Composition and the Winter Anomaly

The winter anomaly in F-region electron density has long been tied to changes in neutral composition between summer and winter. In practice, the appearance of the winter anomaly depends primarily on the balance between the effect of the solar zenith angle on ionization and the neutral composition in the winter hemisphere. Although the ionization rate changes over a solar cycle, this has little bearing on the winter anomaly as the latter depends on the relative difference in the ionization rate between the winter and summer hemispheres. The variations of this balance between the zenith angle and composition with solar cycle are described here. This relationship also produces the quasi-equinoctial peaks in NmF2. The possibility that these changes in neutral composition may affect the annual anomaly will also be considered as will the vertical variation of composition. Finally, some implications of these changes in the winter anomaly over a solar cycle are explored.