Empirical specification of field-aligned plasma density profiles for 
plasmasphere refilling

J. Tu, P. Song, B. W. Reinisch, J. L. Green, X. H. Huang 


Sounding measurements from the radio plasma imager (RPI) on the IMAGE satellite
are used to derive electron density distributions along magnetic field lines in
the nightside plasmasphere and plasma trough for three mid-latitude passes.
These passes occurred during 1) a magnetic storm, 2) a prolonged quiet time, and
3) a sudden commencement of a storm, respectively. It is found that the density
profiles of filled (in the inner plasmasphere) and depleted (in the plasma
trough or outer plasmasphere) flux tubes have different field line dependence. A
multi-variant least square fit with a simple functional form is utilized to
model the density profiles. The fitting parameters in the functional form define
field line dependence of a density profile, i.e., the steepness of the density
profile at high latitudes and flatness at low latitudes. In each pass the
density profiles along the filled and depleted flux tubes can be well modeled
with this functional form, with two different sets of fitting parameter values
for filled and depleted flux tubes. For the three passes examined, the fitting
parameter values are not sensitive to the geomagnetic activity for the inner
plasmasphere density profiles but vary slightly for the plasma trough or outer
plasmasphere density profiles from case to case. The equatorial densities
extrapolated from the measured density profiles approximately have a power law
relation with L values. The results suggest that the functional form used has
potential of being able to construct realistic global empirical
plasmasphere/plasma trough models.  Furthermore, it is now feasible to
empirically determine the density profiles along the depleted flux tubes for
plasmasphere refilling studies. 

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Journal of Geophysical Research, 110, A12210, doi:10.1029/2005JA011328, 2005