ADVANCED COMPOSITION EXPLORERADVANCED COMPOSITION EXPLORER

A C EA C E2nd Edition

ACE Facts

Mission:• Launch: August 25, 1997• Launch Vehicle: Delta II • Primary Mission: Measure the composition of energetic

particles from the Sun, the heliosphere, and the Galaxy• Orbit: Halo orbit around the Earth-Sun libration point, L1• Mission Lifetime: There is sufficient hydrazine for ACE to

remain in an L1 orbit until 2019, depending on the details of the orbit.

Spacecraft:• Mass: 785 kg (includes 195 kg fuel at launch)• Structure: Two octagonal decks,

1.6 m across, 1.0 m high• Propulsion: Hydrazine fuel for insertion and

maintenance in orbit• Power: 443 W, four fixed solar arrays• Attitude Subsystem: Spinning spacecraft (5 rpm),

spin axis is Earth/Sun pointing• Communication Subsystem: S-band, 7 kbps (real time),

2 Gbit (total) solid state recorders• Instrumentation: Eight instruments that measure plasma

and energetic particle composition, and one to measure the interplanetary magnetic field

ACE Science Investigations

Advanced Composition Explorer

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“What canst thou see elsewhere which thou canst not see here? Behold the heaven andthe earth and all of the elements; for of these are all things created.”

Thomas à Kempis, c. 1420

ho among us has not asked, “Where did I comefrom?” This question is usually one about life, but

behind it are scientific questions about the material ofwhich we are made, the elements in the atoms and mol-ecules of our bodies. The answer to the question“Where did the matter we are made of come from?” isnot so easy to find. Some could be satisfied with ananswer such as “We are made of the same elementsthat are found on the Earth we live on.” But where didthat material come from? The Earth is but one planet inthe solar system, and most of the solar system materialis inside the Sun. How can we find out what the Sun ismade of? Where did the Sun come from? One can evengo further and ask, “What is the Galaxy made of?” Thereis a whole series of related questions that are involved inunderstanding the cycles the matter goes through as theuniverse and the structures within it evolve.

The material that Earth and the solar system aremade of has been changed and rearranged during thebillions of years since its creation, so measuring its com-plete composition, or makeup, is difficult. We have goodevidence that the first elements to appear in the earlyuniverse were the lightest ones, hydrogen and helium.Most of the gas found between the stars in our MilkyWay galaxy, and, we think, in other galaxies throughoutthe universe as well, are hydrogen and helium. We alsoknow that the Sun is made chiefly of hydrogen and heli-um. We understand that stars, including our Sun, shineby the process of combining, or “burning”, lighter ele-ments into heavier ones, hydrogen into helium, and heli-um into carbon, and so on. In fact, in the outer layers ofthe Sun, we can see the light emitted by heavy ele-ments, like carbon, silicon, and iron. These elementswere formed by an earlier generation of stars.

Scientists have attempted to answer questions aboutwhere this matter came from and how it evolved in avariety of ways. Meteorites that have hit the Earth canbe studied since, in some respects, they seem to be likethe solar system when it formed. Another way to studythe material is by going into space, above Earth’s atmos-phere, to study particles that come from the Sun. Earlyin the 20th century, scientists learned that energetic par-ticles from space are bombarding the Earth. With theadvent of space missions, we learned that they comenot only from the Sun, but also from the distant reachesof the Galaxy. It has been recently discovered thatsome of these particles come from the gas clouds out-side our solar system. The primary purpose of theAdvanced Composition Explorer, ACE for short, is tostudy these particles. We are learning more about whatmatter is there: about its composition, where it comesfrom, and what it tells us about the evolution of the largeruniverse.

High-speed gas from a supernova explosion slams intodark cooler clouds of interstellar material. Shocked andheated by this tidal wave of energy, the clouds glow inbright, neon-like colors.

W

2

T H E M I S S I O N

The ACE spacecraft was built at the Applied PhysicsLaboratory of The Johns Hopkins University (JHU/APL).

Expanded view of the ACE spacecraft

ACE at L1, between the Earth and the Sun

3

MISSION OVERVIEWThe instruments on the ACE spacecraft are designed to

sample the matter that comes near the Earth from theSun, from the apparently (but not actually) empty spacebetween the planets, and from the Milky Way galaxybeyond the solar system. They do so with a collectingpower 10 to 1000 times greater than previous experi-ments. Particles are identified by their type (which atomthey are), by their mass (which isotope they are), by theirelectric charge or ionic state, and by their energy. Evenvery rare isotopes can be studied. The information gath-ered by ACE is compared with that from other missions,past and present, for a better understanding of the inter-action between the Sun, the Earth, and the Galaxy.

In order to measure solar particles and plasma twentyfour hours a day without being affected by the Earth'smagnetic field, ACE has traveled about 1.5 million km(about a million miles) from the Earth to the Earth-Sunlibration point, L1. This is the point where the centripetalforce and the gravitational pulls of the Earth and Sun bal-ance. This balance keeps ACE at an ideal location forthese studies. From its vantage point, 1/100 of the dis-tance from the Earth to the Sun, ACE performs measure-ments over a wide range of energy. By orbiting the L1point in a halo, ACE can follow the Earth as it revolvesabout the Sun, always staying between them.

The ACE mission has a goal of lasting at least fiveyears. Overall NASA responsibility for the mission is inthe hands of the Space Science Mission OperationsProject Office of NASA Goddard Space Flight Center(GSFC) in Greenbelt, MD. The lead scientific institution isthe California Institute of Technology (Caltech) inPasadena, CA. The Applied Physics Laboratory of TheJohns Hopkins University (JHU/APL) in Laurel, MD wasresponsible for building the spacecraft.

ACE SPACECRAFT AND INSTRUMENTSThe ACE spacecraft consists of a two-deck irregular

octagon, about 1.6 m (65 inches) across and about 1.0 m(40 inches) high. It spins about its axis so that one endalways points toward the Sun and the other toward theEarth. It contains redundant equipment for collecting andstoring data, and transmitting the data back to Earth.Data is transmitted via a highly directional parabolic dishantenna mounted on the aft (bottom) deck of the space-craft. Four other broad-beam antennas, capable of trans-mitting data at lower rates, are also available if needed.Twenty four hours worth of science and housekeepingdata (about 1 Gigabit), recorded on one of two solid-staterecorders, is transmitted to Earth in one three- to four-hour telemetry pass each day. Spacecraft attitude (theorientation of the spacecraft) is provided by a star trackerand digital Sun sensors.

Mounted to the spacecraft are eight scientific instru-ments which measure a variety of different particle types.Four arrays of solar cells power the spacecraft and theinstruments. These arrays provide sufficient power toallow ACE operations to continue for at least five years.Attached to two of the solar panels are booms, or longarms, for the ninth instrument, a pair of magnetometers.

Measuring a particle’s type, charge, mass, energy,direction of travel, and time of arrival provides the cluesneeded to help determine its source and the processes bywhich it has gained energy. The ACE instruments coveran unprecedented range of particle type and energy;simultaneous measurements from these instruments arecoordinated to create a comprehensive picture of theenergetic particles that pervade the inner solar system.

ACE was launched on a Delta II rocket in August1997 from the Kennedy Space Center in Florida. Delta II is an expendable two-stage, liquid-fueledrocket that stands at a height of 38 m (126 ft) andweighs 232,000 kg (511,000 lb). It is currently theworld’s most reliable launch vehicle.

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What’s New on ACE

• Coordinated measurements of three distinct samples of matter

- Solar- Local interstellar- Galactic

• 10 - 1000 times larger collecting power

• Measures all solar elements from carbon to zinc

• Determines the masses of individual atomic nuclei over a wide range of velocities

• Real-time transmission of solar wind parameters and interplanetary magnetic field a half hour or more prior to arrival of solar wind at Earth’s magnetosphere

TO THE SCIENTIFIC COMMUNITY:A primary goal of ACE is to study the composition of

material from the Sun, the local interstellar medium, and theGalaxy to better understand the formation and evolution of thesolar system. Each of these samples tells a different story:pickup ions and anomalous cosmic rays are samples of thepresent-day interstellar medium; galactic cosmic rays providea sample of matter from the Galaxy that was acceleratedmillions of years ago; and solar matter represents an oldersample of interstellar matter that has been stored in the Sunfor the last 4.6 billion years. With coordinated observationsthat extend from solar wind to cosmic ray energies forelements ranging in mass from hydrogen to zinc, ACE is amajor extension of composition studies by earlier spacecraftand balloon-borne instruments. Precise studies of even rareisotopes are possible due to vastly increased capabilities tocollect particles and more accurately identify them.

ACE is providing new determinations of the composition ofthe Sun, which comprises more than 99% of the matter in thesolar system. By measuring how many electrons remainattached to solar wind and higher energy ions ACE ismeasuring the (several million degree) temperatures ofregions from which these particles originate. Elemental andisotopic composition measurements reveal the composition ofthe solar atmosphere, as well as composition patterns thatarise when some particles are accelerated more easily thanothers. The broad range of composition measurements thatACE provides now makes it possible to identify the origin ofenergetic particle populations observed in interplanetaryspace and understand the processes by which they areaccelerated. Comparisons of the composition of solar windand higher energy solar particles with that of meteorites,comets, the moon, planetary atmospheres, and galacticmaterial are providing key information on the history of oursolar system.

Measurements of radioactive isotopes in the galacticcosmic rays by ACE have shown that cosmic rays must have

been accelerated at least 100,000 years after they weresynthesized in supernova explosions. Other isotopemeasurements show that cosmic rays typically spend about15 million years in our Galaxy before leaking out, implying thatthey must be replenished continually. The relativeabundances of the stable isotopes of Mg, Si, Ca, Fe, and Ni incosmic rays are found to be very similar to those in solarsystem material, indicating that the effects of galacticevolution since the creation of the solar system are not large.

ACE was launched during solar minimum conditions andthen observed the transition to solar maximum. During thisperiod the number of solar flares and coronal mass ejectionsincreased dramatically, including some of the largest solarparticle events observed since the dawn of the space age.The new capabilities provided by the fleet of spacecraft now inspace have combined to make this one of the most productivesolar maximum periods in history in terms of providing newunderstanding of the Sun. Studies of solar wind, solarparticles, and cosmic rays by ACE, in combination with otherspacecraft such as Ulysses and Voyager, are providing newinsight into the bubble of solar wind that envelops our solarsystem, and the nature of its interactions with the Galaxy.

ACE has become a key component of NASA’s new “Livingwith a Star” Program, which seeks to understand how solarvariations affect life and society, and to provide a scientificbasis for improved forecasting of “space weather.” From itsposition at L1 ACE is able to measure directly Earth’s ever-changing solar wind and solar particle environment, includinginterplanetary disturbances that disrupt Earth’s magnetic fieldand cause the aurora. The combination of ACE data from L1and magnetospheric data from the Polar, Geotail, SAMPEX,and IMAGE spacecraft has made it possible to determine howthe magnetosphere and upper atmosphere respond to solarvariations.

WHAT’S IMPORTANT ABOUT THIS MISSION?

During coronal mass ejections (CMEs),magnetic fields become unstable, releasingpreviously constrained hot material from theupper atmosphere of the Sun. As a result,hot gas streams out into the solar system,impacting planets, moons, and spacecraft,and causing space weather that can causestorms in Earth’s magnetosphere andendanger spacecraft and astronauts.

5

ACE’s Collecting Power Compared

TO THE WORLD AT LARGE:In addition to the information base provided to the sci-

ence community, ACE has many benefits for the rest of us. In a partnership with the National Oceanic and

Atmospheric Adminstration, the Real-Time Solar Wind(RTSW) system on ACE functions as a “space weather sta-

tion”, allowing a half-hour or more advancewarning of magnetic storms. With proper noti-fication, the harmful effects of these stormscan be minimized, since ACE provides informa-tion about solar wind plasma, the interplanetary

magnetic field, and intermediate and high-energy particleinformation to NOAA for its predictions. ACE, SOHO, andGOES work together to provide space weather data on theinternet that is used by space agencies around the world.

ACE is also involved in a public education partnershipwith the Cooperative Satellite Learning Project (CSLP).CSLP is a successful and award-winning partnership join-

ing NASA, Honeywell Technology Solutions,Inc., and schools nationwide. It is designed tomotivate students of all ages into science,engineering, math, and careers in the spaceindustry through hands-on involvement in a

NASA scientific satellite mission. Through CSLP, ACE hasbeen adopted by the Old Bridge High School in Old Bridge,New Jersey. Students there are involved with, and learnfrom, the various stages of the ACE mission.

ACE was chosen as the first mission to follow NASA’sRenaissance approach. “Renaissance” stands forReusable Network Architecture for Interoperable SpaceScience, Analysis, Navigation, and Control Environment.This approach involves re-engineering the process bywhich flight projects build space communications, data, andinformation systems. The goal is development and opera-tions that are cost-effective with maximum flexibility. ACEmade the best use of existing hardware designs and flight

spares from other NASA missions. These instruments useflight-proven technologies, requiring less development andverification time and effort.

Scientists from Germany and Switzerland, in addition toCalifornia, Delaware, Illinois, Maryland, Missouri, NewHampshire, and New Mexico were involved in buildingACE. The ACE team is currently working with scientistsfrom around the world and the United States on a broadrange of investigations. Such cooperation makes it possi-ble to address questions with diverse approaches, datasets, and models, and leads to a freer flow of ideas and tomore efficient and robust solutions to problems.

A comparison with ACE of the collecting power ofprevious spectrometers designed to measure theisotopic composition and charge states of solar andinterplanetary particles. Note that the collectingpower scale is logarithmic.

In addition to the charge state instruments shownabove, the STOF sensor on SOHO measures chargestates from ~0.02 to ~0.2 MeV/nucleon with acollecting power similar to that shown for ISEE.

Full-scale model of ACE spacecraft built by OldBridge High School in Old Bridge, NJ

ACE

Typical Balloon

ISEE-3

<0.5

~0.5

EHICUlysses

CRRESSAMPEX

GeotailWind

Collecting Power ( Events/year of silicon-14)

Cosmic Ray Isotope Spectrometers

Kin

eti

c E

nerg

y (G

eV

/nu

cle

on

)

ISEEEHIC

SAMPEX

GeotailWind

ACE (ULEIS)

GeotailWind

ACE (SIS & ULEIS

ISEEEHIC

SAMPEXGeotail

WindACE (SIS & ULEIS)

ISEEACE (SEPICA)

Relative Collecting Power

Solar Energetic Paticle Isotopes

Corotating Event Isotopes

Anomalous Cosmic Ray Isotopes

Solar and Interplanetary Charge States

1 101 102 103 104 105

10 102 103 104 105 1

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Periodic Table of the Elements

6

BASIC COMPOSITIONThe particles studied by ACE are atoms or pieces of

atoms. Atoms are composed of three major buildingblocks: protons (with a positive charge), neutrons (withno charge), and electrons (with a negative charge). Thenucleus of an atom contains the protons and neutrons,while the electrons orbit the nucleus. The number of pro-tons determines to which element the atom belongs:hydrogen has one proton, carbon has six, etc.

The number of neutrons tells us which isotope of theelement is present. The isotope number is the total ofthe number of protons plus the neutrons. Take carbon, forexample, which is a very common element necessary forlife, and is found in nature as diamonds and graphite. Wefind several different isotopes of carbon in nature.Carbon-12 has an equal number of protons and neutrons,six of each. Carbon-14 (see isotopes diagram on the left)contains two more neutrons (eight) than carbon-12, butstill has only six protons. This makes carbon-14 anisotope of carbon, but it is different from carbon-12. Whilecarbon-12 is a stable isotope, carbon-14 is unstable, orradioactive. It is much less common in nature, but isfound where carbon-12 is found. Because of itsradioactive nature, carbon-14 is used to datearchaeological artifacts.

In an atom, the number of electrons that orbit thenucleus equals the number of protons in the nucleus, thusmaking an atom electrically neutral. When bombarded byultraviolet (UV) radiation or after being struck by energeticparticles, atoms can lose one or more of their electrons.The positively-charged remains of these atoms are calledions. The number of electrons lost determines the chargestate of the particle. For example, alpha particles arehelium nuclei with a double positive charge; they have twoprotons and two neutrons, but no electrons.

Together with their freed electrons, the ions form aplasma. Plasma is a fourth state of matter, not a liquidsolid, or gas. Matter in the Sun is in a plasma state. Plasma is the most common state of matter in the uni-verse. More than 99% of all matter is plasma, so what we

see on Earth is the exception. Since plasmas consist ofelectrically-charged particles, electric and magnetic forcesaffect a plasma.

When we measure the elemental, isotopic, and chargecomposition of ions, it helps us to understand how natureselected the particles and accelerated them to the ener-gies at which we find them. The “composition” of the elec-trons is not interesting, since all electrons look the same.But the number, energy, and direction of travel are impor-tant.

ENERGETIC PARTICLESThe particles that ACE investigates have a lot of kinetic

energy (they are moving very fast). An electron volt (eV), isa unit of energy used to describe the total energy carried bya particle. (See the energy comparison in the upper leftcorner of this page.)

1 keV = 1 kilo-electron volt = 1,000 eV — typical of dental X-rays

1 MeV = 1 mega-electron volt = 1 million eV— typical of radioactive decay particles

1 GeV = 1 giga-electron volt = 1 billion eV — the equivalent energy of a proton

(hydrogen nucleus) at rest

The molecules in our atmosphere have kineticenergies around 0.03 eV. The Sun’s plasma and Earth’smagnetosphere (the area around Earth where its ownmagnetic field dominates) contain particles that are muchmore energetic. Protons in the magnetosphere range inenergy from a few keV (magnetospheric plasma) to 1 GeV(inner Van Allen belt). And particles having still higherenergies are quite common throughout the universe.

T H E S C I E N C E

H

Li

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Br

Ra

Sc

Y

La†

Ac*

Ce

Th

Ti

Zr

Hf

Rf

Pr

Pa

Y

Nb

Ta

Ha

Nd

U

Cr

Mo

W

106

Pm

Np

Fe

Ru

Os

108

Eu

Am

Co

Rh

Ir

109

Gd

Cm

Ni

Pd

Pt

110

Tb

Bk

Cu

Ag

Au

Dy

Cf

Zn

Cd

Hg

Ho

Es

B

AI

Ga

In

TI

Er

Fm

C

Si

Ge

Sn

Pb

Tm

Md

N

P

As

Sb

Bi

Yb

No

O

S

Se

Te

Po

Lu

Lr

F

CI

Br

I

At

He

Ne

Ar

Kr

Xe

Rn

1

3

11

19

37

55

87

4

12

20

38

56

88

21

39

57

89

58

90

22

40

72

104

59

91

23

41

73

105

60

92

24

42

74

106

61

93

26

44

76

108

63

95

27

45

77

109

64

96

28

46

78

110

65

97

29

47

79

66

98

30

48

80

67

99

5

13

31

49

81

68

100

6

14

32

50

82

69

101

7

15

33

51

83

70

102

8

16

34

52

84

Lu

Lr

9

17

35

53

85

2

10

18

36

54

86

†Lanthinide Series

IA

IIA

IIIB IVB VB VIB VII IB IB

IIIA IVA VA VIA VIIA

0

*Actinide Series

Mn

Tc

Re

107

Sm

Pu

25

43

75

107

62

94

VIIB

Hydrogen and carbon isotopes–deuterium is an isotope of hydrogen

Energy comparison:1 keV1 MeV1 GeV (large gray circle)

Hydrogen (H)11

21

Carbon146

1 Proton0 Neutrons1 Electron

++

+ n+

n+

+nn n

n+

n

n-

Deuterium (D)

1 Proton1 Neutron1 Electron

+-

-

-

-

-

-

-

6 Protons8 Neutrons6 Electrons

n

7

THE SUNOne of ACE’s primary goals is to learn more about par-

ticles from the Sun. It contains the vast majority of allmatter in our solar system. The Sun is mostly hydrogen,with some helium and smaller amounts of other elements.

The visible surface of the Sun is called the photos-phere. The Sun’s atmosphere has two transparent lay-ers. The chromosphere is just above the photosphere.The corona is the outer part of the Sun’s atmosphere. Inthe outer region of the corona, particles travel away fromthe Sun and stretch far out into space. The chromosphereand corona can only be seen during solar eclipses, or withinstruments that simulate a solar eclipse.

The SOHO (Solar and Heliospheric Observatory)spacecraft is also in position at the L1 point. One of itsinstruments, LASCO, is a visible-light coronagraph, adevice that blocks the bright light from the Sun’s surface,allowing the details in the corona to be clearly seen. Inthe LASCO image on the right, blobs of plasma are seenemitting from the Sun. The material in these blobs of thinionized gas is an example of what ACE studies.

NUCLEOSYNTHESISA star’s energy comes from the combining of light ele-

ments into heavier elements in a process known asfusion or “nuclear burning”. It is generally believed thatmost of the elements in the universe heavier than heliumare created, or synthesized, in stars when lighter nucleifuse to make heavier nuclei. This process is called nucle-osynthesis.

Nucleosynthesis requires a high-speed collision, whichcan only be achieved with very high temperature. Theminimum temperature required for the fusion of hydrogenis 5 million degrees. Elements with more protons in theirnuclei require still higher temperatures. For instance, fus-ing carbon requires a temperature of about one billiondegrees! Most of the heavy elements, from oxygen upthrough iron, are thought to be produced in stars that con-tain at least ten times as much matter as our Sun.

Our Sun is currently burning, or fusing,hydrogen to helium. This is the processthat occurs during most of a star’s lifetime.After the hydrogen in the star’s core isexhausted, the star can burn helium to formprogressively heavier elements, carbon andoxygen and so on, until iron and nickel areformed. Up to this point the processreleases energy. The formation of elementsheavier than iron and nickel requires theinput of energy. Supernova explosionsresult when the cores of massive stars haveexhausted their fuel supplies and burnedeverything into iron and nickel. The nucleiwith mass heavier than nickel are thoughtto be formed during these explosions.

ACCELERATIONThe acceleration of charged particles to extremely

high energies takes place almost everywhere in theuniverse, very far away from us and at our front door.Particles are accelerated on the Sun, in interplanetaryspace, at the edge of the solar system, at the blast wavesof supernova remnants, in neutron stars, and probably inblack hole systems. The last two are remains from thecollapse of large stars, either to the density of atomicnuclei (neutron star), or even further to a point such thateven light cannot escape (black hole). Sampling a widerange of accelerated particles from local and distantsources with the ACE instruments and comparing theirfeatures provides crucial information for understandingthe sources, acceleration, and transport of these high-energy particles.

Image of a solar wind stream as recorded bythe outer LASCO coronagraph (C3) on 24 December 1996 at 02:50 UT

Cosmic rays include nucleosynthetic productsfrom other regions of our Galaxy.

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SEPICAThe Solar Energetic Particle Ionic Charge

Analyzer (SEPICA) determines the ionic chargestate, elemental composition, and energy spectra ofenergetic solar ions. This is vital information instudying the material accelerated in solar events. Italso allows us to learn more about element and iso-tope selection processes and particle accelerationon the Sun. During solar quiet times, SEPICAdirectly measures the charge of ACR ions(described later), including nitrogen, oxygen, andneon. It covers a range from 0.5 MeV/charge toabout 5 MeV/charge for charge state composition,and up to 10 MeV/nucleon for element analysis (anucleon is a particle from the nucleus, either a protonor a neutron).

SEPICA contains three multi-slit collimators thatselect the arrival direction of ions so that they arefocused to a line in the detector. The detector systemholds an electrostatic analyzer and gas-proportionalcounters that measure the point of impact andenergy loss of the particle through a gas. Theremaining energy is measured in solid-statedetectors behind the proportional counter. Thecombination of energy loss and remaining energyallows the identification of different elements in theincoming particles. SEPICA achieves improvementsof a factor of 3 in charge resolution and of a factorof 20 in collecting power over previous instruments.SEPICA is a new instrument developed by theUniversity of New Hampshire and the Max PlanckInstitute forExtraterrestrialPhysics,Germany.

he output of the Sun in all forms - light, solar wind,and energetic particles - is not constant. It varies with

both time and position on the Sun. These changes arecalled solar activity and are reflections of changes belowthe Sun’s surface. Scientists can study the output and howit varies to probe the workings of theSun.

The electric currents in the Sun, aswell as in planets and galaxies, gener-ate magnetic fields. Magnetic fieldlines describe the structure of mag-netic fields in three dimensions. Acompass needle will always try to pointalong a field line. Lines close togetherrepresent strong magnetic forces andweak forces are represented as linesfurther apart.

Sunspots, temporary disturbancesin the photosphere, are the most visi-ble advertisement of the solar mag-netic field. They appear dark because temperatures areconsiderably lower than in surrounding areas. Sunspotsoccur where the magnetic field lines emerge from theinside of the Sun to form expanding loops above its sur-face.

A solar flare is an enormous explosion in the solaratmosphere. It results in sudden bursts of particle accel-

eration, heating of plasma to tens of millions ofdegrees, and the eruption of large amounts ofsolar mass. Flares are believed to result from the

abrupt release of the energy stored in magnetic fields inthe zone around sunspots.

This acceleration of solar flare particles to extremelyhigh energies involves all the different elements in thesolar atmosphere. Ions of elements such as carbon, nitro-

gen, oxygen, neon, magnesium, sili-con, and iron, excited in this way arecalled solar energetic particles(SEPs). In order to understand theacceleration processes involved andto measure the composition of theSun, ACE instruments study the quan-tity and type of these particles.

Another form of solar activity is theeruption of huge amounts of massfrom the Sun, which may or may notbe associated with solar flares. Thesecoronal mass ejections (CMEs) areballoon-shaped bursts of solar windrising above the solar corona, expand-

ing as they climb. Solar plasma is heated to tens of mil-lions of degrees, and electrons, protons, and heavy nucleiare accelerated to near the speed of light. The super-heated electrons from CMEs move along the magneticfield lines faster than the solar wind can flow. A shockwave may form ahead of the CME loop, and SEPs areaccelerated at these shock waves, too. Each CMEreleases up to 100 billion kg (about 100 million tons) ofthis material, and the speed of the ejection can reach1000 km/second (2 million mph) in some cases. Solar

S O L A R A C T I V I T Y

SEPICA

1700 1710 1720 1730 1740 1750 1760 1770 1780 1790 1800

DATE

0

100

200

300

SUN

SPO

T N

UM

BER

1810 1820 1830 1840 1850 1860 1870 1880 189

T

Sunspots

9

flares and CMEs are currently the biggest “explosions” inour solar system, occasionally approaching the power inONE BILLION hydrogen bombs!

Solar plumes are long, feathery jets that extend fromnear the poles of the Sun to more than 13 million milesinto space. They may be the origin of high-speed solarwind. Solar plumes expel a high-speed stream of plasmafrom the corona that can reach several million degrees!The base of the plume contains churning magnetic fieldsand solar gases. At its base, a plume is about 2500 km(1600 miles) wide.

These various solar events can interact and interferewith each other, creating a very complex system. Theirfrequency varies with time. The smaller flares tend to fol-low the eleven-year solar activity cycle and peak at sev-eral tens of flares per day. The largest solar events usually

occur only a few times during solar maximum, the periodof maximum solar activity during the eleven-year cycle.Sunspots increase with solar maximum, and are rela-tively rare during solar quiet times. During its first fewyears, ACE observed the transition from solar minimum tosolar maximum, including some of the largest solar eventsever observed.

ULEISThe Ultra Low Energy Isotope Spectrometer

(ULEIS) measures element and isotope fluxes(rates of particle flow) over the range of hydrogenthrough nickel, from about 45 keV/nucleon to sev-eral MeV/nucleon. Studies of ultra-heavy particles,those heavier than iron, are also performed in amore limited energy range near 0.5 MeV/nucleon.ULEIS also allows the study of SEP compositionand the way SEPs are energized in the solarcorona.

The ULEIS instrument is a time-of-flightmass spectrometer. A time-of-flight system usesthe difference in travel time through a chamber toseparate ions of different masses. Along with thetime-of-flight, the spectrometer simultaneouslymeasures the energy of particles entering thetelescope and stopping in one of the arrays ofseven silicon solid-state detectors in thetelescope. This instrument has a collecting powerfor SEP isotopes more than 10 times greater thanany previous instrument. ULEIS provides morethan a one thousand-fold improvement indetection for the study of CIR events, and it is asignificant advance in the research of ACRisotopes (CIRs and ACRs are described later).

ULEIS is a new instrument built by theUniversity of Maryland and JHU/APL.

A CME event in progress. The images were taken a few minutesapart. The dark disk in the upper right corner of each frame is notthe Sun, but the occulting, or Sun-blocking, disk of the SolarMaximum Mission coronagraph, used to take these images.

ULEIS

Monthly averages of the sunspot numbersshow that the number of sunspots visibleon the Sun waxes (during solar maximum)and wanes (solar minimum) with anapproximate 11-year cycle.

90 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

SWIMSThe Solar Wind Ion Mass Spectrometer

(SWIMS) is a versatile instrument that providessolar wind composition data for all solar windconditions. It clearly determines, every fewminutes, the quantities of most of the elements anda wide range of isotopes in the solar wind.The abundances of rare isotopes aredetermined every few hours, providinginformation crucial to the understanding ofpickup ions and ACRs (described later).SWIMS is extending knowledge of solar windcomposition to additional elements andisotopes.

The instrument consists of an electrostaticdeflection system that selects a narrow range ofenergy or charge, followed by a time-of-flight High-Mass Resolution Spectrometer (HMRS). TheHMRS determines the mass of a solar wind ion withhigh accuracy. The sensor measures speedsdepending on particle mass, ranging from about200 - 1500 km/s for helium, and from 200 - 500km/s for iron.

SWIMS was built by the University of Marylandand the University of Bern, Switzerland. It is a copyof portions of the CELIAS experiment from the SOHOmission, adapted only slightly to optimize it for ACE.

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This image, taken March 7, 1996, by theExtreme-ultraviolet Imaging Telescope (EIT)

on the Solar and Heliospheric Observatory(SOHO), shows an ultraviolet image of the

"quiet" solar atmosphere close to the surface.

S O L A R W I N DComet Hale-Bopp as seen from Shenandoah National Park, VAApril 9, 1997

SWIMS

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olar wind is the plasma of charged particles(protons, electrons, and heavier ionized atoms)

coming out of the Sun in all directions at very highspeeds — an average of about 400 km/sec, almosta million mph! It is responsible for the anti-sun-ward tails of comets and the shape of the mag-netic fields around the planets. Solar wind canalso have a measurable effect on the flightpaths of spacecraft.

The composition of the solar windreflects the composition of the solar corona,modified by solar wind processes. Theexact mechanism of solar wind formationis not known. Accurately measuring itscomposition, as ACE does, aids in sep-arating the effects of these processesfrom the original makeup of thecorona.

Coronal holes are largeregions in the corona that areless dense and cooler than sur-rounding areas. The open struc-

ture of their magnetic field allows aconstant flow of high-density plasma to stream out

of the holes. Solar plumes can appear in coronal holes.There is an increase in the intensity of the solar windeffects on Earth when a coronal hole faces us. Coronalholes do not last as long during solar maximum. ACE waslaunched at solar minimum and continues its work throughthe solar maximum, following the evolution of theseevents.

The heliosphere is the immense magnetic bubblecontaining our solar system, solar wind, and the entiresolar magnetic field. It extends well beyond the orbit ofPluto. While the density of particles in the heliosphere isvery low (it’s a much better vacuum than is created in alaboratory), it is full of particles of interest to ACE scien-tists. The heliopause is the name for the boundarybetween the heliosphere and the interstellar medium out-side the solar system. As the solar wind approaches theheliopause, it slows suddenly, forming a shock wave. Thissolar wind termination shock is exceptionally good ataccelerating particles.

In spite of its low density, the solar wind is strongenough to interact with the planets and their magneticfields to shape magnetospheres. Because the ions in thesolar plasma are charged, they interact with these mag-netic fields, and solar wind particles are swept aroundplanetary magnetospheres.

The shape of Earth’s magnetosphere is the directresult of being blasted by solar wind. Solar windcompresses its sunward side to a distance of only 6 to 10times the radius of the Earth. A supersonic shock wave iscreated sunward of Earth, somewhat like a sonic boom.This shock wave is called the bow shock and is normallylocated at about 15 Earth radii out. Most of thesolar wind particles are heated and slowed at thebow shock and detour around Earth. Solar winddrags out the night-side magnetosphere to possibly1000 times Earth’s radius; its exact length is notknown. This extension of the magnetosphere isknown as the magnetotail. Many other planets inour solar system have magnetospheres of similar,solar wind-influenced shapes.

SWEPAMThe Solar Wind Electron, Proton, and Alpha

Monitor (SWEPAM) measures the solar windplasma electron and ion fluxes as functions of direc-tion and energy. These data provide detailed knowl-edge of the solar wind conditions every minute.SWEPAM also provides real-time solar wind obser-vations that are continuously sent to the ground forspace weather purposes.

Electron and ion measurements are made withseparate sensors. The ion sensor measures particleenergies between about 0.26 and 36 KeV, and theelectron sensor’s energy range is between 1 and1350 eV. Both sensors use electrostatic analyzerswith fan-shaped fields-of-view. The electrostaticanalyzers measure the energy per charge of eachparticle by bending its flight path through thesystem. The fields-of-view are swept across allsolar wind directions by the rotation of thespacecraft.

SWEPAM was built by the Los Alamos NationalLaboratory in New Mexico. It was built from thespare solar wind electron and ion analyzers fromthe Ulysses mission, with selective modificationsand improvements.

This X-ray image shows the Sun as viewed by the Yohkohsatellite. A large coronal hole, extending from the northern intothe southern hemisphere is near the Sun's center. The bright-est region shows hot loops that remain after a solar flare.

SWEPAM (Electron sensor)

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he electric currents in the Sun generate a com-plex magnetic field which extends out into inter-

planetary space to form the interplanetary magneticfield. In space, charged particles tend to becomeattached to magnetic field lines, spiraling aroundthem while sliding along them, like beads on a wire.Because of this attachment, the behavior of ener-getic particles in space is dictated by the structureof field lines. Magnetic field lines are frozen intoplasmas and move as the plasmas move.

As the Sun’s magnetic field is carried out through thesolar system by the solar wind, the Sun is rotating. Itsrotation winds up the magnetic field into a large rotatingspiral, known as the Parker spiral, named after the scien-tist who first described it.

The magnetic field is primarily directed outward fromthe Sun in one of its hemispheres, and inward in theother. This causes opposite magnetic field directions in theParker spiral. The thin layer between the different fielddirections is described as the neutral current sheet.

T H E H E L I O S P H E R EEPAMKnowledge of the fluxes and energy of high-

energy protons, alpha particles, and electrons isessential in understanding the dynamic behavior ofsolar flare, CIR, and interplanetary shock particle(CIR and ISP are described later) events. Thesemeasurements are made by EPAM: the Electron,Proton, and Alpha Monitor. This instrument pro-vides information that can reflect changes in bothcoronal and interplanetary magnetic fields, andinformation on solar flares. EPAM covers the rangeof energies from 30 keV/nucleon up to 4MeV/nucleon. It measures the composition of ele-ments up through iron.

EPAM includes five telescopes of three differenttypes. Two Low Energy Foil Spectrometers (LEFS)measure the flux and direction of electrons above30 keV. Two Low Energy Magnetic Spectrometers(LEMS) measure the flux and direction of ionsgreater than 30 keV. And the Composition Aperture(CA) measures the composition of the ions. Solid-state detectors on each telescope analyze theenergy of the incoming particles. These telescopesuse the spin of the spacecraft to sweep the full sky.

The EPAM instrument was built by JHU/APLwith Dr. L.J. Lanzerotti of Lucent Technologies asPrincipal Investigator. It is the flight spare of the HI-SCALE instrument from the Ulysses spacecraft.

The heliosphere is the immense, magnetic bubblecontaining our solar system, the solar wind, andthe entire solar magnetic field. The bow shock ofthe heliosphere is much like the bow shock ofEarth's magnetosphere (see next page). It isshaped by interstellar wind rather than solar wind.

EPAM

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The Heliosphere

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Field lines show the magnetic field around a bar magnet.

The neutral current sheet, sometimesknown as the "ballerina skirt". TheParker spiral magnetic field is indicat-ed by the arrows.

Since this dividing line between the outward and inwardfield directions is not exactly on the solar equator, therotation of the Sun causes the current sheet to become“wavy”, and this waviness is carried out into interplanetaryspace by the solar wind.

In addition, every eleven years the entire magneticfield of the Sun “flips”- the north magnetic pole of the Sunbecomes the south, and vice versa. The flip takes placeat solar maximum. The last maximum was in 2001.

MAGPrecise examination of the interplanetary and

solar magnetic fields and their dynamics providesessential supporting information for the other ACEinstruments. The two magnetometers (MAG) onACE detect and measure the magnetic fields in thevicinity of the spacecraft. As the magnetometerssweep through a field when the spacecraft rotates,electrical signals are produced which are propor-tional to the strength and direction of the field.

MAG consists of two wide-range triaxial magne-tometers. Triaxial means that all three axes (x, y,and z) of the magnetic field are measured. Thisallows for the determination of its exact direction.The magnetometers are mounted out from thespacecraft on separate long booms to reduce theeffects of any magnetic fields from the spacecraftand instruments. MAG measures the strength anddirection of the interplanetary magnetic field 30times per second and can calculate any pattern ofvariations in it.

The scientific institutions involved in buildingMAG were the Bartol Research Institute in Newark,Delaware and GSFC. It is a flight spare from theWind mission.

MAG

The Earth’s Magnetosphere

THE HEL IOSPHERE- PART 2

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Typical Energy Spectra

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Stream

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Background?

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ULEIS

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ISP

Galactic

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SIS

Heliopause(outside edge of heliosphere)

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he solar wind near our Sun’s surface contains alternat-ing streams of high and low speed. The sources of

these streams corotate with the Sun; that is, they rotatealong with it. The high-speed streams originate in coronalholes and extend toward the solar poles; the low-speedstreams typically come from near the equator. ACE seesmostly the low-speed solar wind due to its location, butcan observe the compositional differences between thehigh- and low-speed wind when coronal holes extenddown near the equator. One objective of ACE is to under-stand how the high- and low-speed winds are accelerated.

With increasing distance from the Sun, the high-speedstreams overtake the slower plasma, producing corotat-ing interaction regions (CIRs) on their leading edges.CIRs are bounded by two shocks at the front and rearedges, called the forward and reverse shocks. At these

shocks, the density, pressure, and magnetic field strengthare all higher. These regions are quite effective as ener-getic particle accelerators. When ions that have beenaccelerated at a CIR are observed, they are called coro-tating ion events.

Interplanetary shock particles (ISPs), accelerated byshocks associated with solar flares and CMEs, areanother example of interplanetary acceleration. ACE isable to directly compare the composition of ISPs with thatof the solar wind to test shock acceleration theories.

SWICSThe Solar Wind Ion Composition Spectrometer

(SWICS) determines not only the charge of ions, butalso the temperature and speeds of all the majorsolar wind ions. SWICS covers solar wind speedsranging from 145 km/s (protons) to 1532 km/s (iron).The information recovered tells scientists about thenature of not only the solar wind, but also of solarflares, ISPs, CIRs, and pickup ions (descibed later).

SWICS combines an electrostatic analyzer withpost-acceleration, followed by a time-of-flight andenergy measurement. Post-acceleration meansthat after the electrostatic analyzer, the particle is re-accelerated to determine its mass.

SWICS was built by the University of Marylandand the University of Bern, Switzerland. The instru-ment is the same as one fully developed, designed,and tested during the Ulysses mission. A flightspare from that mission was used for ACE.

ACE is studying the many different types of speeding(energetic) particles in the solar system. These include:

• the solar wind, high-speed streams (high-speed solarwind), coronal mass ejections (CMEs), and solar energetic particles (SEPs) coming from the Sun,

• interplanetary shock particles (ISPs) and corotating ion events (CIRs) from interplanetary space,

• anomalous cosmic rays (ACRs) from the edge of the solar system (the solar wind termination shock),

• interstellar neutral gas, and the pickup ions which originate in the gas right outside the solar system (the very local interstellar medium),

• and galactic cosmic rays (GCRs) from the far reaches of the Galaxy.

These energetic particles are described in this brochure.

The smaller figure (opposite page) is an illustration ofthe number of particles observed (for a standard areaand energy interval) versus energy, for the many differentenergetic particles observed by ACE. Most of theseparticle types actually vary with time by large factors. Asa result, these “spectra” are not really accurate for anyparticular element; the figure is just an example.

In the larger figure (opposite page), the color used foreach type of particle or event matches the color of itscorresponding energy population, shown in the graph inthe smaller figure.

SWICS

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he relative numbers of different isotopes found in theGalaxy are established by the life cycle of massive

stars. Star formation, evolution, and explosion results inthe creation of many of the heavier isotopes ACE finds inspace. The process is illustrated by the figures below.

In a part of the Galaxy where the composition of theinterstellar gas is much like that of our own solar system(a), a cloud of gas collapses under the influence of its own

gravity, and creates a new star (b). Inside the star (c),fusion converts some of the original hydrogen and heliuminto particles like carbon-12 and oxygen-16. At the sametime, the carbon, nitrogen, and oxygen nuclei that wereoriginally present in the stellar fuel are converted intoheavier, neutron-rich nuclei, like neon-22 andmagnesium-25.

When this quiescent burning has exhausted all of thenuclear fuel in the core of the star, the starexplodes as a supernova (d). The shock wavegenerated by the explosion synthesizes additionalheavy nuclei and ejects most of the products ofnucleosynthesis back into the interstellar gas.

Repetition of these events in each generation ofstars steadily enriches the interstellar gas in car-bon, nitrogen, and oxygen, and in heavy nucleiwith an excess of neutrons.

Some of the nuclei in the gas are accelerated tocosmic ray speeds, possibly by the shock wavesfrom supernovae (e). Cosmic ray acceleration

could also occur directly as the supernova is ejecting mat-ter into interstellar space, as in (d).

While interstellar plasma is kept outside theheliosphere by an interplanetary magnetic field, theinterstellar neutral gas flows through the solar system likean interstellar wind, at a speed of 25 km/sec. Whencloser to the Sun, these atoms undergo the loss of oneelectron in photo-ionization or by charge exchange.

Photo-ionization occurs when an electron is knocked offby a solar UV photon, and charge exchange involvesgiving up an electron to an ionized solar wind atom. Oncethese particles are charged, the Sun’s magnetic fieldpicks them up and carries them outward to the solar windtermination shock. They are called pickup ions duringthis part of their trip. By measuring the distribution ofthese pickup ions, ACE determines the composition, flowand temperature of the neighboring interstellar gas.

The ions repeatedly collide with the termination shock,gaining energy in the process. This continues until theyescape from the shock and diffuse toward the innerheliosphere. Those that are accelerated are then knownas anomalous cosmic rays (ACRs). Cosmic rays arethe particles that bombard the Earth from anywherebeyond its atmosphere.

There may also be additional sources of particles,which ACE can shed light on, that are accelerated at thesolar wind termination shock.

SISThe Solar Isotope Spectrometer (SIS)

measures the elemental and isotopic compositionof nuclei from helium to zinc in the energy rangefrom about 10 to 100 MeV/nucleon. During solaractive periods the flux of solar energetic particlescan suddenly increase by a factor of 10,000 ormore, providing an opportunity for SIS to measurethe composition of solar material. The SIS detectorsystem consists of two identical particle telescopes.Each telescope has a pair of silicon-strip detectorsto sense the trajectory (path) of incident nuclei,followed by a stack of large-area silicon solid-statedetectors that measure the energy lost by nuclei asthey slow and stop. By combining these measure-ments it is possible to compute the nuclear charge,mass, and kinetic energy of each incident particle.

An innovative feature in SIS is the use of cus-tom low-power VLSI (very large-scale integration)circuitry to measure signals from the 512 strips ofthe trajectory system. This circuitry, which usesonly about 1/10 the power of previous instrumenta-tion, enables measurements of both position andenergy loss for multiple particles that may arrivewithin a few microseconds of each other duringlarge solar particle events. The collecting power ofSIS is about 100 times greater than that of previoussolar particle isotope spectrometers.

SIS is a new instrument developed by Caltech,GSFC, and NASA’s Jet Propulsion Laboratoryat Caltech (JPL).

LOCAL INTERSTELLARMEDIUM

SIS

Nucleosynthesis in massive stars

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indicate that the seed population for GCRs is neither theinterstellar gas nor the shards of giant stars that becamesupernovae. ACE is performing a detailed survey of theisotopic makeup of GCRs, which should shed light on this

intriguing puzzle.Included in the cosmic rays are a

number of radioactive nuclei whosenumbers decrease over time. As in thecarbon-14 dating technique, measure-ments of these nuclei by ACE are beingused to determine how long it has beensince cosmic ray material was synthe-sized in the Galaxy before leaking outinto the vast void between the galaxies.These nuclei are called "cosmic rayclocks".

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alactic cosmic rays (GCRs) come from outside thesolar system but generally from within our Galaxy.

GCRs are atomic nuclei from which all of the surroundingelectrons have been stripped away during their high-speed passage through the Galaxy.They have probably been acceleratedwithin the last few million years, andhave traveled many times across theGalaxy, trapped by the galactic mag-netic field. As they travel through thevery thin gas of interstellar space,some of the GCRs interact and emitgamma rays, which is how we knowthat they pass through the Milky Wayand other galaxies.

GCRs have been accelerated tonearly the speed of light, probably bysupernova remnants. But exactlywhich particles the supernova rem-nants accelerate is one of the ques-tions that ACE is trying to answer.

The elemental makeup of GCRs has been studied indetail by earlier experiments, and is very similar to thecomposition of the Earth and solar system. But previousstudies of the composition of the isotopes in GCRs may

G A L A C T I CC O S M I C R AY S

CRISThe Cosmic Ray Isotope Spectrometer (CRIS)

is measuring the abundances of galactic cosmic rayisotopes, with energies from ~100 to ~600MeV/nucleon over the element range from helium tozinc, with a collecting power more than 50 timesgreater than previous instruments of its kind.

CRIS determines the nuclear charge, mass, andkinetic energy of incident cosmic rays that stop inone of four identical stacks of large-area siliconsolid-state detectors. Particle trajectories are mea-sured with scintillating optical fibers arranged in sixlayers above the four detector stacks. When acharged particle passes through one of the 7000plastic fibers, it sparkles, and a portion of this light ispiped to the ends of the fiber. This is where it isamplified and read by two CCD (charge-coupleddevice) cameras that record the pattern of “hits” ineach fiber layer. An onboard microprocessor thenuses these data to determine the position coordinatesof the incident particles. A primary objective for CRISis to measure the abundances of various radioactiveisotopes, such as cobalt-57 and nickel-59, that canbe used to measure the time elapsed betweennucleosynthesis and acceleration in supernovae;and isotopes such as beryllium-10, aluminum-26,and chlorine-36 that measure how long cosmic raysare confined by galactic magnetic fields.

CRIS is a new instrument developed by Caltech,GSFC, JPL, and Washington University in St.Louis.

EGRET gamma-ray all-sky survey - above 100 MeV. Some GCRs interact with the interstellar mediumand produce gamma rays.

CRIS

This false color composite picture of the bright supernova remnantSN1006 (above) was taken by the ASCA satellite. The expandinggas from the star collides into the surrounding material. The colli-sion generates a violent shock, which produces X-ray light. Thebright regions in the picture show the locations of this shock alongthe rim of the remnant. The energy spectrum produced in SN1006provides the first clear link between particle acceleration at supernova shock fronts and high-energy cosmic rays.

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S P A C E W E A T H E R

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Astronaut William McArthur appearssuspended over the blue and white Earth during space walk activitiesnear the Space Shuttle Discovery.

Solar activity produces many noticeable effects on and near the Earth, including thenorthern and southern lights. This photograph of the aurora australis (southernlights) is from Spacelab 3.

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he Sun’s activity causes large changes in the Sun’splasma and energetic particle populations, and these

changes are responsible for the “space weather” thataffects Earth. Space weather can impact the upperatmosphere and may influence long-term climate trends.The effects are related to CMEs, SEPs, and coronal holes,the source of high-speed streams. The largest stormsoccur when a fast CME hits Earth shortly after its shockarrives.

Geomagnetic storms (magnetic storms on Earth dueto solar activity) produce the awe-inspiring aurora borealisand aurora australis — the northern and southern lights.However, they can also cause a variety of highlyundesirable consequences. “Killer” electrons acceleratedin the magnetosphere during geomagnetic storms cancause communications satellites to fail. Electrical currentsurges in power lines, interference in the broadcast ofradio, television, and telephone signals, and problems withdefense communications are all associated with magneticstorms. Odd behavior in air and marine navigationinstruments has been observed, and a compass anywhereon Earth is certainly affected. These storms are known toalter the atmospheric ozone layer. Even increased pipelinecorrosion has been attributed to them.

Major solar activity is a very serious concern in spaceflight. Communications may be disrupted. Large solar dis-turbances heat the upper atmosphere, causing it to expandand create increased drag on spacecraft in low orbits, short-ening their orbital lifetime. Spacecraft could potentially tum-ble and burn up in the atmosphere. Intense SEP eventscontain very high levels of radiation, more than a milliontimes the normal daily dose of a human on Earth.Radiation sickness can result when humans are outside theprotective magnetosphere of the Earth, as in missions tothe moon and to Mars.

High-energy solar protons can produce increased radia-tion in the atmosphere at altitudes where supersonic aircraftfly. This is especially true for flights over the north andsouth magnetic poles, areas unprotected by the Earth’s

magnetic field, where the radiation has direct access to theatmosphere. To reduce the risk to aircraft crews and pas-sengers, and reduce risk to the aircraft, routine forecastsand alerts are sent through the Federal AviationAdministration so that a flight in potential danger can con-sider what course of action to take to minimize radiationexposure. The National Oceanic and AtmosphericAdministration (NOAA) forecasts high-speed solar wind andsolar particle events from the Space Environment Center.

The continuous broadcast of solar wind, magnetic field,and SEP data from ACE allows very accurate forecasts ofmajor activity up to one hour beforehand. In particular, ACEdetects large CMEs and their associated shocks beforethey reach Earth, just like weather stations on Earth mea-sure major storms as they move across the continent. Thisremoves much of the guesswork from space weather fore-casts and represents a major advance in NOAA's forecastability, and furthers our understanding of the scientificprocesses involved.

NASA has recently embarked on a new “Living with aStar” program that has the goal of improving the scientificbasis for space weather predictions. The role of ACE in thisprogram is to measure the solar wind input into the magne-tosphere, thereby covering one of the key links in the chainby which solar variations cause geomagnetic storms. Itappears that ACE has enough fuel onboard to remain at L1through the next solar maximum.

Real-Time Solar Wind (RTSW)Geomagnetic storms are a natural hazard that

NOAA forecasts for the benefit of the public, as it doeshurricanes and ground tornadoes. The location ofACE enables it to provide about one-hour advancewarning of impending major geomagnetic activity.

The ground system developed by NOAA receivesthe data broadcast by ACE in real-time (as it is hap-pening). This system includes NOAA stations atWallops Island, Virginia and Boulder, Colorado; dedi-cated stations at the Communications ResearchLaboratory in Japan and Rutherford AppletonLaboratory in England; with additional tracking by theUS Air Force Satellite Control Network, India SpaceResearch Organization in India, and NASA’s DeepSpace Network. A subset of data is sent fromSWEPAM, EPAM, MAG, and SIS during the time thatACE is not transmitting its full telemetry.

For about 21 of 24 hours per day, ACE broad-casts the real-time solar wind data. The data isreceived by the ground stations around the world andsent directly to NOAA. During the three hours perday that NASA ground stations are receiving full ACEtelemetry, NOAA receives a copy of the data. Thisgives them 24 hour per day forecasting. NOAAprocesses the data at its Space Environment Centerin Boulder, CO, which issues alerts of any potentialgeomagnetic problems. Data is put on the Webwithin seconds, and many other groups around theworld also use these data to make their own predic-tions.

The ACE RTSW data can be found at http://sec.noaa.gov/ace/

DATA ANALYSIS AND DELIVERYThe ACE Science Center at Caltech obtains the space-craft telemetry from the ACE Flight Operations Team andGSFC. It then produces a data set that is more appropri-ate for science data processing. The Center makesinteresting plots available to scientists around the worldand takes advantage of emerging World Wide Web tech-nology, making it possible for users to request specificinformation as needed.

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World Wide Web Access

ACE home page: http://www.srl.caltech.edu/ACE/

ACE project page: http://helios.gsfc.nasa.gov/ace/ace.html

ACE News:http://www.srl.caltech.edu/ACE/ACENews_curr.html

ACE Real Time Solar Wind (RTSW):http://sec.noaa.gov/ace/

NOAA Space Environment Center’s“Space Weather Now”: http://www.sec.noaa.gov/SWN/

NASA home page:http://www.nasa.gov/

“Cosmic and Heliospheric Learning Center” at NASA GSFC:http://helios.gsfc.nasa.gov/

CSLP home page: http://cslp.gsfc.nasa.gov/

CONCLUSIONS

This is a ground-based image of the entire Crab nebula,the remnants of a supernova explosion over 900 yearsago. The green, yellow and red filaments toward theedges are the remains of the star that were ejected intospace by the explosion. The blue glow in the inner partis light emitted by energetic electrons as they spiralthrough the Crab’s magnetic field.

cience has barely scratched the surface in examiningthe actual sources of the particles traveling through

space around us. The mix of particles that ACE mea-sures is the result of a complex history. The ability ofACE’s nine instruments to measure a wide range of parti-cle types and energies at the same time and location iswhat enables scientists to separate the many processesthe matter has undergone on its way to ACE.

The prime purpose of ACE is to study the compositionof several distinct sources of matter, the Sun and solarsystem, the local interstellar space, and the Galaxy as awhole. This, in turn, will lead us to a betterunderstanding of the origin of the elements, and thesubsequent evolutionary processing of matter (how it haschanged since it was created). Along the way, ACE islearning more about particle acceleration and transport inthe universe, information needed to separate thechanges in composition during the particles’ travel.Learning the differences in composition between thesolar wind and the Sun helps answer questions abouthow the solar corona is formed and how solar wind isaccelerated. All of these interesting problems are part ofthe larger question “Where did we come from?” ACE isproviding several pieces of the enormous puzzle.

As new information becomes available, from bothspacecraft and Earth-based instruments, the picturebecomes clearer. Theories are upheld or upset, and newtheories take their place. ACE provides an abundance ofinformation to further our understanding of the way oursolar system, Galaxy, and universe were created and howthey continue to evolve.

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ACE Management TeamNASA HeadquartersDr. Charles Holmes

— Senior Program ExecutiveDr. W. Vernon Jones

— ACE Program Scientist

NASA Goddard Space Flight Center

Space Science Mission Operations ProjectOverall project management

Mr. Ronald Mahmot — Project Manager

Mr. Robert Sodano — Mission Director

Mr. Edward Nace — Flight Control Services Manager

Mr. Craig Roberts — Trajectory Analyst

Ms. Jacqueline Snell — Lead Engineer, Flight Operations Team

Science managementDr. Tycho von Rosenvinge

— ACE Project ScientistDr. Eric Christian

— Deputy Project Scientist

Explorer Project OfficeOverall project management, pre-launch

Mr. Donald Margolies— ACE Mission Manager

California Institute of Technology (Caltech)Space Radiation LaboratoryScience payload management

Dr. Edward Stone — Principal Investigator

Dr. Richard Mewaldt — ACE Mission Scientist

Mr. Allan Frandsen — Pre-launch Payload Manager

ACE Science Center Dr. Andrew Davis

— ACE Science Center Manager

The Johns Hopkins University AppliedPhysics Laboratory (JHU/APL)Spacecraft development Observatory integration and test

Ms. Mary Chiu — Program Manager

Ms. Judi von Mehlem — Spacecraft Systems Engineer

Scientific Co-InvestigatorsPrincipal Investigator:Dr. Edward Stone — Caltech/JPL

Project Scientist:Dr. Tycho von Rosenvinge — NASA GSFC

Co-Investigators:Dr. Walter Binns

— Washington University in St. LouisDr. Peter Bochsler — University of BernDr. Leonard Burlaga — NASA GSFCDr. Alan Cummings — CaltechDr. William Feldman

— Los Alamos National LaboratoryDr. Johannes Geiss — University of BernDr. George Gloeckler

— University of MarylandDr. Robert Gold — JHU/APLDr. Dieter Hovestadt — Max Planck

Institute for Extraterrestrial PhysicsDr. Berndt Klecker — Max Planck

Institute for Extraterrestrial PhysicsDr. Stamatios Krimigis — JHU/APLDr. Glenn Mason — University of MarylandDr. David McComas

— Los Alamos National LaboratoryDr. Richard Mewaldt — CaltechDr. Eberhard Möbius

— University of New HampshireDr. Norman Ness — University of DelawareDr. Tycho von Rosenvinge — NASA GSFCDr. Mark Wiedenbeck — JPL

Acknowledgments

• The Boeing CompanyDelta II rocket photograph

• E. Christian, NASA GSFCACE model photograph

• F. Espenak, NASA GSFCComet Hale-Bopp photograph

• C. Fichtel and the EGRETInstrument Science TeamEGRET gamma-ray all-sky survey

• D. Hathaway, NASA MSFC Sunspot cycle diagram

• High Altitude Observatory, National Center for Atmospheric Research (NCAR), Boulder, Colorado- NCAR is sponsored by the National Science FoundationImages of CME event in progress

• Hubble Space TelescopeCygnus Loop image

• J. Jokipii, Univ. of AZ Neutral current sheet image

• JHU/APLACE logo ACE science investigations artwork ACE spacecraft photograph Expanded ACE diagramACE at L1 artworkAll individual instrument photographs

• R. Mewaldt, E. Stone, and M. WiedenbeckGalactic evolution images

• NASA ArchivesSun image with sunspotsPhotograph of astronaut William McArthurCrab nebula image

• R. Overmyer/NASAAurora australis image

• R. Petre and E. Gotthelf, NASA GSFCSN1006 image

• SOHO/LASCO and SOHO/EITconsortia - SOHO is a project of international cooperation between ESA and NASA.CME imageSOHO/LASCO coronagraph imageImage of “quiet” solar atmosphere

• Yohkoh mission of ISAS, Japan -The X-ray telescope was prepared by the Lockheed Palo Alto Research Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo, with the support of NASA and ISAS.Sun image with coronal holes

• GSFC Graphics DepartmentAll additional graphics

Edited by: S. B. Jacob, E. R. Christian, D. L. Margolies,R. A. Mewaldt, J. F. Ormes, P. A. Tyler, and Tycho von Rosenvinge

Design by: T. B. Griswold

Second Edition, March 2002