Abstract
Abstract. Observations of polar mesosphere summer echoes (PMSE) have been carried out during the summer periodes 1999–2001 and 2003–2004 at the very high latitude of 78° N using the SOUSY Svalbard Radar (53.5 MHz) at Longyearbyen. Although the measurements could not be done continuously in these seasons, PMSE have been detected over more than 6600 h of 9300 h of observation time overall. Using this data base, particular PMSE occurrence characteristics have been determined. PMSE at Svalbard appear from the middle of May to the end of August with an almost permanent total occurrence in June and July. Diurnal variations are observable in the height-depend occurrence rates and in PMSE thickness, they show a maximum around 09:00–10:00 UTC and a minimum around 21:00–22:00 UTC. PMSE occur nearly exclusively between a height of 80 km and 92 km with a maximum near 85 km. However, PMSE appear not simultaneously over the entire height range, the mean vertical PMSE extension is around 4–6 km in June and July. Furthermore, typically PMSE are separated into several layers, and only 30% of all PMSE are single layers. The probability of multiple layers is greater in June and July than at the beginning and the end of the PMSE season and shows a marked 5-day-variation. The same variation is noticeable in the seasonal dependence of the PMSE occurrence and the PMSE thickness. We finally discuss potential geophysical processes to explain our observational results.
Highlights
Strong mesospheric VHF radar echoes are observed during summer months at polar latitudes and with a lower occurrence rate at middle latitudes as well
They are called polar mesosphere summer echoes (PMSE) and mesosphere summer echoes (MSE), respectively. The phenomenon of these radar echoes is observed more or less sporadically for nearly three decades since the first detections in mid latitudes (Czechowsky et al, 1979) and polar latitudes (Ecklund and Balsley, 1981). The latter authors first pointed out their particular traits, which discriminate them from standard VHF radar echoes resulting from backscatter by ionisation irregularities created by neutral turbulence in the mesosphere
In the height-time image plot in Fig. 1 we present the signalto-noise ratio (SNR) of a 24 h period on 20 July 2001 as a typical observation for this location
Summary
Strong mesospheric VHF radar echoes are observed during summer months at polar latitudes and with a lower occurrence rate at middle latitudes as well. The phenomenon of these radar echoes is observed more or less sporadically for nearly three decades since the first detections in mid latitudes (Czechowsky et al, 1979) and polar latitudes (Ecklund and Balsley, 1981) The latter authors first pointed out their particular traits, which discriminate them from standard VHF radar echoes resulting from backscatter by ionisation irregularities created by neutral turbulence in the mesosphere. The ionisation irregularity destruction is prevented when the diffusion of electrons is reduced in the presence of heavy aerosol or ice particles This process shifts the turbulence-driven spectrum of the ionisation irregularities to markedly shorter scales than those observed in the neutral turbulence (Kelley et al, 1987; Cho et al, 1992; Rapp and Lubken, 2003) and results in the strong PMSE backscatter
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