A very bright SAR arc: implications for extreme magnetosphere-ionosphere couplingReport as inadecuate

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1 CSP - Center for Space Physics Boston 2 Department of Atmospheric 3 MIT Haystack Observatory

Abstract : In contrast to the polar aurora visible during geomagnetic storms, stable auroral red SAR arcs offer a sub-visual manifestation of direct magnetosphere-ionosphere M-I coupling at midlatitudes. The SAR arc emission at 6300 Å is driven by field-aligned magnetospheric energy transport from ring current-plasmapause locations into the ionosphere-thermosphere system. The first SAR arc was observed at the dawn of the space age 1956, and the typical brightness levels and occurrence patterns obtained from subsequent decades of observations appear to be consistent with the downward heat conduction theory, i.e., heated ambient F-layer electrons excite oxygen atoms to produce a spectrally pure emission. On very rare occasions, a SAR arc has been reported to be at brightness levels visible to the naked eye. Here we report on the first case of a very bright SAR arc ~13 kilo-Rayleighs observed by four diagnostic systems that sampled various aspects of the sub-auroral domain near Millstone Hill, MA, on the night of 29 October 1991: an imaging spectrograph, an all-sky camera, an incoherent scatter radar ISR, and a DMSP satellite. Simulations of emission using the ISR and DMSP data with the MSIS neutral atmosphere succeed in reproducing the brightness levels observed. This provides a robust confirmation of M-I coupling theory in its most extreme aeronomic form within the innermost magnetosphere L~2 during a rare superstorm event. The unusually high brightness value appears to be due to the rare occurrence of the heating of dense ionospheric plasma just equatorward of the trough-plasmapause location, in contrast to the more typical heating of the less dense F-layer within the trough.

Author: J. Baumgardner - J. Wroten - J. Semeter - J. Kozyra - M. Buonsanto - P. Erickson - M. Mendillo -

Source: https://hal.archives-ouvertes.fr/


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