Further Exploration of the Relationship Between Subauroral Proton 
Precipitation and Plasmaspheric Plumes

M. Spasojevic (UC Berkeley, Berkeley, CA 94720), G. K. Parks, P. Chi, 
M. F. Thomsen, H. U. Frey, S. P. Gary, B. R. Sandel

Observations of detached subauroral proton arcs by the FUV instrument on the
IMAGE spacecraft have been reported and shown to be produced by ring current
ions precipitating in the afternoon local time sector during geomagnetically
disturbed periods. Data from June 18, 2001 show a direct association between
a subauroral proton arc and a global observation of a plasmaspheric plume by
the IMAGE EUV instrument. The link between the proton arc and plume suggests
that the precipitation may be due to pitch angle scattering of energetic
protons by electromagnetic ion cyclotron (EMIC) waves, which may be
preferentially generated within the plume since the presence of cold, dense
ions lowers the threshold for the EMIC instability. However, for the event 
of June 18, no direct measurements of EMIC waves are available. To further
investigate the relationship between the cold plasma and proton 
precipitation, we explore observations of EMIC waves by the Magnetic Field
Experiment on the Polar spacecraft. In particular, during March of 2001 
Polar crossed the magnetic equator in the afternoon local time sector at 
radial distances of 7-8 RE, while the IMAGE spacecraft was positioned with 
apogee at high northern latitudes imaging both the proton aurora and the 
cold plasma distribution.

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Presentation at the Yosemite Conference of Inner Magnetospheric Interactions,
3 - 6 February 2004, Yosemite, California, USA