Unlike the previous studies (Le et al. 2009; Huba and Drob 2017) to simulate the ionospheric responses to the solar eclipses by applying a solar EUV radiation mask, this paper only assimilates the GNSS TEC observations into the physical model without using any mask to obscure the solar EUV effect in the model. In order to catch the rapid change of ionospheric electron density during the solar eclipse, the ionospheric TEC observations are assimilated into the system every 2 min. The assimilation analysis results on the eclipse day are compared with the previous day and show the TEC reductions around the maximum obscuration of the 2017 August total solar eclipse. The results obtained from the ionospheric data assimilation system are consistent with previous papers demonstrating TEC reductions by comparison of the solar eclipse day to a reference day during the same eclipse event (Coster et al. 2017; Zhang et al. 2017; Cherniak and Zakharenkova 2018; Nayak and Yiğit 2018; Sun et al. 2018). Furthermore, the related neutral temperature in the assimilation system was also decreased around the range of solar eclipse (figure not shown), which is consistent with the expectation of temperature reduction due to the solar eclipse. This indicates that the present ionospheric data assimilation system has the capability to improve the thermospheric responses to the solar eclipse.
Our results show enhanced EIAs at the low-latitude regions prior to the arrival of maximum obscuration. It is followed by the electron density reductions in the obscuration latitudes with the EIA enhancements at the adjacent lower latitudes. Both features appeared in the conjugate southern hemisphere and are consistent with the conjugated responses reported by Huba and Drob (2017). The major difference is that our assimilation results show not only electron density reductions at the maximum obscuration latitudes but also the lower latitude EIA enhancements. The difference may come from the fact that our assimilation system considers the effects of the thermospheric responses (temperature and winds) to the eclipse.
The ionospheric electric field effects on the electron density variations are shown in Fig. 2. Before the arrival of solar eclipse at 1730 UT (top panel of Fig. 2c), the electric field had been enhanced in the eastward direction around the geophysical latitude of − 5~20° N compared with the previous day. This eastward enhanced electric field on the eclipse day relatively uplifted the ionospheric F layer and resulted in the enhancement of electron density around geophysical latitude 10° N. At 1830 UT, the enhancement and reduction of eastward electric field perturbations further appeared at low- and mid-latitude regions, respectively, in the northern hemisphere. Since the electric fields at low- and mid-latitude regions are calculated with the assumption of equal potential along the magnetic field line (Richmond et al. 1992), the feature of enhanced and reduced electric field perturbations overall appeared along the magnetic file lines, but had the smaller perturbations in the southern hemisphere (also seen in Fig. 3c). At 1930 UT, the electric field perturbations further intensified the enhancement and the reduction of electron density at the low- and mid-latitudes, respectively.
According to the assimilation results of the electric field on the eclipse day (Fig. 4b), it is seen that the enhanced eastward electric field before the maximum obscuration of eclipse in the northern hemisphere produced the TEC enhancements around the low- and mid-latitude regions due to the effect of upward plasma (E × B) drift. Later, the strong westward electric fields occurred right after the period of maximum obscuration of solar eclipse around the mid-latitude region. This westward electric field decreased the ionospheric electron density during the eclipse window (Fig. 3c) by the downward movement of plasma vertical drift. The aforementioned features can also be seen in the longitude-latitude perturbations of the electric field shown in Fig. 5b. On the other hand, the occurrence time of maximum TEC reduction was around 1850 UT at 36° N latitude shown in Fig. 3c, revealing a time delay of 20 min with the solar eclipse obscuration (white circle in Fig. 3c at 1830 UT). These results imply that the ionospheric electron density morphology during the solar eclipse period is controlled by vertical plasma drift, upward (eastward electric field) at the low-latitude region and downward (westward electric field) at the mid-latitude region, respectively.
Figure 6 illustrates the possible mechanism of the abovementioned modification of electric field perturbations in the ionospheric data assimilation system. First of all, the Moon’s shadow decreases the ionizing radiation from the Sun during the solar eclipse, which causes a reduction in electron concentration. Conductivity becomes lower in the obscuration range of solar eclipse (Fig. 5a). Therefore, the charge particles accumulate at the boundary of high and low conductivity. Due to the direction of electric fields at the low-latitude region, the positive (negative) charge particles accumulate at the southwestern (southeastern) boundary of the eclipse, which induces the eastward electric field at lower latitudes to further enhance the original background eastward electric fields. On the other hand, at higher latitudes, the background westward electric fields are also enhanced by the same mechanism. The aforementioned mechanism is confirmed by the assimilation results in Figs. 4b and 5c, showing the enhanced eastward (westward) electric field at the low-latitude (mid-latitude) region.
Another interesting ionospheric feature during the 2017 August solar eclipse is shown in Fig. 3 is the early appearance of EIA (compared with the control run) in both hemispheres. The eclipse-induced early formation of EIA was observed and reported by Tsai and Liu (1999) during the solar eclipse of 24 October 1995, showing the pre-ascension of TEC observations before the solar eclipse. They suggested that this pre-ascension of TEC is possibly caused by the decrease of equatorial plasma fountain strength, which results in the equatorward shift of the EIA crest and the early forming of the EIA crest. However, this mechanism cannot fully explain the early EIA formation. Richmond et al. (2015) and Richmond and Fang (2015) proposed that an evening equatorial plasma vortex and the pre-reversal enhancement (PRE) of the vertical drift are influenced by the distributions of conductivity in the E and F regions in relation to the thermospheric neutral wind. The temperature difference within and outside the moon shadow area can produce a zonal pressure-gradient force to drive the thermospheric zonal wind. The electric field perturbations in Fig. 4 show the enhancement of the equatorial eastward electric field at the eastern region of the solar eclipse obscuration. This low-latitude eastward electric field might result from the sudden darkness induced by the solar eclipse at the termination region, like the eastward convection in the evening, and then further cause the early appearance of the EIA.