Dynasonde observations of ionospheric modification experiments with the HIPAS observatory

This paper presents experimental results derived from NOAA HF radar (dynaso nde) measurements, done during the 1992 ionospheric modification campaign ( Campaign'92) conducted at the UCLA HIPAS facility near Fairbanks, AK. Durin g this campaign, the HIPAS transmitter illuminated the ionosphere with high frequency electromagnetic waves at 2.85 and 4.53 MHz using a peak power of 1 MW, an antenna gain of 18.4 dBi, and a half-power beamwidth of 22 degree s. As a tool for ionospheric diagnostics, the dynasonde utilizes interferom etry techniques to provide high-resolution echolocation, line-of-sight Dopp ler velocity, and digital ionograms as fundamental data products. Here we p resent results, which suggest that high-power, ordinary mode pump waves tra nsmitted during "over-dense" conditions can generate small-scale field-alig ned irregularities and anomalous absorption, affecting diagnostic radio wav es having both ordinary and extraordinary polarization. Furthermore, large- scale plasma density changes of similar to 30% were observed when the 2.85 MHz heater frequency matched that of the ionospheric critical plasma freque ncy. (C) 2001 Elsevier Science Ltd. All rights reserved.