The winter anomaly (or seasonal anomaly) at middle latitudes is a phenomenon during which the daytime plasma density at the F-peak height (NmF(2)) 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 NmF(2) 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.
