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Ann. Geophys., 24, 2519-2532, 2006
www.ann-geophys.net/24/2519/2006/
© European Geosciences Union 2006
Simulations of the neutral structure within the dusk side aurora
H. F. Parish and L. R. Lyons
Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, 405, Hilgard Avenue, Los Angeles, CA 90095-1565, USA
Abstract. Observations of neutral winds from rocket release experiments within the
premidnight and postmidnight substorm recovery phase aurora, show very
large E-region neutral winds of several hundred m/s, where winds measured
on the dusk side are even larger than those on the dawn side. These large
winds are also associated with strong shears, and there is evidence that
some of the regions below these shears may be unstable. The mechanisms which
generate this strong vertical structure are not well understood.
It is also not known whether the acceleration conditions in the
pre and post midnight sectors of the aurora may produce
significantly different neutral responses on the dawn and dusk sides.
Simulations have been performed using a three-dimensional high resolution
limited area thermosphere model to try to understand the neutral
structure within the dawn and dusk side aurora. When simulations
are performed using auroral forcing alone, for equivalent conditions
within the dawn and dusk sectors, differences are found in the simulated
response on each side. When measured values of auroral forcing parameters, and
background winds and tides consistent with recent observations, are used
as model inputs, some of the main features of the zonal and meridional wind
observations are reproduced in the simulations, but the magnitude of the
peak zonal wind around 140 km tends to be too small and the maximum
meridional wind around 130 km is overestimated. The winds above 120 km
altitude are found to be sensitive to changes in electric fields and ion
densities, as was the case for the dawn side, but the effects of background
winds and tides on the magnitudes of the winds above 120 km are found to be
relatively small on the dusk side. The structure below 120 km appears to be
related mainly to background winds and tides rather than auroral forcing, as
was found in earlier studies on the dawn side, although the peak magnitudes of
simulated wind variations in the 100 to 120 km altitude range are smaller
than those observed. The source of the strong shears measured around 110 km
altitude on the dusk side is uncertain, but may be related to different
kinds of oscillations, such as gravity waves, non migrating semidiurnal
tides, or secondary oscillations produced by non linear interactions
between waves.
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