Winter anomaly in NmF2 and TEC: When and where it can occur

Abstract

For the first time, by using a regression procedure, we analyzed the solar activity dependence of the winter anomaly intensity in the ionospheric F2-layer peak electron density (NmF2) and in the Total Electron Content (TEC) on a global scale. We used the data from global ionospheric maps for 1998-2015, from GPS radio occultation observations with COSMIC, CHAMP, and GRACE satellites for 2001-2015, and ground-based ionosonde data. The fundamental features of the winter anomaly in NmF2 and in TEC (spatial distribution and solar activity dependence) are similar for these parameters. We determined the regions, where the winter anomaly may be observed in principle, and the solar activity level, at which the winter anomaly may be recorded in different sectors. A growth in geomagnetic disturbance or in the solar activity level is shown to facilitate the winter anomaly intensity increase. Longitudinal variations in the winter anomaly intensity do not conform partly to the generally accepted Rishbeth theory. We consider the obtained results in the context of spatial and solar cycle variations in O/N2 ratio and thermospheric meridional wind. Additionally, we briefly discuss different definitions of the winter anomaly.