Polar Mesosphere Winter Echoes – height dependence of spectral widths at 224 MHz and 52 MHz and the infrasound theory

Kirkwood, S., Belova, E. and Nilsson, H.

Swedish Institute of Space Physics, Kiruna, Sweden

Thin layers of strongly enhanced radar echo power are often seen from mesospheric heights. At high latitudes, in summer, these are the well known Polar Mesosphere Summer Echoes, PMSE, at ~80-90 km height. Slightly weaker scattering layers also appear at high latitudes in winter, whenever the electron density is enhanced above the quiet level by energetic particle precipitation. These are Polar Mesosphere Winter Echoes, PMWE. When such winter echoes were first observed (in the 1980’s) they were assumed to be caused by thin layers of turbulence in the neutral atmosphere. Recent studies have found two characteristics of PMWE which are incompatible with this explanation. The first is that the scatterering features at 52 MHz move horizontally at very high speeds of several 100s of m/s. The second is that the spectral width of the echoes at 224 MHz is just the same inside the PMWE as it is from the adjacent background plasma. This has led to a proposed explanation of PMWE as scatter by enhanced ion-acoustic waves caused by partial reflection of infrasound at windshears or temperature gradients. This presentation will focus on spectral widths. We examine the height dependence of PMWE spectral widths at 52 MHz (ESRAD, Kiruna) and 224 MHz (EISCAT VHF, Tromsø) during the solar proton event 10-13 November 2004 and compare to theoretical expectations.