Dispersion characteristics for plasma resonances of Maxwellian and Kappa
distribution plasmas and their comparisons to the IMAGE/RPI observations

Adolfo F. Vinas, Richard L. Mace, and Robert F. Benson


The Radio Plasma Imager (RPI) on the IMAGE satellite stimulates short-range
plasma wave echoes and plasma emissions, known as plasma resonances, which are
then displayed on plasmagrams. These resonances are used to provide measurements
of the local electron density ne and magnetic field strength jBj. The
RPI-stimulated resonances are the magnetospheric analog of plasma resonances
stimulated by topside ionospheric sounders. These resonances are stimulated at
the harmonic of the electron cyclotron frequency fce, the electron plasma
frequency fpe, and the upper-hybrid frequency fuh (where fuh^2 = fpe^2 + fce^2).
They are also observed between the harmonics of fce (i.e., nfce) both above and
below fpe, where they are known as Qn and Dn resonances, respectively.
Calculations of the Qn resonances in the ionospheric environment, based upon a
thermal Maxwellian plasma model, provided confidence in the resonance
identification between the observations and the estimated values within the
experimental errors. However, there is often an apparent difference between
these resonances in the magnetospheric environment and those predicted by
calculations based on a Maxwellian plasma model. For example, the Qns are often
(and perhaps consistently) observed at frequencies slightly lower than expected
for a Maxwellian plasma. We present a new set of resonance calculations using
the dispersion characteristics of these resonances based upon a nonthermal kappa
distribution. We then compare these calculations and those based on a
traditional Maxwellian thermal plasma model with the IMAGE/RPI observations. The
calculations based on the kappa distribution model appear to resolve the
aforementioned frequency discrepancy. In addition, the results also provide
insights into the nature of the electron distribution function in the
magnetosphere.

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Journal of Geophysical Research, 110, A06202, doi:10.1029/2004JA010967.