[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.

Abstract. This letter reports for the first time the simulated error distribution of radio occultation (RO) electron density profiles (EDPs) from the Abel inversion in a systematic way. Occultation events observed by the COSMIC satellites are simulated during the spring equinox of 2008 by calculating the integrated total electron content (TEC) along the COS-MIC occultation paths with the "true" electron density from an empirical model. The retrieval errors are computed by comparing the retrieved EDPs with the "true" EDPs. The results show that the retrieved NmF2 and hmF2 are generally in good agreement with the true values, but the reliability of the retrieved electron density degrades in low latitude regions and at low altitudes. Specifically, the Abel retrieval method overestimates electron density to the north and south of the crests of the equatorial ionization anomaly (EIA), and introduces artificial plasma caves underneath the EIA crests. At lower altitudes (E-and F1-regions), it results in three pseudo peaks in daytime electron densities along the magnetic latitude and a pseudo trough in nighttime equatorial electron densities.

We report discovery of a solar‐terrestrial connection between rotating solar coronal holes and density variations in Earth's thermosphere. Specifically, during 2005, a 9‐day recurrence of fast streams in the solar wind exists due to solar coronal holes distributed roughly 120 degrees apart in longitude; this periodicity is transmitted to the geospace environment where it modulates geomagnetic activity and thermospheric densities derived from accelerometer measurements on the CHAMP satellite. Our discovery demonstrates a solar‐terrestrial connection that has not been appreciated before, and by its nature is characterized by an element of predictability. Its potential predictability has practical relevance for collision avoidance and other applications affected by density variability in the terrestrial space environment.

[1] We have investigated the thermospheric and ionospheric response to the 14-15 December 2006 geomagnetic storm using a Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model simulation. In this paper we focus on observations and simulations during the initial phase of the storm (about 8 h), when the shock was driving changes in geospace. The global ionospheric maps of total electron content (TEC), ionosonde data at four stations and Millstone Hill incoherent scatter radar (ISR) observations are compared with the corresponding simulation results from the CMIT model. The observations showed significant positive storm effects occurred in the Atlantic sector after the onset of this storm. The CMIT model is able to capture the temporal and spatial variations of the ionospheric storm effects seen in the GPS TEC observations, although the model slightly underestimates the daytime positive ionospheric storm in the South American sector. The simulations are also in agreement with the ionosonde and ISR ionospheric measurements. Term analysis of the ion continuity equation demonstrates that changes in the electric fields play a dominant role in generating the observed ionospheric positive storm effect in the American sector during the initial phase, although neutral winds and composition changes also contribute. The difference in the strength of the enhancements over North and South America can be explained by the slope of the topside electron density profiles in the two hemispheres. In the southern hemisphere electron densities decrease slowly with altitude, whereas the decrease is much more rapid in the northern (winter) hemisphere. The electric fields, therefore, cannot cause large increases in electron density by uplifting the plasma, so positive storm effects are small in the southern hemisphere compared with the northern hemisphere, even though the increase in h m F 2 is greater in the southern hemisphere. Nighttime changes in electron density in other longitude sectors are small, because the topside electron densities also decrease slowly with altitude at night.