Observation of VHF incoherent scatter spectra disturbed by HF heating

We report on EISCAT ionospheric heating experiments carried out on 13 September 2010 at Tromsø, Norway. This paper focuses on the long-lasting enhanced ion- and plasma-lines observed by the VHF incoherent scatter radar. In previous experiments, the enhanced ion-lines observed by VHF radar lasted ~200 ms. Observations of long-lasting enhanced ion- and plasma-lines in the VHF radar have not been reported before. The enhanced lines may be caused by Parametric Decay Instability (PDI). In addition, the VHF data show that the enhanced ion-lines are descending in altitude with time during the heater-on period. Likewise, the temporal and spatial evolution of the enhanced plasma-lines also show a similar morphology and generally occur at the same height. At last a possible mechanism for the altitude evolution is given in this paper.