Magnetospheric electron density from IMAGE/RPI wave observations during 
magnetic storms

Bodo W. Reinisch, Xueqin Huang, Paul Song, Ivan Galkin, Patrick Nsumei,
James L. Green, and Robert F. Benson


The Radio Plasma Imager (RPI) on the IMAGE spacecraft is the first long distance
radio sounder deployed for the remote sensing of plasma density structures in
the inner magnetosphere. With its extremely long thin-wire antennas (originally
500 m tip-to-tip) RPI was able to receive echoes of the transmitted pulses from
distances of several RE. In the receive-only mode, RPI functioned as a very
sensitive spectrometer in the frequency range from 3 kHz to 3 MHz. During RPI
sounding operations magnetic field-guided echoes were frequently observed. These
echoes are attributed to waves that propagate along magnetic field lines for
frequencies f >> fpe, where fpe is the electron plasma frequency, which allow
the calculation of the instantaneous electron density distribution Ne(s) along
the magnetic field. These measured distributions led to the development of
empirical models of Ne in the plasmasphere [Huang et al., Adv. Space Res., 33,
6, 829-832, 2004] and the polar cap  [Nsumei et al., J. Geophys. Res., 108, A2,
2003]. The quasi-instantaneous measurement of the Ne distribution in the inner
magnetosphere allowed us to investigate the time evolution of the plasmasphere
structure in response to magnetic storms. The large storm of March 31, 2001, in
particular, substantially depleted the equatorial plasma in a range of L-shells.
The flux tubes were refilled after the storm. The filling ratio, defined by the
equatorial plasma density normalized by its quiet-time value before the storm,
was introduced to assess the time evolution of the depletion and refilling
processes.
The VLF transmissions also stimulate plasma resonances that accurately determine
the local plasma and gyro frequencies. During the same storm, the electron
number density and the magnetic-field strength, derived from the resonance
frequencies, increased dramatically near 8 RE.

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Spring Joint Assembly, American Geophysical Union, Baltimore, Maryland, U.S.A., 
23-26 May 2006