EMIC waves observed by Cluster near the plasmapause

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Abstract
We analyze a long‐duration electromagnetic ion cyclotron (EMIC) wave event seen in the inner magnetosphere in order to understand the propagation characteristics of these waves in the vicinity of the plasmapause. The study takes advantage of the south to north orbit of the four‐satellite Cluster constellation as it passed through perigee at L ~ 4.2 at ~08 magnetic local time on 2 November 2001. Cluster traversed from a low‐density magnetosphere (−3) through a gradual plasmapause into a high‐density plasmasphere (~80 cm−3) where the waves were seen over about 50 min and ceased on exiting the plasmapause in the Northern Hemisphere. The waves were observed over 1.8–3.5 Hz, above the local helium cyclotron frequency, between magnetic latitudes ±18°, and confined to a radial source region size estimated at 0.77 RE. Wave polarization appeared to be associated with plasma density, with left hand in the equatorial region, right hand at higher latitudes nearer the plasmapause, and a mixture between. Wave normal angles were typically <60°, and Poynting flux measurements show that wave energy was predominantly directed along the geomagnetic field toward high latitudes in both hemispheres. These results suggest that the plasma density and its gradient play a significant role in confining the wave source region and affecting the wave properties, which will help understand wave generation and propagation mechanisms in the magnetosphere plasma environment.

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