Abstract
AbstractEquatorial noise in the frequency range below the lower hybrid resonance frequency, whose structure is shaped by high proton cyclotron harmonics, has been observed by the Cluster spacecraft. We develop a model of this wave phenomenon which assumes (as, in general, has been suggested long ago) that the observed spectrum is excited due to loss cone instability of energetic ions in the equatorial region of the magnetosphere. The wavefield is represented as a sum of constant frequency wave packets which cross a number of cyclotron resonances while propagating in a highly oblique mode along quite specific trajectories. The growth (damping) rate of these wave packets varies both in sign and magnitude along the raypath, making the wave net amplification, but not the growth rate, the main characteristic of the wave generation process. The growth rates and the wave amplitudes along the ray paths, determined by the equations of geometrical optics, have been calculated for a 3‐D set of wave packets with various frequencies, initial L shells, and initial wave normal angles at the equator. It is shown that the dynamical spectrum resulting from the proposed model qualitatively matches observations.
Highlights
Equatorial noise in the frequency range below the lower hybrid resonance frequency, whose structure is shaped by high proton cyclotron harmonics, has been observed by the Cluster spacecraft
We develop a model of this wave phenomenon which assumes that the observed spectrum is excited due to loss cone instability of energetic ions in the equatorial region of the magnetosphere
The frequency range of the emission falls in the interval between proton gyrofrequency and lower hybrid resonance (LHR) frequency; it has been observed in the near-equatorial region on L shells from about 4 to 4.4
Summary
The term equatorial noise is used to describe the wave phenomenon, but not the plasma mode, and is independent of how the wave mode is called It has been suggested by several authors that equatorial noise is generated by an unstable ring-like distribution of energetic protons (see, for example, Perraut et al, 1982). We should mention that the waves under discussion, which are generated by an unstable proton distribution, are believed to play an important role in the dynamics of energetic electrons of the radiation belts (see, e.g., Horne et al, 2007; Li et al, 2014; Mourenas et al, 2013, and references therein) This problem, is beyond the scope of the present study which is devoted exclusively to generation and spectral features of these waves
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