Sub-auroral proton precipitation as a result of plasmapause-ring-current 
interaction

H.U. Frey, S.B. Mende, T. Forrester, M. Spasojevic


The proton aurora imager SI-12 on the IMAGE spacecraft occasionally observes
subauroral proton precipitation in the morning and the afternoon sectors.
Sub-auroral Morning Proton Spots (SAMPS) rotate with 80-90% of the EarthÕs
corotation speed, while the detached afternoon proton arcs do not move in local
time. Coincident particle measurements by DMSP of both phenomena confirm the
source as pure high energy precipitating protons with energy likely above the
detector limit of 30 keV. The spots appear after magnetic storms in the recovery
phase and last for 1-4 hours in the magnetic local time region of 0600-1200
hours. The subauroral arcs appear after a direction change of the southward or
westward components of the interplanetary magnetic field and they last for 0.5-2
hours.

IMAGE-EUV observations of the plasmasphere indicate a relationship with density
gradients in the plasmapause. This is supported by nearby geosynchronous
observations of increased cold plasma density. We interpret these phenomena as
the result of the interaction of ring current protons with electromagnetic
ion-cyclotron (EMIC) waves caused by the dense, cold plasmasphere ions. The
spots are therefore a consequence of plasmasphere refilling after geomagnetic
storms. In contrast, the afternoon arcs are the result of enhanced EMIC waves
during quiet geomagnetic conditions.

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Presented at the 28th URSI General Assembly, New Delhi, India, 
23 - 29 October, 2005