Upper Limit of Proton Anisotropy and Its Relation to Electromagnetic Ion Cyclotron Waves in the Inner Magnetosphere. Issue 5 (5th May 2021) - Noh, Sung‐Jun
Abstract: Proton anisotropy in velocity space has been generally accepted as a major parameter for exciting electromagnetic ion cyclotron (EMIC) waves. In this study, we estimate the proton anisotropy parameter as defined by the linear resonance theory using data from the Van Allen Probes mission. Our investigation uses the measurements of the inner magnetosphere ( L < 6) from January 2013 to February 2018. We find that the proton anisotropy is always clearly limited by an upper bound and it well follows an inverse relationship with the parallel proton β (the ratio of the plasma pressure to the magnetic pressure) within a certain range. This upper bound exists over wide spatial regions, AE conditions, and resonance energies regardless of the presence of EMIC waves. EMIC waves occur when the anisotropy lies below but close to this upper bound within a narrow plasma β range: The lower cutoff β is due to an excessively high anisotropy threshold and the upper cutoff β is possibly due to the predominant role of a faster‐growing mirror mode instability. We also find that the anisotropy during the observed EMIC waves is unstable, leading to the linear ion cyclotron instability. This result implies that the upper bound of the anisotropy is due to nonlinear processes. Key Points: Proton anisotropy has a clear upper bound that follows an inverse relationship with the plasma beta within a certain range EMIC waves occur with an anisotropy below but close to the upper bound within a narrower plasma beta range The anisotropy associated with EMIC waves is unstable to the linear instability, requiring a nonlinear process to set the upper bound