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Ann. Geophys., 23, 75-85, 2005
www.ann-geophys.net/23/75/2005/
© European Geosciences Union 2005
Observations of time dependence and aspect sensitivity of regions of enhanced UHF backscatter associated with RF heating
R. S. Dhillon and T. R. Robinson
Department of Physics and Astronomy, University or Leicester, University Road Leicester, LE1 7RH, UK
Abstract. The EISCAT incoherent radar system, which is collocated with the EISCAT
heating facility, is used to diagnose the ionosphere while heating
experiments are conducted. In late September 2002, an experiment was
performed in which the heater transmitted a 2-min-on/2-min-off cycle
while its pointing direction was kept fixed and the UHF beam was cycled
through five pointing directions. This UHF cycle was used for three heater
beam-pointing directions. For field-aligned heater beam and UHF pointing,
UHF data indicated a gradual decrease, with time, in the altitude at which
enhanced ion-line scatter occurred. This was accompanied by a reduction in
the intensity of the scatter. For field-aligned heater pointing and the UHF
elevation angle of 6° in the field-aligned direction, a persistent
high-amplitude signature was observed, which remained at a fairly constant
altitude throughout the period that the heater remained switched on.
Different time histories of the backscatter amplitude were observed in other
UHF pointing directions, including the "ion-line overshoot", which is
characterized by an increase and subsequent decrease in the heater-enhanced
backscatter just after heater switch-on. It is suggested that these
signatures may be caused by the presence or absence of field-aligned
irregularities and reduced recombination caused by heating. The CUTLASS
coherent radar system, which operated simultaneously with the UHF radar and
the heater, observed backscatter from field-aligned irregularities created
by the heater. The intensity of this backscatter was highest from the
regions of the ionosphere that were excited by the central part of the
heater beam.
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