THEMIS observations and modeling of multiple ion species and EMIC waves: Implications for a vanishing He+ stop band

Abstract

Observations of left‐hand polarized electro‐magnetic ion cyclotron (EMIC) waves with significant wave power across fHe+ are presented. Concurrent field and particle measurements reveal pitch angle scattering of thermal ions during the interval of maximum wave power. The observations suggest the presence of multiple cold ion species (H+, He+) at this time, below the sum of spacecraft potential and minimum energy of the ion instrument (i.e., <∼20 eV). Fortuitously, immediately following the most intense wave activity, the ambient plasma flow velocity became sufficiently large such that cold ions can be accelerated into the energy range of the particle instrument. Thus, two low energy ion components are revealed in the particle measurements at energies consistent with cold protons and He+. Maxwellian fits provide approximations of these cold ion species component densities and temperatures. The linear dispersion relation of parallel propagating EMIC waves for the observed conditions shows a vanishing cold plasma He+ stop band, consistent with the wave measurements during this interval. These results provide the first observational evidence of the importance of the cool ion species temperature/density in controlling linear wave growth through the heavy ion stop bands in warm plasma theory.