Convergence Effects on the Ionosphere During and After the Annular Solar Eclipse on 21 June 2020

Abstract

A moon shadow passed through the region of eastern and southern China and ends near sunset (∼09:30 UT) under the solar minimum condition on 21 June 2020. It is an opportunity to study the ionospheric behaviors influenced by the solar eclipse at low latitudes. Wang et al. (2021) analyzed the behaviors of the sporadic-E (E s ) layer during the solar eclipse on 21 June 2020. The continuously upward and downward movement of the E s layer reveals a prominent transport process occurring in the vertical direction of the ionosphere during the obscuration.It is known that the transport effect dominants the F region during an eclipse (Dear et al., 2020;Rishbeth, 1970;Tsai & Liu, 1999). Dang et al. (2020) simulated the thermospheric and ionospheric responses to the annular solar eclipse on 21 June 2020 using a coupled thermosphere-ionosphere-electrodynamics model. They suggested that the solar eclipse could cause the total electron content (TEC) enhancement in the equatorial ionospheric anomaly (EIA) area for nearly 4.5 hr under the influence of the transequatorial plasma transport. Using the ground-and space-based observations, Zhang et al. ( 2020) and Huang et al. (2020) showed that the ionospheric responses to the annular solar eclipse, such as the disturbances of TEC, electron density and temperature, and maximum plasma density of F 2 layer (N m F 2 ), are much complicated at low latitudes. They suggested that the disturbed electric field, neutral winds, thermal conduction, interhemispheric photoelectron transport process, are mainly responsible for these complicated behaviors. Aa et al. ( 2021) also observed these similar ionospheric behaviors and suggested that the perturbations in the field-aligned thermal conduction during the solar eclipse lead to the possible interconnections between the eclipse and the conjugate ionospheres. Sun et al. (2021) analyzed the TEC data from the ground-based Global Navigation Satellite System (GNSS) receivers over east and south Asia as well as the ionospheric F 2 layer critical frequency (f o F 2 ) and its height (h m F 2 ) recorded by the ionosonde at Puer (22.7°N, 101.05°E, 72% obscuration) on 21 June 2020. Their results showed that the eclipse-induced TEC perturbations behave as a large-scale terminator wave propagating northwestward after sunset. The previous studies comprehensively examined the effects of the eclipse on the ionosphere. However, the effect of the vertical plasma drifts on the F-layer induced by this solar eclipse was not clearly identified yet.

Occurrence of Nighttime Irregularities and Their Scale Evolution in the Ionosphere Due To the Solar Eclipse Over East Asia on 21 June 2020

“…Wang et al. (2022) reported the long‐lasting convergence effects that transport plasma toward the obscuration path from both the northern and southern sides, and form density enhancements that persist in the F 2 region till midnight.…”

“…The occurrence of spread F over Xiamen coincides with the coarse structures extending there after ∼14:00 UT (∼22:00 LT) (Figure 1f). On the other hand, irregularities occur at midlatitudes mainly between 30° and 45°N (Figures 1, 3, and 4), but PRE was not clearly observed over Wuhan (30.4°N, 114.4°E) (Wang et al., 2022). It was known that the occurrence of PRE benefits the development of irregularities at low latitudes in the pre‐midnight hours (Sun et al., 2012).…”

“…The ionosonde at Puer (22.7°N, 101.05°E) observed the PRE near the sunset and the spread F after ∼15:30 UT (∼22:30 LT) (Sun, Chen, et al., 2021) that coincides with the occurrence of the coarse structures evolving from the major enhancement at low latitude after 15:00 UT (Figures 1g and 2b). The PRE is obscure over Xiamen (24.2°N, 118.07°E), but the spread F there after ∼14:30 UT (∼22:30 LT) is much stronger than that over Nanning (22.7°N, 109.25°E) where the PRE occurs (Wang et al., 2022). The occurrence of spread F over Xiamen coincides with the coarse structures extending there after ∼14:00 UT (∼22:00 LT) (Figure 1f).…”

“…However, the PRE is not the only reason for the occurrence of irregularities on 21 June. The density enhancement persisting in the F 2 region (Wang et al., 2022) can be another reason for irregularities at the low and middle latitudes. The high density in the F 2 region above the low density at lower altitudes can enlarge the vertical density gradient in the nighttime, which can increase the growth rate of instabilities (Kelley, 2009).…”

“…However, whether the solar eclipse can induce irregularities in the nighttime remains unknown. Solar eclipses can induce prereversal enhancement (PRE) or large‐scale ionospheric perturbations with a duration or period of near one to several hours near and after sunset (Chen et al., 2011; Liu et al., 2019, 2020; Sridharan et al., 2002; Sun, Chen, et al., 2021; Tsai & Liu, 1999; Wang et al., 2022; Yeh et al., 1997). It can be attributed to the changes in ionospheric electrodynamics due to the shadow‐induced thermospheric disturbances on the dayside propagating into the nightside after the eclipse ends (Chen et al., 2019; Cnossen et al., 2019; Dang et al., 2020; Harding et al., 2018; McInerney et al., 2018; Wang et al., 2022).…”