Distribution of density along magnetospheric field lines

[ 1] This paper examines the field line distribution of magnetospheric electron density and mass density. The electron density distributions from IMAGE RPI active sounding are generally monotonic. The density increases with increasing MLAT slightly faster than the dependence found from the field line dependence model of Denton et al. (2002b); in general, a power law dependence n(e) = n(e0) (LRE/R)(alpha) with alpha similar to 1 appears to be appropriate within the plasmasphere, at least for geocentric radius R > 2 R-E. Our comparison to RPI data included also one field line distribution at L-T = 7.4, which we fit with alpha = 2.5, a value typical of the plasmatrough based on previous studies. We calculated the average electron density field line distribution at low MLAT using the CRRES plasma wave data and found that the density was relatively flat near the magnetic equator with no convincing evidence for an equatorial peak. Using the average values of toroidal Afven frequencies, we calculated the mass density field line distributions and found that they were roughly monotonic for L-T < 6, with alpha = 2 appropriate for L-T = 4 - 5 and alpha = 1 appropriate for L-T = 5 - 6. At L-T = 6 - 8, the distribution was nonmonotonic, with a local peak in mass density at the magnetic equator. Dividing the frequency data into different groups based on activity, we found that the inferred average mass density field line dependence was insensitive to geomagnetic activity at L-T = 4 - 6 but that at L-T = 6 - 8, the tendency for the mass density to be peaked at the magnetic equator increased with respect to larger Alfven wave amplitude and more negative Dst. The average frequency ratios at L-T = 6 - 8 did not change if we limited the data to cases with MLT = 8 - 16, for which the assumed perfect conductor boundary condition was better justified. Taken together, these results imply that heavy ions are preferentially peaked at the magnetic equator for L-T = 6 - 8, at least during more geomagnetically active periods.