A plasmaspheric mass density model and constraints on its heavy ion 
concentration

D. Berube, M. B. Moldwin, S. F. Fung, J. L. Green


The first empirical model of the equatorial mass density of the plasmasphere is
constructed using ground-based ULF wave diagnostics. Plasmaspheric mass density
between L = 1.7 and L = 3.2 has been determined using over 5200 hours of data
from pairs of stations in the MEASURE array of ground magnetometers. The least
squares fit to the data as a function of L shows that mass density falls
logarithmically with L. Average ion mass as a function of L is also estimated by
combining the mass density model with plasmaspheric electron density profiles
determined from the IMAGE Radio Plasma Imager (RPI). Additionally, we use the
RPI electron density database to examine how the average ion mass changes under
different levels of geomagnetic activity. We find that average ion mass is
greatest under the most disturbed conditions. This result indicates that heavy
ion concentrations (percent by number) are enhanced during large geomagnetic
disturbances. We also find that the average ion mass increases with increasing L
(below L = 3.2), indicating the presence of a heavy ion torus during disturbed
times. Heavy ions must play an important role in storm-time plasmaspheric
dynamics. The average ion mass is also used to constrain the concentrations of
He+ and O+. Estimates of the He+ concentration determined this way may be useful
for interpreting IMAGE Extreme Ultraviolet Imager (EUV) images. 

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Journal of Geophysical Research, 110, A04212, doi:10.1029/2004JA010684, 2005