Global Imaging of Shrinking Plasmaspheres

Bill R. Sandel, W. T. Forrester, and Robert A. King
Lunar and Planetary Laboratory
The University of Arizona
Tucson

Global imaging of the distribution of He+ in the plasmasphere by the
Extreme Ultraviolet Imager of the IMAGE mission offers new insight into
the dynamics of the plasmasphere. Here we examine the way that the
plasmasphere shrinks in response to increasing convection strength, a
phenomenon long known to occur. EUV images confirm that the first
response of the plasmasphere to increasing convection is an inward
motion of the plasmaspause near midnight. The inward motion progresses
nearly symmetrically toward dawn and dusk over the course of a few
hours. Near midnight, the rate of motion can be 0.5 to 1.0 RE/hour, and
changes of position from outside 6 RE to within 3 RE have been observed.
The motion is typically non-uniform, progressing in steps correlated
with substorm activity. In one instance, our measurements of the total
He+ content of the anti-sunward half of the plasmasphere show that it
decreased sharply at first, reflecting the inward motion of the
plasmapause. Within a few hours, the total content began to increase,
even though the size of the plasmasphere did not grow in proportion. A
typical part of these events is a sharpening in the plasmapause (i.e.,
an increase in the steepness of the intensity gradient) in the
transition from the quiet-time extended plasmasphere to the smaller
plasmasphere.