.

[175] Mars [ Acun a et al., 1999] and the Moon [ Fuller ,1974] have permanently magnetized patches of rock ontheir surfaces, suggesting that even if they now lack adynamo field, at some time in the past they might havepossessed one. That would agree with the giant volca-

noes (apparently extinct) observed on Mars, which sug-gest a hot interior, although the volcanoes themselvesare not associated with magnetic patches. On Mars, inparticular, these patches (as observed by the MarsGlobal Surveyor) create fields about 20 times strongerthan the surface magnetization of the Earth (as distinctfrom the Earth’s core magnetic field) would create at thesame distance of observation.

[176] Planetary magnetic fields thus seem to be therule, not the exception, at least in our solar system,though the origin of those fields may be quite differentfrom that of ours. Researchers who feel frustrated bytheir inability to conduct direct observations on the

Earth’s core should note that the source regions of thosefields are even less accessible. Thus, as the study of planetary magnetic field enters its second millennium, itfaces more unanswered riddles than ever before.

20. ASSESSMENT

[177] Often in a field of science the lode of researchseems to run out, as major problems are resolved andattention turns increasingly to details. Geomagnetismshows how disciplines may rejuvenate themselves by

shifting their focus to new targets and new methods.[178] Geomagnetism started with the mapping and

monitoring of the global magnetic field. Then in theearly 1800s a new class of phenomena entered the pic-ture, magnetism caused by electric currents. Work byFaraday in that direction, together with astronomicalstudies of sunspots, led by 1919 to the notion of theself-exciting dynamo. Though this seemed like a prom-ising way of explaining the Earth’s internal field and its

time variations, nearly another century passed beforemodern computers allowed the terrestrial dynamo to beeven approximately modeled. Meanwhile, the observa-tion of rock magnetism suggested the occurrence of magnetic reversals, and dynamo theory turned out to beconsistent with reversals, too. In the 1960s these observations combined with Wegener’s ideas of moving con-tinents and with studies of mid-ocean ridges to producethe science of plate tectonics. Magnetic storms (firstobserved in the 1700s) and related northern lights re-mained a mystery until after 1958, when artificial satel-lites started probing the Earth’s distant magnetic field.

And finally, planetary magnetic fields observed by dis-tant space probes suggest that several different pro-cesses of planetary magnetization may exist.

[179] All these changes helped geomagnetism stay inthe forefront of geophysics, in one form or another. Oneis reminded of the Japanese board game of Go, wheretwo opponents using black and white counters take turnsplacing them on the intersections of a board ruled insquares, each trying to surround and “choke” pieces of the opponent. A player’s “soldiers” survive only as longas they have access to some unoccupied “breathingspace.” Disciplines of science are like that, too. They

only “live” as long as they touch some unsolved areas, aslong as they can lay claim to yet unsolved mysteries. Ithas been the good fortune of geomagnetism to succeedin doing so over four centuries. The past never guaran-tees the future, but it is to be hoped that the field still hasa long way to go.

APPENDIX A: A CHRONOLOGY OF

GEOMAGNETISM

[180]

100 0 (approx) Chinese discover that lodestonefloating on “boat” prefers south-northdirection.

118 7 Alexander Neckham describes pivotedcompass.

126 9 Letter by Petrus Peregrinus describesproperties of magnets.

1492 Columbus sails for America, notesdeclination changes in mid-ocean fromeasterly to westerly.

1581 Robert Norman publishes “The Newe Attractive,” announcing the discovery of

magnetic dip (inclination).1600 William Gilbert’s De Magnete: Earth




itself is a great magnet.1634 Henry Gellibrand discovers the secular

variation of declination.169 9 Edmond Halley conducts the first

magnetic survey.172 2 George Graham discovers diurnal

variation of declination.

174 1 Graham in London and Celsius inSweden observe simultaneous magneticperturbations due to the polar aurora.

1777 Coulomb introduces his torsion balance,later shows magnetic forces (electricones, too) obey an inverse squares law.

1801 Alessandro Volta demonstrates his“ voltaic pile,” the first battery.

182 0 Oersted discovers magnetism due toelectric currents.

1820 Andre-Marie Ampere explainsmagnetism in terms of forces between

electric currents.182 8 von Humboldt urges Gauss to study

magnetism. Later Gauss developsmethod to measure magnetic intensityand an electrical telegraph.

1831 Faraday discovers electrical induction,later introduces disk dynamo. TheRosses and Sabine reach the northernmagnetic pole.

183 2 Faraday tries to detect a dynamo currentin water flowing in the Earth’s field.

1834 Gauss founds “Gottingen Magnetic

Union,” later (1836 –1839) developsspherical harmonic analysis of the scalarmagnetic potential.

184 3 Heinrich Schwabe publishes firstevidence for the sunspot cycle.

185 1 von Humboldt publishes Schwabe’s work;sunspot cycle widely accepted.

1852 Sabine finds evidence that magneticstorms follow the sunspot cycle.

1859 Richard Carrington observes white lightsolar flare, followed by large magneticstorm.

189 2 George Ellery Hale introduces

spectroheliograph.1895 Kristian Birkeland begins experimenting

with electron beams and terrellas.Henri Poincare calculates a simplemotion of trapped particles.

189 6 Pieter Zeeman discovers splitting of spectral lines emitted in magnetic field.

190 3 Birkeland proposes the existence of “polar magnetic storms.” He alsosuggests aurora is caused by electronbeams emitted from the Sun.

190 6 Bernard Brunhes publishes first evidence

of reversely magnetized rocks.1908 Hale uses the “Zeeman effect” to show

sunspots are intensely magnetic.190 9 Douglas Mawson reaches the southern

magnetic pole, at the edge of Antarctica.191 2 Arthur Schuster proposes magnetic

storms are evidence for a “ring current”in space, circling the Earth.

1918 Alfred Wegener publishes The Origin of

the Continents and Oceans.191 9 Joseph Larmor proposes that magnetic

fields of sunspots may be produced by aself-sustaining dynamo action.

192 9 Motonori Matuyama produces evidencethat reversely magnetized rocks mayhave originated when the Earth’smagnetic polarity had reversed.

193 0 Chapman and Ferraro suggest magneticstorms are due to plasma clouds fromthe sun (not electron beams), envelopingthe Earth’s magnetic field.

1931 Alfred Wegener dies in the snows of Greenland.

193 3 Thomas Cowling proves self-sustaineddynamos are never axisymmetric.

194 6 Walter Elsasser tries to calculate dynamosolutions.

194 7 Giovanelli: magnetic neutral points nearsunspots are site of flare energy release.

195 1 Jan Hospers publishes study of Icelandiclavas, concludes from theirmagnetization that reversals were real.

1952 Runcorn promotes “polar wandering” to

explain magnetic reversals.1955 Franklin and Burke detect radioemissions from Jupiter. Eugene Parkerproposes way for solar toroidal fields tostrengthen the poloidal field.

1957 Sputnik 1 and 2 begin the era of spaceflight. Fred Singer proposes a ringcurrent carried by trapped low-energyparticles.

1958 Explorers 1 and 3 discover the innerradiation belt. Eugene Parker predictsthe solar wind.

1959 Tom Gold coins word “magnetosphere.”

Drake proposes Jupiter has a radiationbelt.

1961 Hess and Dietz propose Earth’s crustspreads out from mid-ocean ridges.Magnetic reconnection and plasmaconvection in the magnetosphereproposed by Dungey, Axford, and Hines.Babcock proposes empirical theory of sunspot cycle.

196 2 Magnetopause crossed by Explorer 12.Mariner 2 maps solar wind and itsstreams.

1963 Morley, Vine, and Matthews proposethat magnetic banding of the ocean floor




is produced by seafloor spreading and

polar reversals.

IMP 1 launched, first mapping of the

Earth’s magnetotail. Vanguard 3 maps

the Earth’s internal field from orbit.

196 4 Akasofu and Meng et al. analyze

morphology of magnetic substorms.Braginsky publishes solutions to the

kinematic dynamo problem.

196 5 Heirtzler produces map of symmetric

magnetic banding of the ocean floor.

196 6 Steenbeck et al. propose “alpha

dynamo,” generalizing an idea of Parker.

1969 Schields, Dessler, and Freeman propose

a system of “Birkeland currents” linkingthe Earth to space.

1971–1972 Lunar magnetic field surveyed by lunarsatellites from Apollo 15, 16.

1972 OGO 7 observes “coronal holes,” laterstudied aboard “Skylab.”197 3 Zmuda and Armstrong publish map of

polar “Birkeland currents.”197 3 Pioneer 10 crosses Jupiter

magnetosphere (4 December), followedby Pioneer 11 (1974), Voyagers 1 and 2(1979), and Galileo (enters orbit, 1995).

1974 Mariner 10 flies by Mercury, observes itsmagnetic field.

197 5 Lowes and Wilkinson demonstratedynamo action in the lab.

197 9 Voyager 1 passes near currents inducedby Io’s motion relative to Jupiter

197 9 Pioneer 11 passes magnetosphere of Saturn, followed by Voyager 1 (1980)and Voyager 2 (1981).

198 1 First precision mapping of the Earth’sfield from space, by Magsat.

198 6 Voyager 2 passes magnetosphere of Uranus.

198 9 Voyager 2 passes magnetosphere of Neptune.

199 4 Ulysses observes fast solar wind abovethe Sun’s south pole.

199 7 Mars Global Surveyor observes Marscrustal magnetization, no core field.Mapping phase, March 1999 to August2000.

199 7 Glatzmaier et al. use computer tosimulate the Earth’s dynamo and itsreversals.

1999 “Oersted” satellite launched to map theEarth’s main field.

[181] ACKNOWLEDGMENTS. The author thanks the

many colleagues whose comments helped shape and improvethis article, in particular, David Barraclough, Stuart Malin,

Jack Connerney, Henry Frankel, Raymond Hide, and thereferees of this article.

[182] Louise Kellogg was the Editor responsible for thispaper. She thanks two anonymous technical reviewers and ananonymous cross-disciplinary reviewer.

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20771, USA. ([email protected])


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