Alfven wave produced auroras during substorms

S.B. Mende, H.U. Frey, C.W. Carlson


IMAGE FUV observations when the FAST satellite transits the aurora have provided
evidence that Alfven wave-dominated aurora play a significant part in substorm
aurora. In the FAST satellite data, Alfven wave dominated aurora can be
distinguished from "inverted V" produced auroras that are accelerated by field
aligned 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 BM fields. We have examples of FAST transits with simultaneous
IMAGE FUV observations during various stages of magnetospheric substorms and in
temporary auroral intensifications so called pseudo-break ups. Most often such
Alfvenic aurora is located at the poleward edge of the substorm surge arc,
however there are examples of Alfven wave produced auroras in the center of the
auroral region while surrounded by other types of aurora. One case when a FAST
pass occurred approximately one minute after substorm onset showed that the most
intense onset aurora was produced by Alfven wave accelerated super-thermal
electrons and that this aurora was located well equatorward of the open closed
field line boundary. Within the auroral feature the low energy ion fluxes showed
periodic fine structure in the ExB direction and the magnetometer measurements
also confirmed the presence of waves in this feature. In contrast to several
substorm models the intense surge was not produced by a field aligned
quasi-static electric field. 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 FAST particle and magnetometer measurements are consistent with a
field-aligned current structure in which the bulk of the upward current was
carried by the "inverted V" precipitation region. The wave accelerated electron
auroras are produced by converting the high intensity wave energy (Poynting)
fluxes into electron energy and therefore the Alfven wave accelerated auroras
are footprints of regions of the magnetosphere where magnetic reconfigurations
take place.

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