EMIC Wave Energy Dissipation as a Source of O⁺ Conics and Warm Plasma Cloak in the Earth's Inner Magnetosphere

This study focuses on a specific source of the O⁺ conics and warm plasma cloak in the Earth's inner magnetosphere due to electromagnetic ion cyclotron (EMIC) wave energy dissipation. We analyze the EMIC wave event observed by Van Allen Probe-A in the early afternoon off equatorial magnetosphere on 18 November 2015, where the two dominant EMIC wave bands, He⁺-band and H⁺-band, were observed for ∼4 min. All the wave and plasma parameters, the DC magnetic field, and ion distributions needed for our analysis are taken from the Van Allen Probe-A observations during the event. The major results of our analysis are as follows. (a) The H⁺ and He⁺ heating by EMIC waves is negligible. (b) A strong heating of O⁺ by the wave energy dissipation around the third, fifth, and sixth harmonics of the O⁺ gyrofrequency is revealed, where the majority of energy dissipated goes into heating of O⁺ with the energies ≲100 eV and pitch angles ∼20°–90°. The estimated energy deposition rate is ∼0.1–3.4 eV/s per O⁺, totaling to the deposition of ∼20–800 eV per O⁺ during the event. (c) EMIC waves substantially contribute to the formation of O⁺ conics and warm plasma cloak by heating the upgoing low-energy ionospheric O⁺ because waves heat ions with the energies and pitch angles that are characteristic of conics and warm plasma cloak, and the energy deposition per O⁺ is comparable to those characteristic energies.