The nighttime ionospheric response and occurrence of equatorial plasma irregularities during geomagnetic storms: a case study

Recent studies revealed that the long-lasting daytime ionospheric enhancements of Total Electron Content (TEC) were sometimes observed in the Asian sector during the recovery phase of geomagnetic storms (e.g., Lei (J Geophys Res Space Phys 123: 3217–3232, 2018), Li (J Geophys Res Space Phys 125: e2020JA028238, 2020). However, they focused only on the dayside ionosphere, and no dedicated studies have been performed to investigate the nighttime ionospheric behavior during such kinds of storm recovery phases. In this study, we focused on two geomagnetic storms that happened on 7–8 September 2017 and 25–26 August 2018, which showed the prominent daytime TEC enhancements in the Asian sector during their recovery phases, to explore the nighttime large-scale ionospheric responses as well as the small-scale Equatorial Plasma Irregularities (EPIs). It is found that during the September 2017 storm recovery phase, the nighttime ionosphere in the American sector is largely depressed, which is similar to the daytime ionospheric response in the same longitude sector; while in the Asian sector, only a small TEC increase is observed at nighttime, which is much weaker than the prominent daytime TEC enhancement in this longitude sector. During the recovery phase of the August 2018 storm, a slight TEC increase is observed on the night side at all longitudes, which is also weaker than the prominent daytime TEC enhancement. For the small-scale EPIs, they are enhanced and extended to higher latitudes during the main phase of both storms. However, during the recovery phases of the first storm, the EPIs are largely enhanced and suppressed in the Asian and American sectors, respectively, while no prominent nighttime EPIs are observed during the second storm recovery phase. The clear north–south asymmetry of equatorial ionization anomaly crests during the second storm should be responsible for the suppression of EPIs during this storm. In addition, our results also suggest that the dusk side ionospheric response could be affected by the daytime ionospheric plasma density/TEC variations during the recovery phase of geomagnetic storms, which further modulates the vertical plasma drift and plasma gradient. As a result, the growth rate of post-sunset EPIs will be enhanced or inhibited.

Role of eddy diffusion in the delayed ionospheric response to solar flux changes

Simulations of the ionospheric response to solar flux changes driven by the 27 d solar rotation have been performed using the global 3-D Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) physics-based numerical model. Using the F10.7 index as a proxy for solar extreme ultraviolet (EUV) variations in the model, the ionospheric delay at the solar rotation period is well reproduced and amounts to about 1 d, which is consistent with satellite and in situ measurements. From mechanistic CTIPe studies with reduced and increased eddy diffusion, we conclude that the eddy diffusion is an important factor that influences the delay of the ionospheric total electron content (TEC). We observed that the peak response time of the atomic oxygen to molecular nitrogen ratio to the solar EUV flux changes quickly during the increased eddy diffusion compared with weaker eddy diffusion. These results suggest that an increase in the eddy diffusion leads to faster transport processes and an increased loss rate, resulting in a decrease in the ionospheric time delay. Furthermore, we found that an increase in solar activity leads to an enhanced ionospheric delay. At low latitudes, the influence of solar activity is stronger because EUV radiation drives ionization processes that lead to compositional changes. Therefore, the combined effect of eddy diffusion and solar activity leads to a longer delay in the low-latitude and midlatitude region.

GPS-TEC observations over Nepal during the Total Solar Eclipse on 22 July 2009

This paper explores the ionospheric response in terms of Total Electron Content (TEC) during the 22 July 2009 Total Solar Eclipse. Using the data stored at Biratnagar (BRN2), Ramite (RMTE), Dhangadhi (DNGD), Nepalganj (NPGJ), and Taplejung (TPLJ) Global Positioning System (GPS) stations, the ionospheric activity was investigated through changes in TEC. Our research is based on GPS-TEC measurements from a widely dispersed GPS network across various geographical locations in Nepal, taking place on July 17-21 as a pre-event, July 22 as the main event, and July 23-27 as a post-event. The analysis reveals that the reduction in the TEC level is proportional to the magnitude of the total solar eclipse. The variation of the TEC depends on latitude as well as longitude. We found that TEC depletion was up to 5% from pre-event to main-event and up to 30% from main-event to post-event during the totality of the eclipse. The eclipse was accompanied by the 10-hour geomagnetic storm in Nepal, which was the explanation for the TEC upgrade to 50% on the main event day from pre-event and decreased by 25% from main-event to post-event. The result obtained in this work demonstrates the influence of the eclipse/storm on the variation of TEC.