Conditions Governing High-Latitude Dayside Aurora During IMF B_y>0

H. Korth, B.J. Anderson, T.J. Immel, H.U. Frey, S.B. Mende


In a recent study of IMAGE FUV data, Frey et al. (2003) examined cases of
high-latitude dayside aurora caused by precipitating electrons predominantly
during northward IMF with a strongly positive IMF By component. Analysis of FAST
data showed that the auroral spots coincide with regions of localized upward
field-aligned currents (FACs). In order to determine the conditions that control
these auroral emissions, we examined field-aligned currents observed by the
Iridium constellation for 25~events of prolonged steady IMF orientation with
clock angles between +60 degrees and +90 degrees. We find that localized FACs
are observed under all solar wind conditions but that the occurrence of high
latitude aurora are restricted to solar wind proton densities below 4 {cm^{-3}}
and peak current densities higher than 1.0 {muA/m^2}. These conditions were met
for the 11~events with auroral emissions while emissions are not observedduring
the remaining 14~events for which these conditions are not met. Our findings can
be qualitatively explained with the Knight relation [Knight, 1973], which states
that the field-aligned potential drop is directly proportional to the
field-aligned current density and inversely proportional to the energy density
of the precipitating population. Auroral emissions are generated, if the
field-aligned potential drop, which accelerates electrons into the ionosphere,
is sufficiently large whereas either low current density or high particle
density leads to lower potential and correspondingly weaker aurora. The results
also indicate that the particle density on field lines mapping to the upward
current region is largely governed by the solar wind density.

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Presentation, Fall Meeting, American Geophysical Union, San Francisco, USA, 
8-12 December 2003