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We rely on the Sun's energy to live on
Earth and the aurora relies on the Sun's energy to drive the currents
that make the aurora. The Sun is our nearest star. It is, as all
stars are, a hot ball of gas made up mostly of Hydrogen. The Sun
is so hot that most of the gas is actually plasma, the fourth state
of matter.
| A
cartoon of the four states of matter as the temperature
increases: solid, liquid, gas, and plasma. (Click
for a larger image) |
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The first state is
a solid and it is the coldest state of matter. As we heat up
a solid it becomes liquid. Liquid is the second state of matter.
As we heat up liquid, the liquid turns to gas. Gas is the third
state of matter. As we heat up the gas, atoms break apart into
charged particles turning the gas into plasma. This is not
the same type of plasma that is found in your blood: same name,
different stuff. |
The Sun's plasma is so hot that the
most energetic charged particles can escape from the Sun's
gravity and fly away, out into space. We call this plasma the solar
wind because it blows out away from the Sun and past the planets, interacting with their magnetic fields and/or
atmospheres. Along with the solar wind comes the Sun's magnetic
field, which reaches from the Sun out to past Pluto and Neptune.
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| Earth's
magnetosphere as it would look if we had "magnetic
field glasses". The shape is created by the interaction
of the solar wind with Earth's intrinsic magnetic
field. (Click
for a larger image) |
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Charged particles and magnetic fields
influence each other. So when the solar wind, which is made
up of charged particles, blows past Earth's magnetosphere,
the shape of the magnetic field changes from the dipole magnetic
field -- shown on Earth's
Magnetosphere page -- to a plasma-swept magnetosphere
that looks more like someone's hair got blown in the wind.
This interaction between the
Sun's plasma wind and Earth's magnetosphere is known as the
Sun-Earth Connection.
The side of the magnetosphere
getting hit by the solar wind is called the "dayside magnetosphere" because
it is facing the Sun. The part of the magnetosphere that stretches
back as though it were streaming with the solar wind is called
the magnetotail. The magnetotail is very important to the THEMIS
mission.
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| The
Sun emits both particles and light. It takes
light eight minutes to reach Earth but most
of the time the particles take about three
days to make the journey from the Sun to Earth.
(Click
for a larger image) |
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The every day interaction between
the solar wind and Earth's magnetic fields causes currents
to flow between Earth's upper atmosphere and the magnetosphere,
mostly the magnetotail. And just as currents flow through a
neon light to light up the gas, the currents flow between the
magnetotail and upper atmosphere light up Earth's gases
to cause the aurora.
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light from the aurora is caused by charged
particles (mostly electrons) that come from
inside the magnetosphere and then speed up
to very high speeds as they barrel down along
magnetic field-lines into the upper atmosphere.
As they collide with the gas, they excite the
atoms and molecules, which emit light when
they relax from their excited state. (Click
for a larger image) |
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Sometimes
the magnetosphere stores more energy than it can release
in a slow manner and something inside the magnetotail "breaks" and
waves and currents are generated that trigger the beautiful
and mysterious dancing aurora that we've been talking
about on these THEMIS science pages. THEMIS will solve
the long standing mystery as to what triggers this
energy release in the magnetotail that leads to an
auroral substorm. |
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