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Global ionospheric total electron content (TEC) is generally derived with ground-based Global Navigation Satellite System (GNSS) observations based on mathematical models in a solar-geomagnetic reference frame. However, ground-based observations are not well-distributed. There is a lack of observations over sparsely populated areas and vast oceans, where the accuracy of TEC derivation is reduced. Additionally, the modified dip (modip) latitude is more suitable than geomagnetic latitude for the ionosphere. This paper investigates the improvement of global TEC with multi-source data and modip latitude, and a simulation with International Reference Ionosphere (IRI) model is developed. Compared with using ground-based observations in geomagnetic latitude, the mean improvement was about 10.88 % after the addition of space-based observations and the adoption of modip latitude. Nevertheless, the data from JASON-2 satellite altimetry and COSMIC occultation are sparsely-sampled, which makes the IRI TEC a reasonable estimation for the areas without observation. By using multi-source data from ground-based, satellite-based and IRI-produced observations, global TEC was derived in both geomagnetic and modip latitudes for 12 days of four seasons in 2014 under geomagnetic quiet conditions. The average root-mean-square error (RMSE) of the fitting was reduced by 7.02 % in modip latitude. The improvement was largest in March and smallest in June. [ABSTRACT FROM AUTHOR]

Ground- and space-based Global Navigation Satellite System (GNSS) receivers can provide three-dimensional (3D) information about the occurrence of equatorial plasma bubbles (EPBs). For this study, we selected March 2014 data (during solar maximum of cycle 24) for the analysis. The timing and the latitudinal dependence of the EPBs occurrence rate are derived by means of the rate of the total electron content (TEC) index (ROTI) data from GNSS receivers in China, whereas vertical profiles of the scintillation index S4 are provided by COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate). The GNSS receivers of the low Earth orbit satellites give information about the occurrence of amplitude scintillations in limb sounding geometry where the focus is on magnetic latitudes from 20° S to 20° N. The occurrence rates of the observed EPB-induced scintillations are generally smaller than those of the EPB-induced ROTI variations. The timing and the latitude dependence of the EPBs occurrence rate agree between the ground-based and spaceborne GNSS data. We find that EPBs occur at 19:00 LT and they are mainly situated above the F2 peak layer which descended from 450 km at 20:00 LT to 300 km at 24:00 LT in the equatorial ionosphere. At the same time, the spaceborne GNSS data also show, for the first time, a high occurrence rate of post-sunset scintillations at 100 km altitude, indicating the coexistence of equatorial sporadic E with EPBs. [ABSTRACT FROM AUTHOR]

A comparison between GPS derived total electron content (TEC) and IRI-2012 is studied at low latitudes in China in 2006. 4 GPS receivers form a small scale GPS network, and the spatial coverage starts at (26.1°N, 119.3°E) and ends at (22.8°N, 108.3°E), the corresponding magnetic coordinate is from (19.6°N, 191.6°E) to (16.2°N, 180.4°E). The network covers a region at or near the northern crest of the equatorial anomaly with ∼11° and ∼3° in longitude and latitude, respectively. The GPS-TEC is computed with the grid-based method. The ABT-2009 and NeQuick are selected for the bottomside and topside profile model for IRI-2012. There are similar monthly and seasonal variation trends between GPS-TEC and IRI-TEC in 2006, while the peak value of GPS-TEC and IRI-TEC appear at different times. The IRI-TEC is almost larger than GPS-TEC at day time, while the difference is small at night time. The largest difference between GPS-TEC and IRI-TEC can reach 16.5 TECU, and the ratio of the largest difference and GPS-TEC is about 65%. The largest latitudinal gradient of GPS-TEC at 22–24°N was 5 TECU/deg, while that of IRI-TEC was 2.5 TECU/deg. These above results indicate that the IRI-2012 can reproduce the TEC variation trends at low latitudes in China, but the IRI-2012 overestimates the GPS-TEC at day time in this region. [ABSTRACT FROM AUTHOR]

The latest version of the International Reference Ionosphere (IRI-2016) is evaluated for ionospheric total electron content (TEC) over China during a high solar activity phase of 2014. TEC is derived from observations by the GNSS receiver network of Crustal Movement Observation Network of China (CMONOC). IRI-2016 uses the NeQuick option for the topside electron density profile. The comparisons shows that IRI-2016 underestimates GPS-TEC slightly in spring and winter, but overestimates slightly in summer and autumn at the mid-latitudes of 35oN-45oN. The deviation of TEC estimation is about 5 TECU at 35oN-45oN. As a whole, IRI-2016 underestimates GPS-TEC at the lower latitudes of 15oN-35oN, and the deviations of TEC estimation are about 4 TECU, 10 TECU, 26 TECU and 33 TECU in summer, autumn, winter and spring, respectively. The largest difference between IRI-TEC and GPS-TEC is about 50 TECU at the northern crest of equatorial ionization anomaly (EIA) in spring. There is an obvious winter anomaly for GPS-TEC, while it is not predicted by IRI-2016. Due to the influence of nighttime enhancement of ionosphere, the extreme latitudinal gradient of GPS-TEC appears after sunset in spring and winter, while it is not shown on that of IRI-TEC. Because of the equatorial fountain effect, the maximum time rate of GPS-TEC appears at the beginning of EIA (∼10:00–11:00 LT) in equinoxes, while it is not shown on that of IRI-TEC. The most rapid decrease of IRI-TEC appears after sunset (∼19:00 LT) in spring and winter, while GPS-TEC quickly decreases at ∼22:00 LT. • There is an obvious winter anomaly for GPS-TEC in 2014, while it is not for IRI-2016. • IRI-2016 has a good description of the ionosphere at mid-latitudes of 35oÑ45oN, where the RMS of ΔTEC is about 5 TECU. • For the ionosphere of 15oÑ35oN, IRI-2016 estimates GPS-TEC inaccurately in spring and winter, and the RMS of ΔTEC can be large as 33 TECU and 26 TECU, respectively. • The description of nighttime enhancement of ionosphere by IRI-2016 is inaccurate in 2014. [ABSTRACT FROM AUTHOR]