Polar mesosphere winter echoes – A review of recent results

Polar mesosphere winter echoes (PMWE) are strong radar returns from heights between 50 and 80 km recorded by high-latitude VHF radars during the winter months. They are seen when ionization is increased above background levels during solar protons events or by high-energy electron precipitation from within the magnetosphere. Interest in PMWE has increased substantially in recent years since it has been suggested that the traditional explanation for their existence – turbulence in the neutral atmosphere – cannot explain their characteristics. An alternative theory has been proposed in which PMWE are caused by infrasound from the oceans being reflected at wind-shears and temperature gradients in the mesosphere. The observed characteristics of PMWE are reviewed in relation to the two theories. Although some characteristics of PMWE are consistent with either theory, other characteristics are not, particularly the spectral characteristics of the radar echoes at 224 MHz and the high horizontal travel velocity of the scatterer diffraction pattern at 52 MHz. The theory based on infrasound can qualitatively explain these characteristics but much remains to be done to test whether there is also quantitative agreement.