[1] On the basis of ionospheric total electron content (TEC) enhancement over the subsolar region during flares, and combined with data of the peak X‐ray flux in the 0.1–0.8 nm region, EUV increase in the 0.1–50 and 26–34 nm regions observed by the SOHO Solar EUV Monitor EUV detector, also with the flare location on the solar disc, the relationship among these parameters is analyzed statistically. Results show that the correlation between ionospheric TEC enhancement and the soft X‐ray peak flux in the 0.1–0.8 nm region is poor, and the flare location on the solar disc is one noticeable factor for the impact strength of the ionospheric TEC during solar flares. Statistics indicate clearly that, at the same X‐ray class, the flares near the solar disc center have much larger effects on the ionospheric TEC than those near the solar limb region. For the EUV band, although TEC enhancements and EUV flux increases in both the 0.1–50 and 26–34 nm regions have a positive relation, the flux increase in the 26–34 nm region during flares is more correlative with TEC enhancements. Considering the possible connection between the flare location on the solar disc and the solar atmospheric absorption to the EUV irradiation, an Earth zenith angle is introduced, and an empirical formula describing the relationship of TEC enhancement and traditional flare parameters, including flare X‐ray peak and flare location information, is given. In addition, the X‐ray class of the flare occurring on 4 November 2003, which led the saturation of the X‐ray detector on GOES 12, is estimated using this empirical formula, and the estimated class is X44.

Using the GPS data from as many as 114 GPS stations of the International GPS Service for Geodynamics (IGS), the morphological features of the ionospheric total electron content (TEC) variations on the sunlit hemisphere during the 4B solar flare on 28 October 2003 is studied. It is found that the strongest sudden increase of TEC (SITEC) happened during the flare, and the magnitudes of SITEC vary at regions with different local solar zenith angle (SZA). In the northern hemisphere, the TEC enhancement is approximately symmetrical to the local noon, and its value is usually greater than 14 TECU (1 TECU = 10¹⁶/m²) if the SZA is less than 60°. On the whole, as the SZA increases, the value of TEC enhancement in the northern hemisphere decreases. It is worth mentioning that even in the regions of SZA between 90° and 100°, the SITEC was still seen from the temporal TEC curves. Using a photochemical model, the electron production rate over the sunlit boundary region is calculated and some obvious features of SITEC over this region are analyzed. In the polar region, the effect of this flare on the ionosphere exceeds the effect of the ionospheric scintillations and it seems that the ionosphere in the northern polar region responses more sensitively to this flare. In the end, superimposed on the curves of the rate of TEC change, there are some small disturbances (spikes) synchronously appearing on all curves and thus indicating an existence of similar structures in the EUV band of the flare.

In situ electron density measurements by the CHAllenging Minisatellite Payload and the Defense Meteorological Satellites Program F17 satellites show that the midlatitude ionization at altitudes of ∼350 and 850 km is enhanced in the late evening. The enhancements increase to maximum around midnight and are clearly observed till early morning as the equatorial ionization decays to minimal level. They appear in the winter hemisphere during June and December solstices and in both hemispheres during equinox. The enhancements are well confined between ±30° and ±50° magnetic latitude, with the magnetic flux tubes of L = 1.3 − 2.4 connecting to the plasmasphere. Furthermore, coincident longitudinal variations exist in both the ionospheric enhancements and the plasmaspheric total electron content, especially during the solstice months. The coincidence may suggest essential plasma transport between the ionosphere and the plasmasphere. These facts support the idea that the plasmasphere provides extra plasma to the midlatitude ionosphere through downward plasma influx along the magnetic field lines to form the nighttime ionization enhancements when the sunlight is absent.

An algorithm has been developed to derive the ionospheric total electron content (TEC) and to estimate the resulting instrumental biases in Global Positioning System (GPS) data from measurements made with a single receiver. The algorithm assumes that the TEC is identical at any point within a mesh and that the GPS instrumental biases do not vary within a day. We present some results obtained using the algorithm and a study of the characteristics of the instrumental biases during active geomagnetic periods. The deviations of the TEC during an ionospheric storm (induced by a geomagnetic storm), compared to the quiet ionosphere, typically result in severe fluctuations in the derived GPS instrumental biases. Based on the analysis of three ionospheric storm events, we conclude that different kinds of ionospheric storms have differing influences on the measured biases of GPS satellites and receivers. We find that the duration of severe ionospheric storms is the critical factor that adversely impacts the estimation of GPS instrumental biases. Large deviations in the TEC can produce inaccuracies in the estimation of GPS instrumental biases for the satellites that pass over the receiver during that period. We also present a semi quantitative analysis of the duration of the influence of the storm.

This study presents a comprehensive observation of the large-scale traveling ionospheric disturbances (LSTIDs) in the eastern Asian sector during the 2015 St. Patrick's Day (17 March 2015) geomagnetic storm. For the first time, three dense networks of GPS receivers in China and Japan are combined together to obtain the two-dimensional (2-D) vertical total electron content (VTEC) perturbation maps in a wider longitudinal range than previous studies in this region. Results show that an LSTID spanning at least 60^∘ in longitude (80–140^∘ E) occurs as a result of possible atmospheric gravity waves (AGWs) propagating from high to lower latitudes at around 09:40–11:40 UT (universal time), and the crest of this LSTID shows a tendency of dissipation starting from the eastern side. The manifestation of the 2-D VTEC perturbation maps is in good agreement with the recordings from two high-frequency Doppler sounders and the iso-frequency lines from eight ionosondes. Then, the propagation parameters of the LSTIDs are estimated by applying least-square fitting methods to the distinct structures in the 2-D VTEC perturbation plots. In general, the propagation parameters are observably longitudinally dependent. For example, the propagation direction is almost due southward between 105 and 115^∘ E, while it is slightly southwest and southeast in the western and eastern sides of this region. This feature is probably related to the regional geomagnetic declination. The mean values of the period, trough velocity (V_t), crest velocity (V_c), and wavelength of the LSTIDs in the studied longitudinal bands are 74.8±1.4 min, 578±16 m s⁻¹, 617±23 m s⁻¹, and 2691±80 km, respectively. Finally, using the VTEC map data from the Madrigal database of the MIT Haystack Observatory, the characteristics of the ionospheric disturbances over the European sector (30–70^∘ N, 10–20^∘ E) are also studied. The results are very different from those in the eastern Asian sector in parameters like the occurrence time, oscillation period, and propagation velocities.