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 COSMIC 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.

Ionospheric F₂ region peak densities (NmF2) are expected to have a positive correlation with total electron content (TEC), and electron densities usually show an anticorrelation with electron temperatures near the ionospheric F₂ peak. However, during the 17 March 2015 great storm, the observed TEC, NmF2, and electron temperatures of the storm‐enhanced density (SED) over Millstone Hill (42.6°N, 71.5°W, 72° dip angle) show a quiet different picture. Compared with the quiet time ionosphere, TEC, the F₂ region electron density peak height (hmF2), and electron temperatures above ~220 km increased, but NmF2 decreased significantly within the SED. This SED occurred where there was a negative ionospheric storm effect near the F₂ peak and below it, but a positive storm effect in the topside ionosphere. Thus, this SED event was a SED in TEC but not in NmF2. The very low ionospheric densities below the F₂ peak resulted in a much reduced downward heat conduction for the electrons, trapping the heat in the topside in the presence of heat source above. This, in turn, increased the topside scale height so that even though electron densities at the F₂ peak were depleted, TEC increased in the SED. The depletion in NmF2 was probably caused by an increase in the density of the molecular neutrals, resulting in enhanced recombination. In addition, the storm time topside ionospheric electron density profiles were much closer to diffusive equilibrium than the nonstorm time profiles, indicating less daytime plasma flow between the ionosphere and the plasmasphere.

Launched in 2006, the Formosa Satellite Mission 3–Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) was the first constellation of microsatellites carrying global positioning system (GPS) radio occultation (RO) receivers. Radio occultation is an active remote sensing technique that provides valuable information on the vertical variations of electron density in the ionosphere, and temperature, pressure, and water vapor in the stratosphere and troposphere. COSMIC has demonstrated the great value of RO data in ionosphere, climate, and meteorological research and operational weather forecasting. However, there are still challenges using RO data, particularly in the moist lower troposphere and upper stratosphere. A COSMIC follow-on constellation, COSMIC-2, was launched into equatorial orbit in 2019. With increased signal-to-noise ratio (SNR) from improved receivers and digital beam steering antennas, COSMIC-2 will produce at least 5,000 high-quality RO profiles daily in the tropics and subtropics. In this paper, we summarize 1) recent (since 2011 when the last review was published) contributions of COSMIC and other RO observations to weather, climate, and space weather science; 2) the remaining challenges in RO applications; and 3) potential contributions to research and operations of COSMIC-2.

The joint Taiwan‐United States FORMOSAT‐3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, hereafter called COSMIC, is the first satellite constellation dedicated to remotely sense Earth's atmosphere and ionosphere using a technique called Global Positioning System (GPS) radio occultation (RO). The occultations yield abundant information about neutral atmospheric temperature and moisture as well as space weather estimates of slant total electron content, electron density profiles, and an amplitude scintillation index, S4. With the success of COSMIC, the United States and Taiwan are moving forward with a follow‐on RO mission named FORMOSAT‐7/COSMIC‐2 (COSMIC‐2), which will ultimately place 12 satellites in orbit with two launches in 2016 and 2019. COSMIC‐2 satellites will carry an advanced Global Navigation Satellite System (GNSS) RO receiver that will track both GPS and Russian Global Navigation Satellite System signals, with capability for eventually tracking other GNSS signals from the Chinese BeiDou and European Galileo system, as well as secondary space weather payloads to measure low‐latitude plasma drifts and scintillation at multiple frequencies. COSMIC‐2 will provide 4–6 times (10–15X in the low latitudes) the number of atmospheric and ionospheric observations that were tracked with COSMIC and will also improve the quality of the observations. In this article we focus on COSMIC/COSMIC‐2 measurements of key ionospheric parameters.