Electron density profiles in the magnetosphere from IMAGE/RPI plasmagrams

B. Reinisch, X. Huang, P. Song, I. Galkin
Center for Atmospheric Research, University of Massachusetts Lowell

R. Benson, S. Fung, J. Green
NASA Goddard Space Flight Center

The Radio Plasma Imager (RPI) on IMAGE conducts echo radio sounding
experiments producing plasmagrams that display echo amplitudes as
function of virtual range and frequency.  These plasmagrams show
different type of echo traces resulting from X, O and Z mode wave
propagation, and the identification of the wave modes requires careful
analysis. Angle of arrival measurements together with the echo trace
characteristics reveal two classes of echoes, direct and field-aligned
propagation (FAP) echoes. In this paper we describe the technique of
inverting the FAP traces into electron density profiles. These profiles
represent the density distribution along the field line that intersects
the spacecraft. In many instances, RPI receives FAP echoes from the
local as well as the conjugate hemisphere. This allows the calculation
of the complete density distribution from one ionosphere to the
conjugate one, beginning at altitudes of ~_ or 1 RE.  Once a profile has
been calculated, in general from the X trace, it is possible to validate
the analysis by calculating the expected Z and O traces and compare the
calculated traces with the measured ones. We found excellent agreement
assuring the accuracy of the profiles. By applying this technique to
different time periods, we were able to assess the effect of CME¹s on
the magnetosphere.

During the March 31, 2001 event the solar pressure was more than 40
times greater than normal with a sustained southward interplanetary
magnetic field, conditions for a major magnetic storm. The Kp index went
up to near 9. During the event, the IMAGE orbit was near the
noon-midnight meridian monitoring the magnetospheric responses at local
noon and midnight. The RPI sounding measurements show that the total
content of the plasma changed dramatically during the event. The dayside
density dropped significantly during the storm and we also observed the
refilling process after the storm. There is potential evidence for an
increased density on the nightside during the event, which may be the
result of the higher solar wind density. Quantitative assessment of the
density variations during the event and the refilling process afterward
will be presented.