Seasonal and solar cycle variations of thermally excited 630.0 nm emissions in the polar ionosphere

Abstract

Solar cycle and seasonal variations have been found in the occurrence of strong thermally excited 630.0 nm emissions in the polar ionosphere. Measurements from the European Incoherent Scatter Svalbard Radar have been used to derive the thermal emission intensity. Thermally excited emissions have been found to maximize at solar maximum with peak occurrence rate of ∼40% compared to ∼2% at solar minimum. These emissions also have the highest occurrence in equinox and the lowest occurrence rate in summer and winter. There is an equinoctial asymmetry in the occurrence rate which reverses with the solar cycle. This equinoctial asymmetry is attributed to variations of the solar wind‐magnetosphere coupling arising from the Russell‐McPherron effect. The occurrence rate of thermal excitation emission on the dayside, at Svalbard, has been found to be higher in autumn than spring at solar maximum and the reverse at solar minimum. Enhanced electron temperatures characterize the strong thermal component for solar minimum and winter, whereas enhanced electron densities characterize the thermal component for solar maximum. The results point to solar wind‐magnetosphere‐ionosphere coupling as the dominant controlling process.