Aspect angle dependence of HF enhanced incoherent backscatter

An HF ionospheric interaction experiment was performed in November and December of 1997 using the EISCAT HF transmitter and 931 and 224 MHz incoherent scatter radars, all co-located near Tromso, Norway. During this experiment the pointing of the UHF radar was varied in a predetermined and repeating cycle between elevation angles of 90 and 77.2 degrees south, that is, between vertical and geomagnetic field aligned. The HF transmitter duty cycle was intentionally kept to the relatively low value of 2% (200 ms every 10 s) in order to minimize the effects of ionospheric irregularities. Here we report on variations in the intensity of the enhanced incoherent scatter ion and plasma Lines observed during the experiment. Bottomside and topside F region enhanced Lines were seen with both radars, and while intensity enhancements observed with the UHF radar were clearly correlated with pointing angles between the Spitze angle and field aligned, no correlation between the intensity of the Lines observed with the scanning UHF radar and the vertically pointing VHF radar was observed. Consistent with HF propagation theory, the field aligned backscatter observed by the UHF radar originated several kilometers below the HF reflection height. (C) 1999 COSPAR. Published by Elsevier Science Ltd.