Excitation and saturation of electron loss cone by upper-hybrid wave and Z mode instabilities in the Earth's magnetosphere

Abstract:

The Earth’s inner magnetosphere and auroral ionosphere is replete with various types of naturally occurring electrostatic and electromagnetic waves and fluctuations. These waves include whistler and electron cyclotron waves that are highly transverse, multiple harmonic electron cyclotron (known as n + 1/2 waves), and upper-hybrid and/or Z mode waves. The Van Allen Probe observations made in the Earth's radiation belt reveal the persistent occurrence of quasi-electrostatic upper-hybrid frequency range fluctuations, which are sometimes accompanied by multiple-harmonic electron cyclotron frequencies above and below the upper-hybrid frequency. Such a feature was recently investigated in the light of the spontaneous thermal emission theory for magnetized plasmas, which is analogous to the quasi-thermal noise spectroscopy developed in the context of the solar wind [Hwang et al., 2017. Yoon et al., 2017]. It was shown that tenuous hot electrons contribute to the peak upper-hybrid emission intensity as well as to its spectral width, so that by measuring the peak and width of the upper-hybrid emission spectrum one may diagnose the underlying hot electron content. One of the assumptions in the theories of Hwang et al. [2017] and Yoon et al. [2017] is that the electron velocity distribution function remains close to isotropic thermal equilibrium, whereas in the actual magnetosphere, the electrons must possess the loss-cone feature. The present paper investigates the excitation of the upper-hybrid and multiple-harmonic cyclotron instabilities by the loss-cone distribution of electrons and the subsequent saturation by quasilinear process. Such an analysis is important in order for determining the time scale of relaxation of free energy associated with the loss-cone electrons, which might have relevance to other energy release phenomena in the radiation belt and inner magnetosphere.

Hwang, J., D. K. Shin, P. H. Yoon, W. S. Kurth, B. A. Larsen, G. D. Reeves, and D. Y. Lee, Roles of hot electrons in generating upper-hybrid waves in the earth’s radiation belt, Phys. Plasmas, 24, 062904 (2017)

Yoon, P. H., S. Kim, J. Hwang, and D.-K. Shin, Upper hybrid waves and energetic electrons in the radiation belt, J. Geophys. Res., 122, 5365 (2017).