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The Earth is a
large magnet
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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.
The Earth's Magnetosphere
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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.
What is the
Magnetosphere?
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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.
The Sun and the Earth are connected
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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.
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
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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.
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.
map of the
magnetosphere
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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.
The Earth's Magnetosphere
Clickfor animation
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.
The Earth's Magnetosphere
The event of July 14, 2000
The event of January 10, 1997
Clickfor animation
Click for animation
First optical evidence
of 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 electricallyconducting 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 to
fast-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
Venus
The 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
Mars
The 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)
Click for animation
www-spof.gsfc.nasa.gov/Modeling/group.html
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).
Storms in Space
Storms in Space
Magnetic 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.
ESA picture
Magnetic reconnection allows particles to enter the magnetosphere
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.
Click for animation
www-spof.gsfc.nasa.gov/Modeling/group.html
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.
Modeling Earth's Magnetosphere
Using Spacecraft Magnetometer Data (3)
www-spof.gsfc.nasa.gov/Modeling/group.html
Click for animation
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.
Magnetic
Reconnection
Magnetic reconnection allows particles to enter the magnetosphere
Plasmasphere
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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.
Charged particles motion
Lorentz force:
FL=qVB

ma= qVB
mV= qVB 
a=2r
V=r
L=qB/m
L  Larmor frequency
RL=mV/(qB)
 Larmor radius
At 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=EB/B2
Charged particles motion
magnetic mirror
Lorentz force:
FL=qVB
RL=mV/(qB)
 Larmor radius
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)
Charged particles motion
If the particle moves to a place where B increases, it is
forced to increase its V decresing its V// in order to keep its
Kinetic 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.
Radiation belts
Trapped Radiation
1
1
2
3
2
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.
Particles drift
around Earth
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.
The Radiation Belts
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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.
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)
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.
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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 ...
Natural wave emissions
in space plasmas(2)
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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....
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.
3
1
Wave map
2
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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.
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
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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....
Jupiter
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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
Saturn
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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.
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.
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
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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.
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
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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 HydroQuebec 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.
where are the Earth's Magnetic Poles?
Click for animation:
mag_pole_animated.gif
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.
Auroras
A spacecraft Constellation
La missione europea CLUSTER
1.
2.
3.
4.
5.
Cluster: la missione
Cluster: le sonde
L’armata solare
Tempeste nello spazio
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
Click for connection:
Today's Space Weather
www.windows.ucar.edu/spaceweather/
Links
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www-spof.gsfc.nasa.gov/Modeling/group.html
www.windows.ucar.edu/spaceweather/
www.spacescience.org
www.spacescience.org/ExploringSpace/1.html
www.spacescience.org/ExploringSpace/PlasmaStateOfMatter/1.html
www-istp.gsfc.nasa.gov/exhibit/index.html