High‐Frequency Thermal Fluctuations and Instabilities in the Radiation Belt Environment

Abstract

This paper overviews the electrostatic and electromagnetic theories of spontaneous emission in magnetized plasma as they relate to measured electric and magnetic field fluctuations in quiet time radiation belt and ring current region. The pervasively detected high-frequency fluctuations in the upper-hybrid frequency range as well as the background low-frequency range spectral profile in the whistler mode range are explained within the context of the spontaneous emission theory. The quasilinear calculation of loss cone instability is also carried out in order to validate the assumption of spontaneous emission model. It is shown that the saturated wave amplitudes associated with the upper-hybrid and multiple-harmonic cyclotron instability are quite low, indicating that the theoretical explanation based upon the assumption of spontaneous emission theory may be adequate for understanding the observed background fluctuations during quiet times. Plain Language SummarySince hot electrons constitute only a small fraction of the total electron number density in the space plasma, the general thought has been that the upper-hybrid fluctuations are useful only as a tool for indirectly measuring the cold electron number density. However, the data from the Van Allen Probes showed that upper-hybrid electrostatic fluctuations pervasively and ubiquitously exist in the radiation belts. From there, we proved that the presence of hot electrons and upper-hybrid fluctuations are mutually related phenomenon. We believe that it is the high-frequency electrostatic fluctuations that are constantly emitted and reabsorbed by the hot electrons, which allow these radiation belt electrons to remain inside the outer radiation belt for such a long time.n 0 e 2 ∕( m e ) and f c = eB 0 ∕(2 m e c) are the plasma frequency and electron cyclotron frequency, respectively. Here e, n 0 , B 0 , m e , and c represent the unit electric charge, ambient number density, ambient magnetic field intensity, electron rest mass, and the speed of light in vacuo, respectively. The multiple harmonic cyclotron emissions are also known in the literatures as the (n + 1∕2)f c emissions, and such emissions have been detected by the GEOTAIL and the Van Allen Probes.

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

“…In addition to the wave environment, we seek to understand the local plasma conditions (e.g., Thaller et al, ; Hwang & Yoon, , in this collection) that lead to the enhancement or suppression of these various wave modes and the consequences therein for the precipitation of the trapped populations. For instance, Greeley et al () in this collection have found that the plasmapause is likely a better indicator of microburst location than L‐shell.…”

Dynamical Coupling of Energetic Electrons and Upper‐Hybrid Thermal Fluctuations in the Earth's Radiation Belt

“…First, we will consider the steady state wave equation in the presence of energetic electrons. The solution is essentially that which was already discussed in our previous papers (Hwang et al, ; Hwang & Yoon, ; Yoon et al, , ; Yoon, Hwang, et al, ), but herewith, we briefly include the essential analysis for the sake of completeness. We will then analyze the steady‐state particle equation in which the thermal noise fluctuation spectrum governs the velocity space diffusion.…”

“…We have further employed this result in the thermal noise spectroscopy taking into account the antenna response function (Yoon, Hwang, et al, ) and successfully reanalyzed the VAP data. Finally, we have extended the basic methodology in order to include electromagnetic effects (Hwang & Yoon, ; Yoon et al, ) and showed that the electromagnetic effects have an appreciable impact on the low‐frequency portion of the fluctuation spectrum.…”

“…The ubiquitously observed fluctuations were recently interpreted within the framework of spontaneously emitted electrostatic fluctuations in thermal plasma that consists of thermal core distribution of electrons plus a hot electron component (Hwang et al, ; Hwang & Yoon, ; Yoon et al, , ; Yoon, Hwang, et al, ). Spontaneous emission in thermal plasma is a concept already available in the literature (Issautier et al, ; Meyer‐Vernet et al, ; Moncuquet et al, ; Sentman, ), hence, is not new.…”