Electrostatic upper-hybrid waves and energetic electrons in the Earth's radiation belt

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, detected in the solar wind. Generally upper-hybrid emissions are used for estimating the background electron density, but the physical mechanism for generating such fluctuations or their possible influence on the energetic electrons 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 intensity associated with the upper-hybrid fluctuations is determined largely by tenuous energetic electrons, and that dense background electrons do not contribute much to the peak intensity. This finding implies that upper-hybrid fluctuations may not only be useful for 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.