Synoptic Studies of Earth's Plasmasphere with the IMAGE Extreme Ultraviolet Imager

Bill R. Sandel, Robert A. King, W. T. Forrester, Dennis Gallagher, C. C. Curtis,
and A. Lyle Broadfoot

The Extreme Ultraviolet Imager (EUV) of the IMAGE Mission records the spatial
distribution of He+ in the plasmasphere with a frame time of 10 minutes. EUV
measures the brightness of 30.4-nm sunlight resonantly scattered by He+
surrounding Earth. The 30.4-nm emission from the plasmasphere is optically thin,
so the measured brightness in each resolution element is directly proportional to
the He+ number density integrated along the corresponding line of sight from the
instrument through the plasmasphere.

The individual image frames, and especially movies made from them, reveal a host
of distinct plasmaspheric features and behaviors. Many of these had already been
inferred during the more than three decades of investigations of the plasmasphere
using ground-based and in situ techniques, but some of them appear to be new. And
global imaging offers new perspectives on even the previously-known features.

The features accessible to study using EUV include:

Plasma tails, which connect to the main plasmasphere in the dusk sector and
extend sunward. Their typical length is 3-4 Re, and they are separated from the
main plasmasphere by a density trough in which brightnesses are reduced by a
factor of roughly 6. The most spectacular example of this phenomenon recorded to
date had a width of about 1 Re.

Regions of plasma detached from the main plasmasphere.

Regions of reduced density in the main plasmasphere, typically located in the
pre-midnight sector.

"Shoulders," that is, sharp variations in radial distance of the plasmapause (by
~1/2 Re) in a narrow range of local time. These usually appear in the morning
sector.

Localized density troughs in the main plasmasphere.

Plasma "fingers" extending radially from the brightest parts of the plasmasphere.

Wavy plasmapause boundaries, with azimuthal scales of roughly 1 hour in local
time.

Rapid decreases in the radial distance to the plasmapause, concentrated mainly
near the midnight sector. Decreases of 1 Re in 30 minutes have been measured.

Generally speaking, these features corotate with the magnetic field as they form
and decay.

Auroral emissions are also found within the passband of EUV, so that the single
data set offers information on plasmasphere-substorm relationships.

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Presented at the Seventh Huntsville Workshop - 
A New View of Geospace: Integration, Interpretation and Synthesis 
October 30 - November 3, 2000, Callaway Gardens, Georgia