Investigation of Multi‐Dimensional Electromagnetic Ion Cyclotron (EMIC) Wave Formation Using Hybrid Simulations

Abstract

Electromagnetic ion cyclotron (EMIC) waves contribute to magnetospheric loss cone scattering, but the fundamental processes during EMIC wave formation and evolution have not been systematically studied. We examine the formation of EMIC waves, driven by temperature anisotropy, in a uniform plasma using one‐dimensional (1D) and two‐dimensional (2D) hybrid simulations in proton‐only and hydrogen‐helium mixed plasmas. In a 1D plasma, a hot proton population with sufficient temperature anisotropy leads to the growth of parallel‐propagating EMIC waves in line with the predictions of the linear‐fit dispersion relation. The changing plasma temperatures and wave structures during EMIC wave evolution are illustrated. Additionally, a standing structure with parallel wave‐vector was found to act as a mechanism for parallel heating of the cold background plasma. This electrostatic structure emerges with half the EMIC wavelength, and is generated by nonlinear EMIC self‐interaction. In the 2D case, we observe the formation of structures possessing perpendicular wavenumbers and a broad‐spectrum overlap between the EMIC and electrostatic waves leading to locally enhanced amplitudes. In simulations with a cold helium population, we note specific contributions of each ion species to the thermal evolution of the system; hot protons carry cross‐field thermal energy and cold protons and helium respond to the electrostatic mode and its harmonics respectively. Harmonics of the electrostatic mode are nontrivial in the mixed plasma and contribute substantially to heating in the cold plasma background. Overall, results show evidence of nonlinear processes contributing to particle heating and modulation of excited EMIC waves.