Explaining solar cycle effects on composition as it relates to the winter anomaly

Abstract

AbstractThe solar cycle variation of F2 region winter anomaly is related to solar cycle changes in the latitudinal winter‐to‐summer difference of O/N2. Here we use the National Center for Atmospheric Research–Global Mean Model to develop a concept of why the latitudinal winter‐to‐summer difference of O/N2 varies with solar cycle. The main driver for these seasonal changes in composition is vertical advection, which is expressed most simply in pressure coordinates. Meridional winds do not change over the solar cycle, so the vertical winds should also not change. The other component of vertical advection is the vertical gradient of composition. Is there any reason that this should change? At solar maximum vertical temperature gradients between 100 and 200 km altitude are strong, whereas they are weak at solar minimum. To maintain the same pressure, the weak vertical temperature gradients at solar minimum must be balanced by weak density gradients and the strong temperature gradients at solar maximum must be balanced by strong density gradients to obtain the same pressure profile. Changes in the vertical density gradients are species dependent: heavy species change more and light species change less than the average density change. Hence, vertical winds act on stronger O/N2 gradients at solar maximum than they do at solar minimum, and a stronger winter‐to‐summer difference of O/N2 occurs at solar maximum compared with solar minimum.