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Although accurate at the time of publication, it is no longer being updated. The page may contain broken links or outdated information, and parts may not function in current web browsers.
Why? -- is it produced
Where do the electrons come from?
Nightside aurora: magnetosphere
Dayside aurora: some from the magnetosphere and others
directly from the solar wind
drawing of the magnetosphere:
This drawing shows that the magnetic field lines
from most of the magnetosphere are focused down to Earth in the northern
and southern high latitude regions. The patterns of electron precipitation
seen in auroral images provide information about the physical processes
involving charged particles (the plasma) and electromagnetic field along
the magnetic field lines. Thus an image of the aauroral oval and
polar region provide a global monitoring of activity in the magnetosphere.
(wavelengths) as function of height:
Altitude of the maximum ionization rate (points)
and the altitudes at which this rate has fallen to 10% of the maximum (bars)
versus energy, for ionization produced by electrons which form the discrete
aurora as modeled by monoenergetic spectra of isotropic incidence [Rees
and IMF plot:
The magnetosphere has to provide energy to the
electrons that precipitate into the atmosphere to produce the aurora.
This energy comes ultimately from the sun via the solar wind and into the
magnetosphere. The ability of energy of the solar wind to enter the
magnetosphere depends very strongly on the direction of the interplanetary
magnetic field that is carried by the solar wind. When this field
has a northward component (Bz is positive), little energy can enter the
magnetosphere and the aurora that is seen is very weak. It is measured
by an index called AL, which on the plot is normalized by the square of
the velocity of the solar wind, so only the effects of the magnetic field
direction are taken into accound. However, when the magnetic field
has a southward component (Bs in the plot), more and more energy enters
the magnetosphere as Bs becomes larger. Thus the direction of the
weak interplanetary magnetic field acts as a valve to letting energy of
the solar wind enter the magnetosphere.
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Last updated: 7/26/00
Above is background material for archival reference only.