In-situ observations of the "preexisting auroral arc" by THEMIS all-sky imagers and the FAST spacecraft

Details In-situ observations of the "preexisting auroral arc" by THEMIS all-sky imagers and the FAST spacecraft In-situ observations of the "preexisting auroral arc" by THEMIS all-sky imagers and the FAST spacecraft

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm33a2125j&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SM33A-2125

Physics

[2704] Magnetospheric Physics / Auroral Phenomena, [2721] Magnetospheric Physics / Field-Aligned Currents And Current Systems, [2790] Magnetospheric Physics / Substorms

Scientific paper

Auroral substorms were first described more than 50 years ago, and their atmospheric and magnetospheric signatures have been thoroughly examined. However, because magnetic mapping from the ionosphere to the equator is highly uncertain, especially during active times, there remains considerable uncertainty regarding the magnetospheric source regions of the features that are observed in the upper atmosphere. In optical images, auroral substorms are observed as brightening and poleward expansion of a preexisting discrete auroral arc. The preexisting arc is usually the most equatorward of several auroral arcs that remain quiescent for a period of time (~10 min to ~ 60 min) prior to the onset of brightening. In the analysis reported here, we identify the magnetospheric source of the preexisting arc by combining auroral images with low altitude spacecraft data and maps of field-aligned currents. We surveyed data from the THEMIS all sky imagers (ASI) array and the FAST spacecraft from 11/2007 to 03/2008 and 11/2008 to 03/2009 and obtained 5 events in which the low altitude FAST spacecraft crossed magnetic flux tubes linked to a preexisting auroral arc imaged by THEMIS ASI. The observations show that: 1) the preexisting arc is embedded within the poleward portion of energetic ion precipitation, 2) the electrons exciting the preexisting arc have an inverted-V structure in energy vs. time spectrum, with characteristic energy ranging from a few hundred eV to a few keV, 3) the pre-existing arc is located at the boundary between the Region 1 and Region 2 field-aligned currents. The localization is confirmed by comparison with maps of field-aligned currents inferred from ground magnetometer records. We have also found that the preexisting arc maps to equatorial locations within the electron plasma sheet and tailward (a few RE) of its inner edge.

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