Annual variations of the ionosphere: A review based on MU radar observations Review Article

A review of the annual variations of the ionosphere, which focuses on the physical mechanisms causing the well-known seasonal anomaly and equinoctial asymmetry, is presented. In this review, the electron density (Ne), electron and ion temperatures (Te and Ti), and field-parallel and field-perpendicular plasma velocities (V∥ and Vt), measured by the MU radar in the 180–600 km altitude range during 1986–1994, are analysed to study the altitude dependence of the seasonal anomaly and equinoctial asymmetry. The meridional component of the thermospheric neutral wind velocity (Uθ) derived from V∥ and neutral densities obtained from MSIS-86 are used to investigate the relative importance of the chemical and dynamical processes causing the anomaly and asymmetry. The review concludes that, although the anomaly and asymmetry involve chemical and dynamical processes, the dynamical processes (mainly through the neutral wind) predominate in the asymmetry while the chemical processes predominate in the anomaly.