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

Abstract

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 ∣B∣. 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 fuh2 = fpe2 + fce2). 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.