Recent EISCAT heating results using chirped ISR

Abstract

The chirp technique has recently become fully operational on the EISCAT UHF radar system and has been used for daytime observations of the HF-modified ionosphere over Ramfjordmoen in November 1992 and March 1993. During certain periods the UHF observations show a difference in the frequencies of the photoelectron-enhanced plasma line and the HF-enhanced plasma line (HFPL) similar to the one seen during chirp observations of heating at Arecibo. The frequency difference seen at EISCAT, however, varied dramatically with time and howetimes vanished completely, which was not the case at Arecibo. This frequency difference indicates that the HFPL source region is located several kilometres above the height where the linear Langmuir dispersion equation indicates a resonance should occur at the HF pump frequency. Simultaneous long-pulse measurements of the HFPL spectrum show a cascade-type structure in the HFPL spectra which, according to current theories, indicates that the HFPL must follow the Langmuir dispersion relation. This may be interpreted to mean that the HF-induced plasma waves are excited within plasma density depletions whenever the frequency difference is present. UHF observations also sometimes show a feature in the HF-modified plasma line spectrum which appears to be the same as that observed in an experiment performed at EISCAT in August 1986 by Isham et al. (1990). This new feature has been dubbed the “outshifted” line as it appears downshifted (upshifted) from the heating frequency in the downshifted (upshifted) plasma line spectrum. The magnitude of the shift is in the range of 100–300 kHz.