The Relationship between Plasma Density Structure and Emic Waves at Geosynchronous Orbit

Recent IMAGE satellite EUV helium density observations of plasmaspheric plumes extending beyond the plasmapause into the plasma trough region of the magnetosphere have been associated with sub-auroral proton arcs observed by the IMAGE FUV instrument. Also proton precipitation has been associated with electromagnetic ion cyclotron (EMIC) waves seen on the ground as Pel-2 ultra-low frequency (ULF) waves. This evidence suggests a relationship between plasma plumes, proton precipitation and EMIC waves, and supports the EMIC wave-particle interaction with ring current ions as a possible ring current loss mechanism. Using high-resolution (0.5 s) fluxgate magnetometer data from the GOES-8 and GOES-10 geosynchronous satellites we show two case studies on 9-10 and 26-27 June 2001, where EMIC waves in the 0.1-0.8Hz frequency range are observed within plasma plumes extending to geosynchronous orbit. These plumes are also seen in LANL geosynchronous satellite MPA data. The results suggest that EMIC waves may be preferentially generated in enhanced plasma density created by the plasma plume. The EMIC waves are unstructured and have the properties of the well-known intervals of pulsations with diminishing period (IPDP) seen on the ground and in space in the afternoon sector. They are classical EMIC transverse waves showing left-hand circular and elliptical polarization below the helium cyclotron frequency. The observation of a slot in the wave spectrum suggests the presence of He⁺ ions with relative concentrations in the range 6-16%, in a predominantly H⁺ plasma. This is consistent with the IMAGEEUV He⁺ observations of plasma plumes.