The winter anomaly in the middle-latitude<i>F</i>region during the solar minimum period observed by the Constellation Observing System for Meteorology, Ionosphere, and Climate

Abstract

[1] The winter anomaly (or seasonal anomaly) at middle latitudes is a phenomenon during which the daytime plasma density at the F-peak height (NmF2) is greater in winter than in summer. Radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites provide a new data source for study of the winter anomaly on a global scale. In this study we investigate the altitude, local time, latitude, longitude, and hemispheric variations of the electron density in the middle-latitude ionosphere by analyzing the COSMIC data measured in 2007 during a magnetically quiet period (Kp ≤ 3). The seasonal mean behavior of the NmF2 obtained from COSMIC data shows the occurrence of the winter anomaly feature during 0800–1600 LT in the Northern Hemisphere but not in the Southern Hemisphere. The intensity of the winter anomaly is variable with longitude, and a more intense winter anomaly is likely to occur at longitudes closer to the magnetic pole. At northern middle latitudes, a greater electron density in the winter than in the summer occurs in the narrow altitude range near the F-peak height. Except for the winter anomaly feature at northern middle latitudes, the electron density at middle latitudes is greater during the summer than during the winter in both hemispheres.