A study of the electromagnetic proton cyclotron instability as a generation mechanism for EMIC waves in the Earth's magnetosphere using SCATHA data

Abstract

Using 59 days of magnetic field data from SCATHA, a study of the electromagnetic proton cyclotron instability as a generation mechanism for EMIC waves in the Earth's magnetosphere has been conducted. From 1181 events found, we selected 373 events that were not influenced by the presence of He+ and O+ ions, i.e., events with frequency above the local He+ cyclotron frequency and that had all the required accompanying data. These events occurred in all magnetic local times (MLT) sectors, with L ranging from 5.6 to 8.1 and magnetic latitudes (MLAT) from −13.7° to 15.6°. Assuming a pure proton plasma, the proton temperature anisotropy, Ap, is determined from the maximum normalized frequency at which power is emitted. Using measured parameters and estimated total number density, the convective growth rate, S, of Electromagnetic Ion Cyclotron (EMIC) waves as a function of frequency depends only on the parallel proton temperature, T∥. Matching the observed peak in the frequency spectrum and the calculated peak in S allows the determination of T∥ and the proton perpendicular temperature, T⊥, for each event. Analysis of the results yields a new empirical expression for Ap as a function of the hot proton parallel beta, β∥h, and the maximum value of the wave temporal growth rate. The relationship between Ap and β∥h is consistent with theoretical predictions. The dependence of Ap on T∥ is consistent with the expectation that the free energy source of EMIC waves is energetic protons with T⊥ > T∥.