Observations of time dependence and aspect sensitivity of regions of enhanced UHF backscatter associated with RF heating

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.