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Abstract: In this paper, we compare the ionogram-derived total electron content (ITEC) obtained at the low latitude station Hainan (geographical coordinates 19.4°N, 109.0°E; Dip=22.8°), China with that derived from GPS signals produced by the International GPS Services for Geodynamics (TECigs) and the TEC data obtained by the altimeter on TOPEX satellite when it passed over nearby Hainan. We also compare them with the results produced by the current International Reference Ionosphere model (IRI2001) TECiri. Data used for the present comparison study are both the daily hourly and monthly median values for the 12 months of the descending solar activity year 2003. Our study showed that (1) in general ITEC is in agreement with TOPEX-TEC, although there are exceptions; (2) the total electron contents produced by all techniques (TECigs, ITEC and TECiri) can reproduce quiet well the remarkable semi-annual variation behavior of the total electron content; (3) the variation trends of ITEC and TECigs are generally in good agreement, however during nighttime to pre-sunrise hours ITEC is systematically lower than TECigs, whereas during daytime hours around local noontime ITEC is usually larger than TECigs. The absolute difference between ITEC and the TECigs (|▵TEC1|=|ITEC-TECigs|) of the monthly median data is usually under the limit of 10–15 TECu (1 TECu=1016 el/m2) during the diurnal variation, except for the time period 09–16UT(∼17–24LT) in September and October when the value of |▵TEC1| is larger than 15 TECu, fluctuating between about 15–25 TECu; (4) during nighttime hours TECiri is generally larger than TECigs, whereas during daytime hours TECiri is generally smaller than TECigs except for the months June and July when TECiri is slightly larger than TECigs; (5) the difference between ITEC and TECigs (ΔTEC1) and that between TECiri and TECigs (ΔTEC2) are almost reverse in their positive/negative signs at a given local time, however, the magnitudes of the maximum absolute values of |▵TEC1| and |▵TEC2| are comparable. [Copyright &y& Elsevier]

The thickness parameters that most empirical models use are generally defined by empirical relations related to ionogram characteristics. This is the case with the NeQuick model that uses an inflection point below the F2 layer peak to define a thickness parameter of the F2 bottomside of the electron density profile, which is named B2. This study is focused on the effects of geomagnetic storms on the thickness parameter B2. We selected three equinoctial storms, namely 17 March 2013, 2 October 2013 and 17 March 2015. To investigate the behavior of the B2 parameter before, during and after those events, we have analyzed variations of GNSS derived vertical TEC (VTEC) and maximum electron density (NmF2) obtained from manually scaled ionograms over 20 stations at middle and low latitudes of Asian, Euro-African and American longitude sectors. The results show two main kinds of responses after the onset of the geomagnetic events: a peak of B2 parameter prior to the increase in VTEC and NmF2 (in ~60% of the cases) and a fluctuation in B2 associated with a decrease in VTEC and NmF2 (~25% of the cases). The behavior observed has been related to the dominant factor acting after the CME shocks associated with positive and negative storm effects. Investigation into the time delay of the different measurements according to location showed that B2 reacts before NmF2 and VTEC after the onset of the storms in all the cases. The sensitivity shown by B2 during the studied storms might indicate that experimentally derived thickness parameter B2 could be incorporated into the empirical models such as NeQuick in order to adapt them to storm situations that represent extreme cases of ionospheric weather-like conditions. [ABSTRACT FROM AUTHOR]