The Earth is a large magnet

All magnetic objects produce invisible lines of force, extending between the poles of the object. The earth acts like a very large magnet. Just as a bar magnet produces field lines, so too does Earth. And, just as the magnetic field of a bar magnet pushes iron filings into a pattern, so too does the Earth's magnetic field. That is why a compass always points North. You can visualize the Earth's invisible magnetic field lines by thinking of the Earth as having a bar magnet running from the North to South poles. Magnetic fields also push things that have been charged by static electricity and are moving in the magnetic field. In the case of outer space, the charged objects pushed by the Earth's magnetic field are ions and electrons. You may remember that ions are atoms that have had one or more electrons knocked off of them.

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The Earth's Magnetosphere

In spite of its low density, the solar wind, and its accompanying magnetic field, is strong enough to interact with the planets and their magnetic fields to shape magnetospheres. A magnetosphere is the region surrounding a planet where the planet's magnetic field dominates. Because the ions in the solar plasma are charged, they interact with these magnetic fields, and solar wind particles are swept around planetary magnetospheres. Life on Earth has developed under the protection of this magnetosphere.

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What is the Magnetosphere?

The magnetosphere is that area of space, around the Earth, that is controlled by the Earth's magnetic field. It is important to learn as much about this space around the Earth as we would about any other part of the Earth's environment. The magnetosphere helps to protect our Earth from the danger of the Sun's solar wind.

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The Sun and the Earth are connected

The solar wind "squashes" the earth's magnetic field. The magnetic field lines on the sun side of earth (left) are squashed and on the side of the earth away from the sun (right), the magnetic field lines are stretched out as if the wind is trying to blow them away. The magnet of the earth pushes the solar wind particles sideways so they don't hit the earth head on.

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Our magnetic field is squashed by the solar wind

The solar wind "squashes" the earth's magnetic field. Earth's magnetic field does a pretty good job of standing up to the solar wind. A great deal of the matter in the solar wind is pushed sideways around the earth by the earth's magnetic field.

The Bow Shock

The bow shock is a standing wave in front of a magnetosphere at which the supersonic solar wind is slowed, heated, and deflected around the planet. The strength of this shock depends on the flow velocity of the solar wind relative to the velocity of compressional waves in the plasma. This latter velocity decreases with increasing distance from the sun while the former remains quite constant. As a result, the strength or Mach number of the bow shock increases markedly from the inner solar system to the outer solar system. At Mercury the bow shock has a Mach number of about 4 but at Neptune it is about 20. At low Mach numbers the shock is found to be quite smoothly varying or laminar in appearance but at high Mach number the shock becomes very turbulent.

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When the solar wind meets the Earth's magnetic field, most of the solar wind is pushed around the earth because of its magnetic field. It begins its journey around in a curve called the "Bow Shock". Just like water makes a curved wave in front of a boat, the solar wind makes a curve in front of the Earth.

map of the magnetosphere

After passing through a shock wave at the bow shock, the wind flows around the magnetosphere and stretches it into a long tail. However, some solar wind particles leak through the magnetic barrier and are trapped inside. Solar wind particles also rush through funnel-like openings (cusps) at the North and South Poles, releasing tremendous energy when they hit the upper atmosphere. The Northern and Southern Lights (auroras) are the evidence we can see of this energy transfer from the Sun to the Earth. The particles then follow a path that goes around the earth in a sort of cover or sheath. This curve is called the "magnetosheath". These particles mix with other particles that come up from the earth's ionosphere to fill the magnetosphere.

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The Earth's Magnetosphere

Real time data from the ACE spacecraft are used to predict the shape and location of these boundaries at the present time and into the near future. The solar wind emanating from the Sun is super-magnetosonic with respect to the Earth, so that a shock wave is formed. As the solar wind flows through the shock it is slowed down, and the pressure of the solar wind is balanced by the pressure from the Earth's magnetic field. The boundary at which this pressure balance is achieved is called the magnetopause. The ACE spacecraft monitors the solar wind from a position about 200 Earth radii (RE) sunward of the Earth. The real time solar wind data from this spacecraft allows us to predict what will happen at the Earth many minutes before the solar wind actually reaches us. Important solar wind values obtained from the ACE observations include the z-component of the interplanetary magnetic field (Bz) measured in units of nano-Tesla, and the dynamic pressure (also called the momentum flux) of the solar wind, measured in units of nano-Pascal.

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The Earth's Magnetosphere

The event of July 14, 2000

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The event of January 10, 1997

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First optical evidenceof Solar Wind – Magnetosphere connection

After the two world wars, people finally sent satellites into space to investigate the ionosphere and magnetosphere. In fact, the first United States satellite, Explorer 1, discovered many belts of high radiation particles. Since the satellites could go up in space and take pictures, we could have pictures like this picture of an aurora around the North Pole. From this picture and other measurements, scientists figure out what the magnetosphere and the solar wind are like.

Solar wind would singe our atmosphere if not for our magnetic field.

We've learned that the solar wind travels past the Earth at well over 1.620.000 km/h. And thanks to the Earth's magnetic field, the solar wind is stopped and deflected around the Earth so that most of it does not hit our atmosphere head on. Ultra-violet rays from the sun ionize the upper atmosphere, creating the electrically-conducting ionosphere and a source of plasma for the magnetosphere. Our neighboring planet, Mars, which has little or no magnetic field, is thought to have lost much of its former oceans and atmosphere to space. This loss was caused, at least in part, by the direct impact of the solar wind on Mars' upper atmosphere. Our other close planetary neighbor, Venus, has no appreciable magnetic field, either. Venus is also thought to have lost nearly all of its water to space, in large part owing to solar wind-powered ablation

Solar wind eroded the martian atmosphere

Earth is shielded from the solar wind by a magnetic bubble extending 50,000 km into space - our planet's magnetosphere.

without a substantial magnetosphere to protect it, much of Mars's atmosphere is exposed directly tofast-moving particles from the Sun.The Martian atmosphere extends hundreds of kilometers above the surface where it's ionized by solar ultraviolet radiation. The magnetized solar wind simply picks up these ions and sweeps them away."

Magnetosphere of unmagnetized planets

Solar extreme ultraviolet radiation ionizes the upper atmospheres of all planets to varying degrees. If the thermal pressure of this ionosphere exceeds the solar wind momentum flux or dynamic pressure, a quantity proportional to the density times the square of the velocity, then the ionosphere can stand off the solar wind and it remains unmagnetized. A magnetic lid or cap forms on the ionosphere called the magnetic barrier and this barrier in turn deflects the solar wind. The solar wind as mentioned above is supersonic and thus this deflection must involve the formation of a detached bow shock. This bow shock, which interestingly forms without the aid of collisions in the gas, slows, heats and deflects the solar wind.

Magnetosphere of unmagnetized planets

Schematic illustration of the formation of a magnetic tail in the interaction of the solar wind with an unmagnetized planet. Field lines from the solar wind which are convected closest to the planet move most slowly as they pass the planet and become stretched the most.

Planetary magnetospheres

Unmagnetized planets

VenusThe magnetic moment of Venus is less than one hundred thousandths of that of the Earth and plays no role in the solar wind interaction with the planet

MarsThe precise size of the magnetic field of Mars is not known but its strength is probably much less than one ten thousandths of that of the Earth and like Venus the intrinsic magnetic field is not significant for the solar wind interaction

Modeling Earth's Magnetosphere Using Spacecraft Magnetometer Data (1)

The Earth's magnetosphere is a very dynamical system. Its configuration depends on internal and external factors. The first factor is the orientation of the Earth's magnetic axis with respect to the Sun-Earth line, which varies with time because of both the Earth's diurnal rotation and its yearly orbital motion around the Sun. The animation shows how the magnetospheric field varies in response to the dipole wobbling. The background color coding displays the distribution of the scalar difference B between the total model magnetic field and that of the Earth's dipole only. Yellow and red colors correspond to the negative values of B (depressed field inside the ring current, in the dayside polar cusps, and in the plasma sheet of the magnetotail). Black and blue colors indicate a compressed field (in the subsolar region on the dayside and in the magnetotail lobes on the nightside).

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Storms in Space

Storms in SpaceMagnetic storms can produce energy equivalent to that released by the atomic bomb that leveled Hiroshima in 1945. In the northern hemisphere, they usually occur when the solar wind's magnetic field is directed southward. This orientation is opposite Earth's field on the dayside boundary of Earth's magnetosphere (which points northward), so that Earth's magnetic field becomes interconnected with the solar wind magnetic field. This acts like a switch, allowing much more solar wind energy to enter the magnetosphere.

Magnetic reconnection allows particles to enter the magnetosphere

ESA picture

Modeling Earth's Magnetosphere Using Spacecraft Magnetometer Data (2)

Another important factor is the orientation and strength of the interplanetary magnetic field , "carried" to the Earth's orbit from Sun. Interaction between the terrestrial and interplanetary fields becomes especially effective, when the interplanetary magnetic field is directed antiparallel to the Earth's field on the dayside boundary of the magnetosphere. In this case the geomagnetic and interplanetary field lines connect across the magnetospheric boundary, which greatly enhances the transfer of the solar wind mass, energy, and electric field inside the magnetosphere. As a result, the magnetospheric field and plasma become involved in a convection, as illustrated in this animation.

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Modeling Earth's Magnetosphere Using Spacecraft Magnetometer Data (3)

In actuality, this kind of stationary convection is rarely realized. The solar wind is not steady: periods of quiet flow are often interrupted by strong "gusts", and the interplanetary magnetic field fluctuates both in magnitude and orientation. This results in dramatic dynamical changes of the entire magnetospheric configuration, which culminate in magnetospheric storms, accompanied by an explosive conversion of huge amounts of the solar wind energy into the kinetic energy of charged particles in the near-Earth space, manifested in polar auroral phenomena and ionospheric disturbances. The animation illustrates the dynamical changes of the global magnetic field in the

course of a disturbance: a temporary compression of the magnetosphere by enhanced flow of the solar wind is followed by a tailward stretching of the field lines. Eventually, the increase of the tail magnetic field results in a sudden collapse of the nightside field (a substorm ) and a gradual recovery of the magnetosphere to its pre-storm configuration.

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MagneticReconnection

Magnetic reconnection allows particles to enter the magnetosphere

Plasmasphere

Artist's concept of the magnetosphere. The rounded, bullet-like shape represents the bow shock as the magnetosphere confronts solar winds. The area represented in gray, between the magnetosphere and the bow shock, is called the magnetopause. The Earth's magnetosphere extends about 10 Earth radii toward the Sun and perhaps similar distances outward on the flanks The magnetotail is thought to extend as far as 1,000 Earth radii away from the Sun.

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Charged particles motion

Lorentz force:

FL=qVB

ma= qVB a=2r V=r

mV= qVB L=qB/m

L Larmor frequency

RL=mV/(qB) Larmor radiusAt 1 AU B~5·10-9 T; if Vth~50 km/sec and m=mp

L= (1.6 ·10-19 C ·5 ·10-9 T)/ (1.67 ·10-27 Kg)=0.5 Hz

RL=(1.67 ·10-27 Kg ·50 ·103 m/sec)/(1.6 ·10-19 C ·5 ·10-9 T)=1.04 ·102 Km

Charged particles motion

In case an electric field E is also present a drift motion of the guiding center is supersimposed to the Larmor motion

The velocity of the guiding center results to be:

Vgc=EB/B2

Charged particles motion

magnetic mirror

Every time the particle moves to a place where magnetic field increses or decreases, the Larmor radius changes inducing a drift in the guiding center.

It can be shown that, if magnetic field intensity varies slowly in space, the quantity

=½mV2/B=constant (magnetic moment)

Lorentz force:

FL=qVB

RL=mV/(qB) Larmor radius

Charged particles motion

Radiation belts

If the particle moves to a place where B increases, it isforced to increase its V decresing its V// in order to keep itsKinetic energy K=1/2 m (V

2+ V//2) constant.

The particle will reach a point where all of its energy will be due to V and at that point it will reverse its motion.

Charged particles--ions and electrons--can be trapped by the Earth's magnetic field. Their motions are an elaborate dance--a blend of three periodic motions which take place simultaneously:

1. A fast rotation (or "gyration") around magnetic field lines, typically thousands of times each second.

2. A slower back-and-forth bounce along the field line, typically lasting 1/10 second

3. A slow drift around the magnetic axis of the Earth, from the current field line to its neighbor, staying roughly at the same distance from the axis. Typical time to circle the Earth--a few minutes.

Trapped Radiation

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2

3

1

2

On typical field lines, attached to the Earth at both ends, such motion would soon lead the particles into the atmosphere, where they would collide and lose their energy. However, an additional feature of trapped motion usually prevents this from happening: the sliding motion slows down as the particle moves into regions where the magnetic field is strong, and it may even stop and reverse.

In addition to spiraling and bouncing, the trapped particles also slowly drift from one field line to another one like it, gradually going all the way around Earth. Ions drift one way (clockwise, viewed from north), electrons the other, and in either drift, the motion of electric charges is equivalent to an electric current circling the Earth clockwise

That is the so-called ring current, whose magnetic field slightly weakens the field observed over most of the Earth's surface. During magnetic storms the ring current receives many additional ions and electrons from the nightside "tail" of the magnetosphere and its effect increases, though at the Earth's surface it is always very small, only rarely exceeding 1% of the total magnetic field intensity.

Particles drift around Earth

The Radiation Belts

The Earth actually has two radiation belts of different origins. The inner belt, the one discovered by Van Allen's, occupies a compact region above the equator (see drawing, which also includes the trajectories of two space probes) and is a by-product of cosmic radiation. It is populated by protons of energies in the 10-100 Mev range, which readily penetrate spacecraft and which can, on prolonged exposure, damage instruments and be a hazard to astronauts. Both manned and unmanned spaceflights tend to stay out of this region. The outer radiation belt is nowadays seen as part of the plasma trapped in the magnetosphere. The name "radiation belt" is usually applied to the more energetic part of that plasma population, e.g. ions of about 1 Mev of energy (see energy units). The more numerous lower-energy particles are known as the "ring current", since they carry the current responsible for magnetic storms. Most of the ring current energy resides in the ions (typically, with 0.05 MeV) but energetic electrons can also be found.

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The Radiation Belts:the inner belt

Cosmic rays are fast positive ions, bombarding Earth from all directions. When these ions smash into nuclei of atmospheric gases, fragments go flying off in different directions, some of them are short-lived particles created by the collision. Some of the fragments are however neutrons. Having no electric charge, neutrons are not affected by the Earth's magnetic field, and usually escape into space.The free neutron is however radioactive: within about 10 minutes it breaks up into a proton, which captures most of the energy, an electron and a massless neutrino. Ten minutes is a fairly long time for a fast particle, time enough for many neutrons to get halfway to Mars. However, decay times are spread out statistically, and while 10 minutes is the average, a few neutrons decay quite soon, while still inside the Earth's magnetic field. The energetic protons which then materialize are grabbed by the Earth's magnetic field, often on trapped orbits which do not return to the atmosphere, in which the proton can stay trapped for a rather long time.

The Radiation Belts:the outer belt

We know that outer-belt ions and electrons probably come from the long "magnetic tail" of stretched field lines on the night side of the magnetosphere.

Now and then a violent outburst, known as a magnetic storm, drives tail plasma earthward, into the near-Earth magnetosphere. Electric fields are essential to this process, to help tail particles break into trapped orbits and to drive them to higher energies. When the outburst ends and the electric field dies away, the particles find themselves locked in trapped orbits of the ring current and the outer radiation belt.

Whereas the inner belt is marked by great stability, the ring current and outer belt constantly change. Sooner or later the particles are lost, e.g. by collision with the rarefied gas of the outermost atmosphere, and on the other hand, new ones are frequently injected from the tail. The electric fields which inject the new particles can also draw oxygen ions upwards from the ionosphere, and the ring current contains such ions, typically a few percent of the total, more during magnetic storms.

Space plasma entering our magnetosphere produces auroras

Auroras

Space plasma entering our magnetosphere produces auroras

Natural wave emissions in space plasmas(1)

It wasn't until the satellites of the 1950's that we discovered what caused the strange whistles in the phone lines in England. This is a photograph of an aurora taken from the space shuttle. Auroras are trails of light that appear near the North and South Poles. Changes in the solar wind can cause changes in the earth's magnetosphere. These changes or "space storms" cause the aurora. When really strong space storms happen, people farther away from the poles can see them. The people in England listening on their telephones during an aurora similar to that shown, might have heard something like this ...

About 100 years ago, people in England heard some strange noises on their newly developed telephones. They heard these strange sounds during a time when aurora borealis or the Northern Lights occurred but didn't really put the two things together.science.nasa.gov/ssl/pad/sppb/

Natural wave emissions in space plasmas(2)

The next reports of strange sounds came during World War I. About 20 years after the people in England heard strange sounds on their telephones, the soldiers in World War I were listening to their enemies using electronic equipment. When they turned on their electronic equipment, they could hear their enemy's conversations but they also heard strange whistling sounds like bombs flying overhead. Here is what the soldiers may have heard....

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Our Magnetosphere as a source of wave Emission

You can compare a whistler to plucking a guitar string. The pluck is like the lightning that disturbs the magnetosphere.

That disturbance runs through the entire magnetosphere and some parts pick up certain frequencies and make them louder.

Like the guitar string whose length, tension and weight pick up a certain note or tone, the plasma in the magnetosphere picks up frequencies and "sings" back to us this whistling sound.

In a real way, the magnetosphere is communicating its structure through these radio signals.

Wave map

There are special places in the magnetosphere where the plasma and the magnet of the earth make light and cause sounds on telephone and radio.

1) The first sounds we heard over the radio were caused by the plasma bouncing back and forth in the magnet of the earth. The plasma then makes the radio or telephone sound like those back and forth motions.

2) The whistler sounds were caused in the part of the magnetosphere shown. The plasma here causes the high pitched sounds to be heard before the lower pitched sounds so that what you hear sounds like a whistle.

3) In the lion's roar region, the plasma bounces back and forth causing the radio and telephone to sound like a roaring lion. Here the signal lasts about 2 seconds and has a low tone or pitch.

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Our Magnetosphere as a source of wave Emission

Whistlers are constant-loudness signals.

The frequency decreases through the complete hearing scale and ends with the lowest tones you can hear.

The whole process lasts almost a full second.

The highest frequencies travel faster. So, the low frequencies arrive later, giving rise to the descending tone that resambles a whistle.Frequency is around several

KHz and, always < C

Jupiter

Other planets have magnets in them and have plasma doing the same things as earth. We certainly cannot see their aurora with a telescope because it would be too small and dim. When Voyager got close to Jupiter, it sent back radio signals that sounded like this....

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Jupiter

The second Voyager space robot found other plasma closer to Jupiter. This sound was made by plasma and Jupiter's magnet. Voyager received it with its radio and sent it back to earth. This sound is very much like the sounds made in the earth's magnetosphere close to the planet

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Saturn

When Voyager 2 got to Saturn, it found a similar kind of radio noise. That means the earth, Jupiter and Saturn all play the same type of sounds through a radio.

When Voyager 2 got closer to Saturn, it picked up this signal.... This "hiss" sound was also picked up from radios on earth satellites when they were in earth's plasma.

When Voyager 2 passed through a gap in Saturn's rings, this sound was heard on the radio.... This sound was caused by dust particles hitting the Voyager 2 radio antenna.

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Space Weather

For a planet to be affected by a blob of material being ejected by the sun, the planet must be in the path of the blob, as shown in this picture. The Earth and its magnetosphere are shown in the bottom right. Disturbances in the solar wind arrive at the Earth within hours to days after a violent event on the Sun. If the Earth were on the other side of the Sun (the top left of the picture), then the blob would miss the Earth, and there would be no geomagnetic storm or powerful aurora.

Space Weather

What is "Space Weather"?Everyone is familiar with changes in the weather on Earth. But "weather" also occurs in space. Just as it effects weather on Earth, the Sun is responsible for disturbances in our space environment as well.

Besides emitting a continuous stream of plasma called the solar wind, the Sun periodically releases billions of tons of matter in what are called coronal mass ejections. These immense clouds of material, when directed towards Earth, can cause large magnetic storms in the magnetosphere and the upper atmosphere.

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Space weather

Magnetic storms produce many noticeable effects on and near Earth:           

•Aurora borealis, the northern lights, and aurora australis, the southern lights •Radio and television interference •Hazards to orbiting astronauts and spacecraft •Current surges in power lines

space plasma storms

The Earth's atmosphere is protected from the solar wind by our magnetosphere. Even so, some solar wind energy does enter our magnetosphere and atmosphere and can cause a small amount of our atmosphere to be launched into space. We need to understand this loss of our atmosphere in order to understand our planet's environmental stability over a long time period. Solar wind energy in our magnetosphere can also cause what are known as space plasma storms. These storms can cause communication and science satellites to fail. They can also cause damage to electric power systems on the surface of the Earth. A large space storm in 1989 made currents on the ground that caused a failure in the Hydro-Quebec electric power system. This prevented 6 million people in Canada and the US from having electricity for over 9 hours. The same storm caused the atmosphere to inflate and dragged the LDEF satellite to a lower orbit earlier than expected.

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where are the Earth's Magnetic Poles?

The location of the magnetic pole is not fixed. It changes slowly with time. The magnetic pole in the geographic north is called the Earth's North Magnetic Pole by convention. The North Magnetic Pole is actually the south pole of the Earth's magnetic field. This came about because the north pole of a compass was defined as the pole that points to the geomagnetic north. However, since opposite poles attract, the north pole of the magnetic needle in the compass must point toward the south pole of the Earth's magnetic field. The Earth's surface magnetic field has a strength and a direction. The sites of the magnetic poles are the locations where the magnetic field lines are completely vertical. Maps with the locations of the magnetic poles are given above. At these locations, harmful radiation from the sun more easily penetrates to the Earth's middle and lower atmospheric layers.

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Auroras

A spacecraft Constellation

La missione europea CLUSTER

1. Cluster: la missione2. Cluster: le sonde3. L’armata solare4. Tempeste nello spazio5. Le aurore

La missione ESA/CLUSTER

La missione ESA/CLUSTER

La missione ESA/CLUSTER

La missione ESA/CLUSTER

La missione ESA/CLUSTER

Space Weather on_time

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Links

helios.gsfc.nasa.gov/ace_info.htmlscience.nasa.gov/newhome/headlines/ast07sep99_1.htmwww-spof.gsfc.nasa.gov/Modeling/group.htmlwww.windows.ucar.edu/spaceweather/www.spacescience.orgwww.spacescience.org/ExploringSpace/1.htmlwww.spacescience.org/ExploringSpace/PlasmaStateOfMatter/1.htmlwww-istp.gsfc.nasa.gov/exhibit/index.html