Recent observations of electromagnetic ion cyclotron waves in space

Abstract

In the past ten years observations of electromagnetic ion cyclotron (EMIC) waves in space have been reported which have greatly increased our understanding of the occurrence distribution and dynamics of Pc 1 waves in the magnetosphere. The most prominent site of EMIC wave activity is now known to be the dayside outer magnetosphere. Transient compressions of the magnetosphere have been shown to be effective in stimulating proton cyclotron wave activity by enhancing the proton temperature anisotropy. New Poynting flux measurements from Viking and IRM both indicate that wave energy is directed toward the ionosphere, in agreement with earlier ATS-6 results. Observations from lower altitude have revealed properties of lower L pulsations that reflect behavior consistent with Pc 1 pearl pulsations including quasi-periodic packets, dispersion, and occurrence in a narrow L range but wide longitudinal extent. Finally, the response of thermal ions to EMIC waves has also been studied showing that He + responds by developing X-type distributions indicative of non-local heating at locations along the field line where the wave frequency approaches the He + gyrofrequency. Further progress is likely in a number of areas: new spacecraft data sets, especially Freja; ground-satellite correlation studies; detailed instability analysis of proton distributions in the source region; and analysis of wave characteristics to assess the wave vector orientation.