Generation mechanism of Z-mode waves in the equatorial plasmasphere

Y Nishimura,T Ono,A Kumamoto,A Shinbori,M Iizima

doi:10.1186/bf03352043

Y Nishimura, T Ono + Show 3 more

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https://doi.org/10.1186/bf03352043

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Abstract

Abstract In order to clarify the generation mechanism of Z-mode waves observed in the equatorial plasmasphere, the growth rate of Z-mode electromagnetic waves has been calculated under the higher-order cyclotron interaction process. Z-mode waves can interact with some tens of keV electrons with large pitch angles even in the dense cold background, and the amplitude is consistent with the Akebono plasma wave measurements. UHR and whistler mode waves are also excited by the same electron distribution, and this is also consistent with observations. The origin of these energetic electrons are identified as the ring current electrons injected into the plasmasphere by the intense large-scale electric field during geomagnetic storms, accelerated perpendicular to the ambient magnetic field and confined around the geomagnetic equator conserving the first and second adiabatic invariants. Since the intensity of Z-mode and UHR waves is associated with the development and decay of the ring current, ring current particles are most possible candidate for the free energy source of these waves.

Highlights

UHR waves are identified as weak continuous emissions and measured in the wide region of the magnetosphere and plasmasphere (Mosier et al, 1973; Christiansen et al, 1978)
UHR and Z-mode waves frequently enhance in the geomagnetic equatorial region of the plasmasphere, which has been called the equatorial enhancement of the plasma wave turbulence (EPWAT) (Oya et al, 1990, 1991)
From the Akebono wave measurements, we have shown that Z-mode electromagnetic waves are excited in the equatorial plasmasphere during geomagnetically disturbed periods

Summary

Introduction

UHR (slow Z-mode) waves are identified as weak continuous emissions and measured in the wide region of the magnetosphere and plasmasphere (Mosier et al, 1973; Christiansen et al, 1978). We discuss how effectively Z-mode electromagnetic waves are excited directly in the equatorial region of the plasmasphere through a higher-order cyclotron resonance. Numerical calculations of wave growth rates is performed by considering energetic electrons with large pitch angles, and we will show that large growth rates can be obtained through the higher-order cyclotron resonance with Z-mode electromagnetic waves in high-ωp/ e regions.

Results

Conclusion