Polar mesosphere summer echoes observed with the EISCAT 933‐MHz radar and the CUPRI 46.9‐MHz radar, their similarity to 224‐MHz radar echoes, and their relation to turbulence and electron density profiles

Abstract

During a campaign to study polar mesosphere summer echoes (PMSE) the European Incoherent Scatter (EISCAT) UHF and VHF radars and the Cornell University portable radar interferometer (CUPRI) VHF radar were operated in Tromsö in the summer of 1988. Also, for the first time the EISCAT UHF radar detected coherent echoes from the mesosphere. Their relation to the echoes recorded simultaneously with the CUPRI radar is studied, and it is shown that these 933‐MHz echoes are of the same nature as the VHF radar polar mesosphere summer echoes. Comparisons are made of the CUPRI PMSE with the electron density profile, measured simultaneously with the EISCAT 933‐MHz radar. A narrow electron density depletion seen in the EISCAT data appears to occur at the same altitude as the CUPRI PMSE and is consistent with earlier rocket measurements. It appears from the turbulence energy dissipation rates, which we estimate from these radar measurements, that the electron gradients related to these depletions have to be considerably steep to explain the large radar reflectivities. Doppler spectra of PMSE observed with the EISCAT UHF radar are much narrower than the incoherent scatter spectra and are used to obtain an independent estimate of the turbulence energy dissipation rates. Some possible origins of the scattering process of the polar mesosphere summer echoes observed at VHF and UHF are outlined, such as enhanced electron density fluctuations as well as steep electron density gradients in the presence of cluster ions in the cold polar mesosphere. We believe that it is impossible to explain the large radar reflectivities at both 46.9 and 933 MHz with a single turbulence mixing mechanism.