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Ann. Geophys., 14, 1274-1283, 1996
www.ann-geophys.net/14/1274/1996/
© European Geosciences Union 1996
Observations of nightside auroral plasma upflows in the F-region and topside ionosphere
C. Foster and M. Lester
Radio and Space Plasma Physics, Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
Abstract. Observations from the special UK EISCAT
program UFIS are presented. UFIS is a joint UHF-VHF experiment, designed to make
simultaneous measurements of enhanced vertical plasma flows in the F-region and
topside ionospheres. Three distinct intervals of upward ion flow were observed.
During the first event, upward ion fluxes in excess of 1013 m–2
s–1 were detected, with vertical ion velocities reaching 300 m s–1
at 800 km. The upflow was associated with the passage of an auroral arc through
the radar field of view. In the F-region, an enhanced and sheared convection
electric field on the leading edge of the arc resulted in heating of the ions,
whilst at higher altitudes, above the precipitation region, strongly enhanced
electron temperatures were observed; such features are commonly associated with
the generation of plasma upflows. These observations demonstrate some of the
acceleration mechanisms which can exist within the small-scale structure of an
auroral arc. A later upflow event was associated with enhanced electron
temperatures and only a moderate convection electric field, with no indication
of significantly elevated ion tem- peratures. There was again some evidence of
F-region particle precipitation at the time of the upflow, which exhibited
vertical ion velocities of similar magnitude to the earlier upflow, suggesting
that the behaviour of the electrons might be the dominant factor in this type of
event. A third upflow was detected at altitudes above the observing range of the
UHF radar, but which was evident in the VHF data from 600 km upwards. Smaller
vertical velocities were observed in this event, which was apparently
uncorrelated with any features observed at lower altitudes. Limitations imposed
by the experimental conditions inhibit the interpretation of this event,
although the upflow was again likely related to topside plasma heating.
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