Interactions between Electromagnetic Ion Cyclotron Waves and Protons in the Magnetosphere: SCATHA Results

Abstract

Electromagnetic ion cyclotron (EMIC) waves and their role in governing proton populations in the ring current and radiation belt regions are of importance in understanding the dynamics of the Earth’s magnetosphere. The availability of SCATHA magnetic field and proton spectra data allowed us to study the electromagnetic proton cyclotron instability as a generation mechanism for EMIC waves and the relationship between EMIC waves and protons in the equatorial region of the magnetosphere. To identify EMIC waves and obtain wave power spectra from the SCATHA magnetic field data, we employed the fast Fourier transform and spectral analysis techniques. First, we studied the conditions under which the electromagnetic proton cyclotron instability acts as a generation mechanism for EMIC waves in the Earth’s magnetosphere. The results are consistent with those of previous studies and/or with theoretical expectations. Especially, the results show an inverse correlation between proton temperature anisotropy and proton parallel beta, that the observed proton temperature anisotropies are above the electromagnetic proton cyclotron instability thresholds, and that the observed waves actually get energy from energetic and anisotropic proton populations. Second, we studied the relationship between EMIC waves and protons in the region, and found that there were 20 short time intervals showing correlations between EMIC waves and proton perpendicular differential fluxes. They indicate that under suitable conditions, i.e., the proton distributions are non-gyrotropic or exhibit gyrophase bunching, EMIC waves indeed pitch angle scatter protons either toward or away from the local magnetic field. To explain the observations, we also established a model that is based on resonant interactions between EMIC waves and protons. In this model individual protons interact resonantly with the whole or a portion of the spectrum of the existing EMIC waves and are then pitch angle scattered with respect to the local ambient magnetic field. Calculations based on this model are in agreement with the observations and suggest that the EMIC waves are responsible for the changes in the proton distribution by pitch angle scattering protons with respect to the local ambient magnetic field.