THEMIS observations of electrostatic ion cyclotron waves and associated ion heating near the Earth's dayside magnetopause

Abstract

We present the first observations of large-amplitude electrostatic ion cyclotron (EIC) waves near the Earth's dayside magnetopause at MLT of ∼14 using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. The EIC waves were identified in a boundary layer in the magnetosphere adjacent to the magnetopause where reconnection was occurring. The EIC wave power was primarily at 2fcH (where fcH is the hydrogen cyclotron frequency) and simultaneously observed with perpendicular ion heating. The EIC waves had electric field amplitudes as large as 30-mV/m peak to peak with significant power both perpendicular and parallel to the magnetic field. These amplitudes were greater than those of previously observed ion cyclotron harmonics at the nightside magnetopause. The EIC waves occurred during an interval of enhancements in the quasi-static electric field and fluctuations in the background magnetic field, plasma density, and temperatures. The observations indicate that a plasma density gradient is a possible source of free energy for the EIC waves. The observed flow shears are not large enough to drive the waves. Whistler mode waves were identified near the EIC wave region but closer to the magnetopause in a region with slightly higher ion and electron temperatures. Key Points First observations of large-amplitude EIC waves near magnetopause Perpendicular ion heating is associated with the EIC waves Plasma density gradient is a possible source of free energy for the EIC waves