Increased magnetospheric fpe/fce values in response to magnetic-cloud
enhancements of the solar-wind quasi-invariant on 31 March 2001

V. A. Osherovich, R. F. Benson, J.  Fainberg, and B. W. Reinisch


The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora
Global Exploration (IMAGE) satellite normally observes a large number of
plasma resonances at the harmonics of the electron cyclotron frequency
fce in the apogee portion of the IMAGE orbit where fce is usually quite
small (a few kHz).  These resonances, which are usually the only ones
observed in this region, are often observed most prominently on the
spin-axis Z antenna. During the large disturbance of 31 March 2001,
however, this situation changed dramatically.  The value of fce
increased considerably (by about a factor of 2), indicating a similar
increase in the magnetic field strength, and the electron plasma
frequency fpe (proportional to the square root of the electron density)
increased by an even larger factor.  The resulting plasmagrams near
apogee on this day revealed about half as many fce resonances as are
normally observed and a rich spectrum of additional resonances due to
the increase in the fpe value.  We will relate these increases to the
large (more than a factor of 10) enhancement of the solar-wind
quasi-invariant associated with this disturbance.  We will demonstrate
the importance of being able to relate the changes in the magnetospheric
fpe/fce ratio (equivalent to the ratio of the electron gyro radius to
the Debye length) to changes in the solar wind as measured by the
solar-wind quasi-invariant.