[1] Electron densities retrieved from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements are compared with those measured by incoherent scatter radars (ISR) and ionosondes in this paper. These results show that electron density profiles retrieved from COSMIC RO data are in agreement with the ISR and ionosonde measurements. The ionospheric characteristics (N m F 2 and h m F 2 ) derived from the COSMIC satellites are also compared with those calculated by the latest International Reference Ionosphere model (IRI-2001) and the National Center for Atmospheric Research Thermosphere-IonosphereElectrodynamics General Circulation Model (NCAR-TIEGCM). The comparison of the magnitude of the COSMIC N m F 2 data with those calculated by the IRI model and the TIEGCM is good. However, features such as the north-south asymmetry and longitudinal variation of the equatorial anomaly that are seen in the COSMIC data and the TIEGCM simulations are not fully present in the IRI model. On the other hand, the TIEGCM produces a stronger winter anomaly than that seen in either the COSMIC data or the IRI model.

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This paper presents monthly variations of the midlatitude summer nighttime anomaly (MSNA) of the ionosphere for the first time by global observations of the FORMOSAT‐3/COSMIC (F3/C) mission. The anomaly is characterized by the greater nighttime (1800 LT ∼ 0200 LT) ionospheric electron density than during daytime (0800 ∼ 1800 LT) at middle latitudes during months around June and December solstices. The anomaly shown during December solstice was known as the Weddell Sea Anomaly (WSA) occurring around the Antarctica and nearby the Pacific Ocean. This paper demonstrates that the WSA‐like feature also exists in the Northern Hemisphere and is most prominent near the Northeast Asia, Europe/Africa, and Central Pacific longitudes around June solstice. In both hemispheres, the anomalies with similar electron density characteristics and variations caused by the similar mechanism prompts us to name this phenomenon the MSNA. The monthly F3/C observations indicate that the anomaly appears as the most prominent structure of the global ionosphere around midnight hours.