Occurrence and characteristics of high-latitude mesospheric echoes at MF: observations by Halley and Tromso dynasondes

The dynasonde at Halley, Antarctica, is operated throughout the year making ionogram soundings every five minutes. The extensive database of observations from 1993 and 1994 has been processed to investigate ionogam echoes from mesospheric heights. The occurrence of these echoes shows a very clear seasonal trend with far more echoes observed during the winter. The echoes are shown to be predominantly dependent on the absence of direct solar illumination. The characteristics of the echoes are compared with similar observations from a northern hemisphere site at Tromso, Norway. The observations from Halley and Tromso are discussed in relation to Polar Mesosphere Summer Echoes (PMSE) which are seen at higher radio frequencies above Tromso. It is found that ionogram mesospheric echoes have different occurrence statistics from PMSE and there is no statistical evidence to suggest that the echoes observed during the summer are different from those observed at other times of year. (C) 1998 Elsevier Science Ltd. All rights reserved.