D-Region Phenomena and Stratospheric-Mesospheric Coupling

Abstract

Coupling effects between D-region plasma and neutral gas in the mesosphere have been found from pronounced seasonal variations in ionospheric absorption and D-region phase heights, as well as from special events and anomalies in the wintertime behavior of the lower ionosphere. The paper presents examples of annual variations of ionospheric absorption and phase height at constant solar zenith angles, the results of investigations of sunrise phenomena in different seasons, and results from ionospheric drift measurements at low frequencies. A quasi semiannual variation in ionospheric absorption is obtained with a wintertime maximum, dominating at high frequencies, and a summertime maximum, dominating at low frequencies. The minima of absorption in spring and autumn indicate a low degree of ionization in the D-region, coinciding with abrupt changes in the phase of the tidal drift component in the lower ionosphere and with the well-known reversals of the general circulation in the stratosphere. The lag of summertime conditions behind the summer solstice is found to be different at different heights.

In addition to the properties of the behavior of ionospheric absorption during the winter anomaly, special coupling periods during stratospheric warming events are considered. There is now strong evidence that sudden stratospheric warmings are sometimes accompanied by pronounced ionospheric absorption anomalies, but the time shift between the two events is not always constant. There are now also some examples available demonstrating that circulation anomalies in the middle stratosphere, connected with stratospheric warmings, are related to distinct drift anomalies at the 95-km level.