A statistical study of EMIC wave-associated He⁺ energization in the outer magnetosphere: Cluster/CODIF observations

Abstract

During the time period of 1 March 2001-1 April 2008, the Composition and Distribution Function (CODIF) Analyzer on board Cluster observed 41 prolonged He⁺ energization events, lasting for 1.10-4.97 h, on average, 3.18 ± 0.91 h. These He⁺ heating events occurred predominantly at low/middle magnetic latitudes (MLAT = -4.3°-51.7°) in the afternoon sector (MLT = 11:32-19:06) in the outer magnetosphere (L = 7.9-14.6). During the events, the He⁺ ions resonantly interacted with electromagnetic ion cyclotron (EMIC) waves and were perpendicularly energized to energies up to 1 keV. Their contribution to total ion density in the energy range of 0.04-1 keV was elevated on average up to 51%. A superposed epoch analysis of the plasma data measured by Cluster during these events indicates the presence of the two EMIC wave-controlling factors: hot anisotropic H⁺ (the wave free-energy provider) and cold dense plasma (the wave generation catalyst). In addition, it is common in the events that the density of the energetic H⁺ is elevated and electron plasma/gyrofrequency ratio (f_pe/f_ce) reaches values higher than 10. Quiet solar wind and geomagnetic activity appear to be favorable conditions for the generation of the EMIC waves and thus the resultant He⁺ energization in the outer-magnetospheric region. The reason is that, under quiet solar wind and geomagnetic conditions, an overlap of hot anisotropic H⁺ from the plasma sheet and cold dense plasma from a plasmaspheric plume or plume-like region could exist in the afternoon sector of the outer magnetosphere.