Systematic variation in observing altitude of enhanced ion line by the pump near fifth gyroharmonic

The observation of ultra high frequency radar during an ionospheric experiment carrying out at the European Incoherent Scatter Scientific Association, demonstrates a systematic variation in the altitude of the pump enhanced ion line, which is quite remarkably dependent on the pump frequency, that is, when the pump frequency sweeps above the fifth gyroharmonic, the altitude of the enhanced ion line is ∼3 to ∼6 km lower than that at the pump frequency very close to the fifth gyroharmonic. The analysis shows that the systematic variation in the altitude of the pump enhanced ion line is principally dependent on the enhanced electron temperature, although the changes in the profile of the electron density brought about by the ionospheric heating are not independent of those systematic altitude variations.