Substorm onset asymmetries derived from conjugate auroral imaging

N. Ostgaard, S.B. Mende, H.U. Frey, J.B. Sigwarth, A. Aasnes


Simultanous imaging in the ultraviolet wavelengths by IMAGE and Polar, enable us
to examine auroral features in the conjugate hemispheres.With an imaging cadence of
2 min and 1 min for IMAGE/FUV and Polar VIS Earth camera, respectively, we have
examined both dynamic features as substorm onsets and cusp precipitation as well
as slowly varying phenomena as theta aurora. For substorm onset locations, we have
found that there exists a systematically displacement in one hemisphere compared to
the other. The relative displacement of onset locations in the conjugate hemispheres
is found to be controlled primarily by the IMF clock-angle. Compared with some of
the existing magnetic field models, the observed asymmetries are an order of magnitude
larger than the model predictions. These results have been compared with the
statistical distribution of substorm onsets in the southern and northern hemispheres for
different clock angles. Based on 3000 substorm onsets in the Northern hemisphere and
1000 in the southern hemisphere observed by IMAGE we find a remarkable support
for our previous findings.

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European Geosciences Union General Assembly, Vienna, Austria, 2-7 April 2006