A comparison of polar mesosphere summer echo at VHF (224 MHz) and UHF (930 MHz) and the effects of artificial electron heating

A polar mesosphere summer echo (PMSE) campaign with a VHF radar at 224 MHz and UHF radar at 930 MHz was carried out from 30 June to 15 July 2004 at the European Incoherent Scatter (EISCAT) radar facility near Tromsø, Norway. The EISCAT Heating facility was used to produce electron heating with the aim to investigate the effects it produces on PMSE, particularly the overshoot phenomenon. A total of 59 and 28 h of observations were gathered with the VHF and UHF radars, respectively. In this study we concentrate on the 8 d when PMSE was simultaneously looked for with both radars. The occurrence rate over the observation period was 95% for the VHF radar, which is exceptionally large, and 11% for the UHF radar, which observed PMSE on parts of 4 d, always in coincidence with VHF PMSE. On all occasions, the altitude of maximum intensity of the UHF PMSE coincided with the maximum of the VHF PMSE, although it is possible that the maximum of the UHF layer could be slightly below that of the VHF layer. The effects of the heating in reducing the PMSE strength as it was switched on, and a subsequent overshoot effect as it was switched off, were clearly present in both VHF and UHF on only 2 of the 4 d where UHF and VHF PMSE were observed simultaneously. The effects of the heating were identical on the data from both radars. The absence of heating on the other 2 d is connected to the level of electron density which controls the occurrence of the overshoot effect, the latter being stronger when the electron density is lower, in agreement with greater electron heating taking place when the electron density is smaller at the altitude of the PMSE layers and below them. Since all the properties of PMSE that we have extracted from the measurements were identical for both radar wavelengths, we conclude that there is most likely only one mechanism active in creating PMSE at all radar wavelengths.