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Space Plasma
The Solar Wind
The Bow Shock
The Magnetosphere
Substorms
Data Analysis
Simulation
Science Topics
Missions
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The Earth's Geomagnetic Tail
The night-side region of the Earth's magnetosphere is known as
geomagnetic tail. It is approximately cylindrical in shape, with a
radius of around 25 Earth radii (RE), although this somewhat reduced
closer to the Earth. It is anticipated that the tail maintains a
coherent structure over lengths of order 1000 RE, although these
distances have not yet been comprehensively surveyed by spacecraft.
The tail acts as a reservoir of energy tapped from the solar wind by
the action of magnetic reconnection at the dayside magnetopause. This
energy is stored as magnetic field energy in the two lobes of the
tail. These are regions of strong and steady magnetic fields which are
directed either towards (north lobe) or away (south lobe) from the
Earth.
The lobes are separated by a region of hot plasma associated
with reduced and more variable magnetic field, known as the plasma
sheet, and its boundary layers. Embedded within the plasma sheet is a
current or neutral sheet across which the reversal in the magnetic
field direction takes place. The occurrence of magnetic reconnection
at this current layer releases the magnetic energy stored in the lobes
and produces accelerated plasma sheet flows jetting both Earthward and
tailward away from the neutral line. The diagram above shows the
average cross section of the tail. However, significant asymmetries in
this structure are known to occur through the action of the y-component
of the interplanetary magnetic field. This may result in asymmetric
plasma entry to the tail and significant twisting of the whole tail
structure around its central axis. In addition, small angular
deflections in the solar wind flow direction may cause a significant
flapping motion of the tail. By ~240 RE down the tail (the largest
distance to which we have been able to extensively survey the tail with
the ISEE-3 and GEOTAIL missions), the tail may undergo transverse
relocations with amplitudes greater than the tail radius. In addition,
the energy storage and release cycle is sporadic, and the tail
undergoes dramatic structural changes during this cycle, which is
known as a geomagnetic substorm.
Cross section of the geomagnetic tail.
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