Is the plasmapause a preferred source region of electromagnetic ion cyclotron waves in the magnetosphere?

Abstract

It has generally been assumed in the past that the plasmapause is a preferred region for the generation and propagation of electromagnetic ion cyclotron (EMIC) waves in the Earth's magnetosphere. One assumption invoked the overlap of the expanding cold plasmapause with the inner edge of the hot ring current during storm recovery to provide favourable conditions for EMIC instability. The plasmapause was also expected to provide a convenient gradient for guiding the waves from equatorial sources to higher latitudes. The paper commences from a historic perspective and reviews the development of the ideas from the 1960s that relate the source of EMIC waves to the plasmapause. CRRES spacecraft observations of EMIC wave events over L = 3.5 –8 and associated plasmapause locations indicate that the plasmapause is a region of wave generation and propagation, with significant wave power seen in the plasmatrough, but is not necessarily the preferred region. Other results show that wave occurrence predominates in the afternoon and increases with radial distance and a He + slot is seen in the data. These agree with earlier AMPTE-CCE results from Anderson et al. (J. Geophys. Res. 97 (1992) 3075, 3089). New results from CRRES show all wave polarisations (left-hand, linear, and right-hand) are seen within 8° of the equator whereas linear predominates over 20–30° latitude. Waves are observed in background plasma densities of 4– 300 cm −3 . Wave frequencies above the He + cyclotron frequency are concentrated outside the plasmapause in lower density regions of 2– 30 cm −3 .