Comparison of modulated UHF PMSE characteristics at different altitude ranges

Abstract Heating of electrons in PMSE region with powerful high frequency (HF) radio waves is called artificial electron heating. This is used as a diagnostic tool to study the interesting radar phenomenon of Polar Mesosphere Summer Echoes (PMSE). Using the PMSE observations of EISCAT UHF (930 MHz) radar from 2003 to 2015, the effect of artificial electron heating on PMSE is studied for the first time in terms of modulated PMSE characteristics. The modulated PMSE characteristics including the PMSE intensity reduction, PMSE intensity recovery, and PMSE overshoot, are analyzed and compared between different height ranges. Our results indicate variability in these characteristics at different height ranges, however, statistics reveal that unlike VHF PMSE these characteristics show a clear difference between the comparatively lower (below 84.0 km) and upper (above 84.0 km) height ranges. It is found that in below 84.0 km height ranges a greater number of heating cycles show PMSE intensity reduction, PMSE intensity recovery, and PMSE overshoot as compared to above 84.0 km height ranges. All characteristics found are in good agreement with literature especially at just after the heater-on time where no opposite behavior of PMSE intensity reduction (onset overshoot) is found, and at the heater-off time where the unheated PMSE intensity never found smaller than the heated PMSE intensity. The observed response of the PMSE to heating by a powerful radio wave is used to estimate the associated electron temperature enhancement and PMSE modulation index. PMSE modulation index is the ratio of heated PMSE to unheated PMSE, which describes that how much the heated PMSE varies around its unheated PMSE. In below 84 km height ranges, the PMSE modulation index and enhanced electron temperatures are greater than that in above 84 km height ranges. The difference in modulated properties of PMSE below and above 84 km is due to the difference in electron temperature enhancement, which depends on the electron density below the PMSE altitudes.