Polar mesosphere winter echoes during solar proton events

Abstract

Thin layers of enhanced radar echoes in the winter mesosphere have been observed by the ESRAD 52 MHz MST radar (67 deg 53 min N, 21 deg 06 min E) during several recent solar proton events. These polar mesosphere winter echoes (PMWE) can occur at any time of day or night above 70 km altitude, whereas below this height they are seen only during daytime. An energy deposition/ion-chemical model is used to calculate electron and ion densities from the observed proton fluxes. It is found that PMWE occurrence correlates well with low values of lambda (the ratio of negative ion density to electron density). There is a sharp cut-off in PMWE occurrence at lambda approximately equal to 10(exp 2), which is independent of electron density. No direct dependence of PMWE occurrence on electron density can be found within the range represented by the solar proton events, with PMWE being observed at all levels of electron density corresponding to values of lambda less than 10(exp 2). Together with results concerning the thickness, echo aspect-sensitivity and echo spectral-width of the PMWE, this observation leads to the conclusion that the layers cannot be explained by turbulence alone. A role for charged aerosols in creating PMWE is proposed.