Proton Aurora in the Cusp

H.U. Frey, S.B. Mende, T.J. Immel, S.A. Fuselier, E.S. Clafin, J.-C. Gerard, 
and B. Huber

Frequently the FUV instrument on the IMAGE spacecraft observes intense 
ultraviolet emission from a localized dayside region poleward of the general 
auroral oval location.  This emission is especially distinct in the Doppler 
shifted emission of hydrogen atoms produced by precipitating protons.  We 
interpret this as a direct signature of proton precipitation into the cusp
after reconnection of magnetospheric lobe field lines.  This cusp signature
appears only when the interplanetary magnetic field (IMF) has a positive
northward Bz component.  However, the intensity of the precipitation and
hence the intensity of UV emission is not controlled by the magnitude of Bz
but rather by the solar wind dynamic pressure.  A statistical analysis of 18
cases observed in summer and fall 2000 shows good correlation between the UV
intensity and the dynamic pressure, and between the location in local time 
and the IMF By component.  A quantitative analysis of observations from all 
3 FUV sub-instruments allows for an estimate of proton and electron energy 
fluxes during these times.  In general, these estimates agree with results 
from in-situ measurements by spacecraft and show, that during these times 
protons may contribute significantly to the overall energy deposition into 
the cusp.

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Accepted for publication, Journal of Geophysical Research, 2002