Experimental evidence for the acceleration of thermal electrons by ion cyclotron waves in the magnetosphere

Ion cyclotron waves (ICWs) at frequencies just above ƒHe+, the helium gyrofrequency, are often observed by the ESA satellites GEOS 1 and GEOS 2. Young et al. (1981) have shown the close connection between these ICWs and the presence of thermal He + ions in the outer magnetosphere. The purpose of this paper is to provide experimental evidence for the acceleration of thermal electrons by the large amplitude ICWs. The wave power in the ultra low frequency (ULF) range near ƒHe+ is compared with the distribution function of low energy electrons, measured by instruments aboard the GEOS satellites. It shows that, near the geomagnetic equator, electrons are accelerated along field lines at times when the ICW energy is large and the cold plasma density is below a threshold value. It is suggested that these accelerated electrons can account for the ELF emissions, modulated at the ICWs frequency, observed by Cornilleau‐Wehrlin (1981). Other ULF events having frequencies close to the proton gyrofrequency ƒ>H+ are also observed and result in a very efficient acceleration of thermal electrons along field lines. The evidence in this case is that medium energy protons having large temperature anisotropies ( T ⊥ ≫ T ∥ ) in the 100‐ to 500‐eV range are responsible for the ICW wave generation.