O+ as a Source of He-band EMIC Waves at Low L shells of Earth's Magnetosphere
Abstract
Observation of linearly polarized He -band electromagnetic ion cyclotron (EMIC) waves at low L shells is a new, and quite unexpected, result from the Van Allen Probes mission. H ere we present a n observational test of generation mechanism(s) for the He-band EMIC waves with a broad spectrum between the O+ gyrofrequency and its second harmonic observed by Van Allen Probe- B in the predusk hours at L≈3 on 13 May 2015. For our test, all the wave and plasma parameters, the DC magnetic field, and energetic ion distributions are taken from the Van Allen Probe- B observations during the analyzed EMIC wave event. First, FFT is applied to magnetic field data to obtain an EMIC wave frequency spectrum, normal angle, and ellipticity. Second, to unveil the microphysics of wave generation, the observational data are used to calculate growth rates for EMIC waves. The calculations of growth rates using in situ observations for all the background plasma parameters and energetic ion distributions during the observed EMIC wave event are done for the first time. From our calculations , we identify the frequencies and normals generated, the energetic ion species responsible for instability, the physical mechanism of instability, and the ion energies and pitch angles that contribute most in the total growth rate. Finally, we analyze how the intrinsic limitations of conventional FFT affect analysis of the real EMIC wave events and how wave superposition affects the resulting minimum variance direction.
Acknowledgments: This paper is supported by NASA g rant 80NSSC18K1221 .- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSM54A..08G
- Keywords:
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- 2720 Energetic particles: trapped;
- MAGNETOSPHERIC PHYSICS;
- 2730 Magnetosphere: inner;
- MAGNETOSPHERIC PHYSICS;
- 2772 Plasma waves and instabilities;
- MAGNETOSPHERIC PHYSICS;
- 2774 Radiation belts;
- MAGNETOSPHERIC PHYSICS