Ground-based noontime D-region electron density climatology over northern Norway

Abstract. The bottom part of the earth’s ionosphere is the so-called D-region, which is typically less intense than the upper regions. Despite the comparably lower electron number density, the ionization state of the D-region has a significant influence on signal absorption for propagating lower to medium radio frequencies. We present local noon climatologies of electron number density in the middle atmosphere at high latitudes as observed by an active radar experiment. The radar measurements cover nine years from the solar maximum of cycle 24 to the beginning of cycle 25. Reliable electron densities are derived by employing signal processing, applying interferometry methods, and the Faraday International Reference Ionosphere (FIRI) model. For all years a consistent spring-autumn asymmetry of the electron number density pattern as well as a sharp decrease at the beginning of October was found. These findings are consistent with VLF studies showing equivalent signatures for nearby propagation paths. It has been suggested that the meridional circulation associated with downwelling in winter could cause enhanced electron densities through NO transport. However, this mechanism lacks to explain the reduction in electron density in early October.