Roles of upper-hybrid waves on the particle dynamics in the Earth's radiation belt

Abstract:

Quasi electrostatic fluctuations near upper-hybrid frequency, which are sometimes accompanied by multiple-harmonic electron cyclotron frequencies above and below the upper-hybrid frequency, are common occurrences in the Earth's radiation belt, as revealed through Van Allen Probe observations. Such a feature is analogous to the quasi-thermal noise, or enhanced Langmuir frequency fluctuations, historically detected in the solar wind. In general, upper-hybrid emissions are used for estimating the background cold electron density in the radiation belt study, but the physical mechanism for generating such higher frequency’s fluctuations or their possible influence on the particle dynamics has not been discussed in detail. The present paper carries out detailed analyses of data from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) suite onboard Van Allen Probes, as well as theoretical calculation of spontaneous thermal emission. It is found that peak of the upper-hybrid fluctuations is determined largely by tenuous energetic electrons, and that dense background electrons do not contribute much to the peak intensity especially during geomagnetically quiet conditions. This finding implies that upper-hybrid fluctuations may not only be useful for cold electron density measurement, but also such a spectrum of electrostatic fluctuations may contribute to the steady-state energy spectrum of radiation belt electrons via wave-particle resonant interaction. We also expand this study to investigate the effects of electromagnetic fluctuations since the low frequency portions of the multiple harmonic cyclotron bands, which are commonly detected together with the upper-hybrid frequency emissions, are characterized by magnetic field polarizations. By making use of the recently formulated fully electromagnetic theory of spontaneous emissions in magnetized plasmas we carry out a comparative study of the upper-hybrid/multiple-harmonic cyclotron emissions for geomagnetically quiet conditions as well as for moderately active cases.