3D Structure and Dynamics of the Plasmasphere

F. Herbert (The University of Arizona, Tucson, AZ 85721) and B.R. Sandel

The Extreme Ultraviolet Imager (EUV) on the IMAGE spacecraft has steadily
observed the 30.4-nm sunlight resonantly scattered by He+ ions in Earth's
plasmasphere for more than 3 years.  During IMAGE's plasmaspheric traversals
its radio plasma imager (RPI) measured electron density all along the
instantaneous local magnetic field line of the spacecraft under a number of
different magnetospheric conditions, and found its latitude dependence to be
approximately invariant (Reinisch et al., 2003). In principle, if we assume
that the lati-tude dependence of the He+ density follows a similar profile,
this 1D information can be combined with the 2D information provided by each
EUV image to deduce the 3D distribution of He+ density throughout the part of
the plasmasphere imaged by IMAGE at that moment. This pseudo-tomographic
inference has proven to be practicable at least in the case of roughly
pole-on EUV images, and we will show in movie form the results of several
such image sequences so analyzed. 

One finding of this analysis is a partial ring of He+ depletion (relative to
nHe+ µ L-4) at L=2 (plasmapause at L=3) or 2-3 (plasmapause at L=4 to 6),
starting near midnight and extending in the anti-corotation direction to
geographic longitude ~250 degrees ± 20 degrees W.  Between the depletion ring 
and the plasmapause, the product (nHe+)(L4) rises before falling again at the
plasmapause. Because the depletion's trailing end at ~250 degrees W is 
corotating and its midnight end is not, it lengthens in time, presumably to 
be erased by infill from the ionosphere.  Thus it appears that we can 
estimate the refilling rate as a function of longitude and latitude.

In one observation of a shoulder, a local outward displacement of the
plasmapause,  the leading edge of the shoulder approximately corotates at
longitude ~0 degrees, and from there to the shadow, He+ is depleted near the
plasmapause also. At smaller L, the depletion ring is also filled in at the
longitude of the shoulder, suggesting that the refilling rate may be higher
there.

For the future, images from several viewpoints in IMAGE's orbit could in
principle be combined to further constrain the 3D He+ distribution, at least
at epochs when the it is reasonably steady.   Moreover, this analysis could
be extended to Jupiter's equivalent of the plasmasphere, the Io plasma torus,
using analogous images from the proposed JMEX plasma imager now in Phase A
study for the Small Explorer Program.

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Presentation at the Yosemite Conference of Inner Magnetospheric Interactions,
3 - 6 February 2004, Yosemite, California, USA