In situ observation at L = 2.3-5 by the Akebono satellite of the plasmaspheric depletion during the September 1998 magnetic storm

From ground-based observations at L = 2.07 of the field line resonance (FLR) during an intense magnetic storm on 25 September 1998, Chi et al. (2000) estimated that the equatorial plasma density at L = 2.07 dropped to 25% of the prestorm value. Such depletion very close to the Earth is unusual. Two possible interpretations of the result are: (1) the plasmapause moved inward past L = 2.07, and (2) the plasmapause remained outside L = 2.07, but the density within the plasmasphere decreased. To distinguish between these possibilities, we examine in situ observations of the electron density made by the Akebono satellite at I = 2.3-5 on four passes during the same storm. The electron density measured by Akebono at a reference L shell of L ∼2.5 changed with time in a manner consistent with the FLR-based estimates. On three of these passes, the plasmapause was located at L > 3, and if the plasmaspheric L profile of the Akebono density data is extrapolated inward, for each pass, its value at L = 2.07 matches the FLR-based estimates. However, on the pass at 2323-2351 UT on 25 September that corresponded to the severe density decrease detected at L = 2.07, Akebono did not detect a plasmapause in the L range (>2.3) for which the electron density data were available. For the same pass, inward extrapolation of the density measured at L > 2.3 was a factor of ∼5 smaller than the FLR-based estimate. This implies that there was a sharp density inward gradient (the plasmapause) at 2.07 < L < 2.3, supporting the second interpretation.