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Radioscience Seminars

EE 350 Radioscience Seminar
Professor Umran S. Inan
Winter 2003-2004

Date: Wednesday, February 18, 2004
Time: 4:15 PM – Refreshments at 4:00
Location: Bldg. 160, Room 323

The role of Alfven wave produced auroras during substorms
Dr. Stephen Mende
Space Sciences Laboratory, University of Berkeley

Simultaneous observations of IMAGE FUV and in situ FAST satellite measurements provided evidence that Alfven wave dominated auroras play significant part in substorm processes. In the FAST satellite data, Alfven wave dominated aurora can be easily distinguished from "inverted V" produced auroras (accelerated by field aligned quasi-static electric fields) by the dominance of high fluxes of low energy electrons with beam like field aligned pitch angle distribution and accompanying strong wave activity. Each FAST transit provides a snapshot of the spatial morphology of the precipitating particles, field aligned current profiles and the fluctuating E and B fields. We have examples of FAST transits with simultaneous IMAGE FUV observations during various stages of magnetospheric substorms. In one example the FAST satellite passed through an arc located westward of an expanding westward traveling surge verifying that the arc prior to break up is indeed inverted V type precipitation. In one case FAST pass occurred approximately two minutes after substorm onset showing that the most intense onset aurora was produced by Alfven wave accelerated super-thermal electrons. Within the auroral feature the low energy ion fluxes showed periodic fine structure in the ExB direction and the magnetometer also confirmed the presence of waves in this feature. Thus the intense surge aurora was not produced by a field aligned quasi-static electric field in contrast to several substorm models. The onset arc was also the poleward boundary of the intense energetic protons. Equatorward of the surge, there was a broader region of electron precipitation with embedded quasi-static "inverted V" electric fields. The Alfven wave surge aurora was located well equatorward of the open closed field line boundary showing that the surge was initiated deep in closed field line regions. In another case the FAST pass occurred later in the substorm when the surge was formed over a wide local time range. The surge consisted also of Alfven wave accelerated electrons accompanied by only weak ion precipitation. The FAST data showed that the surge was at the boundary of the closed field line region therefore at the nightside reconnection region. Other FAST passes showed that this is a typical configuration in later substorm phases. This picture is consistent with the substorm surge being produced by wave accelerated electrons denoting intense Poynting fluxes carried by kinetic Alfven waves from the magnetospheric regions where dipolarization or reconnection occurs.