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Generation of EMIC Waves Observed by Van Allen Probes at Low L Shells
Author  Gamayunov, Konstantin; Min, Kyungguk; Saikin, Anthony; Rassoul, Hamid; 
Keywords  effects of wave superposition on EMIC waves; EMIC wave generation; EMIC waves at low L shells; growth rate calculations for EMIC waves; polarization properties of EMIC waves; Van Allen Probes; Van Allen Probes observations at low L shells 
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. Here we analyze the two EMIC wave events observed by Van Allen Probes at low L shells and put forward a newgeneration mechanism for the lowL EMIC waves. Both events were observed at L \~ 3 but one of them has a discrete spectrum near the O+ gyrofrequency and its second harmonic, whereas the second event has a broad spectrum between the O+ gyrofrequency and its second harmonic. For both events, the major conclusions of our analysis can be summarized as follows. (1) Only O+ causes EMIC wave generation, and instability is driven by the positive derivatives of distribution functions over perpendicular component of velocity. (2) The timing and frequencies of generated waves are in agreement with observations. The generated wave normal angles, however, are highly oblique being in strong disagreement with the minimum variance angles obtained from Fast Fourier transform. (3) The wave step analysis shows that a signal nonstationarity is not a major cause for disagreement between the minimum variance angles and theoretical predictions for normal angles. (4) A superposition of plane sine waves with the same frequency and normal angle but with different azimuthal angles for wave vector around the background magnetic field can reconcile the polarization properties of EMIC waves obtained from Fast Fourier transform and/or the wave step analysis with those predicted by the linear theory of EMIC waves. 
Year of Publication  2018 
Journal  Journal of Geophysical Research: Space Physics 
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Number of Pages  
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Date Published  10/2018 
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URL  https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JA025629 
DOI  10.1029/2018JA025629 