Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm53a..04s&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM53A-04
Physics
[2455] Ionosphere / Particle Precipitation, [2744] Magnetospheric Physics / Magnetotail
Scientific paper
The optical brightening associated with a substorm expansive phase onset is typically observed in a region slightly poleward of the peak in proton auroral luminosity. The location of proton aurora is thought to be a projection of the inner Central Plasma Sheet (CPS) where there is sufficient particle energy to cause proton auroral luminosity and still strong enough pitch angle scattering due to field line curvature and/or wave-particle interactions. This observation has been used by many to argue that the auroral brightening is driven by processes occurring in the transition region between the highly stretched magnetotail and the less stretched inner magnetosphere. Unfortunately, this topological mapping of the proton aurora has an inherent ambiguity when compared to satellite observations. It is often impossible to know where a satellite is relative to topological boundaries that we can reasonably associate with auroral features. Using FAST (ElectroStatic Analyzer) ESA ion observations and proton auroral brightnesses observed from ground-based photometers, we have derived a empirical relationship between integrated downward energy flux just above the ionosphere and the resulting proton auroral brightness. Since the precipitating ions are understood to be pitch angle scattered into the loss cone in the vicinity of the neutral sheet, it should also be possible to relate fluxes observed in the loss cone measured in the CPS to those measured in the topside ionosphere. Given our empirical relationship between the fluxes in the topside ionosphere and the proton auroral brightness, we take fluxes measured in the CPS and infer what the corresponding proton auroral intensity would be at the magnetic footprint of the satellite location. In this paper, we present the results of a statistical study of THEMIS ESA ion observations made in the night-side CPS from which we assert we can infer at least the average location of the magnetospheric region that gives rise to the proton aurora. Our initial results indicate the not surprising result that the peak in the proton aurora intensity maps to inside of 10Re. What is surprising, however, is that there are frequently ion fluxes in the loss cone tailward of 10 Re that when mapped to ionospheric altitude correspond to significant proton auroral brightnesses. These results have important implications for mapping auroral features to the magnetotail, the interpretation of substorm onset auroral evolution, and for our understanding of the source mechanism(s) of auroral arcs.
Angelopoulos Vassilis
Donovan Eric F.
Jackel Brian J.
Lui Alberto
McFadden James P.
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