Finland HF and Esrange MST radar observations of polar mesosphere summer echoes

Peculiar near range echoes observed in summer with the SuperDARN HF radar in Finland are presented. The echoes were detected at four frequencies of 9, 11, 13 and 15 MHz at slant ranges of 105-250 km for about 100 min. Interferometer measurements indicate that the echoes are returned from 80-100 km altitudes with elevation angles of 20°-60°. Echo power (≤ 16 dB), Doppler velocity (between -30 and + 30 m s^-1) and spectral width (≤ 60 m s^-1) fluctuate with periods of several to 20 min, perhaps due to short-period atmospheric gravity waves. When the HF radar detected the echoes, a vertical incidence MST radar, located at Esrange in Sweden (650 km north of the HF radar site), observed polar mesosphere summer echoes (PMSE) at altitudes of 80-90 km. This fact suggests that the near range HF echoes are PMSE at HF band, although both radars did not probe a common volume. With increasing radar frequency, HF echo ranges are closer to the radar site and echo power becomes weaker. Possible mechanisms to explain these features are discussed.