Reverse Fountain and the Nighttime Enhancement in the Ionospheric Electron Density Over the Equatorial Region: A Case Study

Abstract

This paper focuses on the linkage between reverse fountain effect and anomalous enhancement in the nighttime electron density over the equatorial ionosphere often referred as the ionospheric nighttime enhancement (INE). The Global Positioning System (GPS)-derived total electron content (TEC) measurements over two Indian equatorial stations and ionosonde measurements are used to investigate the occurrence characteristics and mechanism of the premidnight and postmidnight INE events. The high-resolution spatiotemporal two-dimensional (2D) TEC maps generated by using satellite-based augmentation system are used to visualize the reverse fountain effect. These TEC maps are found to be effective in envisaging the motion of equatorial ionization anomaly (EIA) in nighttime ionosphere and its impact on the distribution of electron density over the equatorial region. Salient features of this case study illustrate that, accompanying enhancements in the nighttime TEC, there exist decrease in the F2 layer height, prevalence of downward vertical plasma drift, and dramatic increase in the F2 region electron density. The 2D-TEC maps reveal the persistence of well-developed EIA until late evening hours and its gradual shift toward equator during the events of postmidnight INEs. It is found out that the premidnight INE is not invariably linked to the prereversal enhancement of the zonal electric field and could be associated with the redistribution of electron density caused by the reverse fountain effect. The present observations indicate that although the westward electric field serves as a prime driver, the INEs over the equatorial regions are associated with plasma motions caused by the retreating anomaly.

Evidence for the Significant Differences in Response Times of Equatorial Ionization Anomaly Crest Corresponding to Plasma Fountains During Daytime and Post‐Sunset Hours

“…However, if one considers the duration of the PRE and 3-4 h of diffusion time (similar to daytime), the ionization over the EIA crest region is not expected to get affected by the PRE-driven fountain. This is because the equatorial zonal electric field changes to westward direction after the PRE and eventually reverse plasma fountain (e.g., Balan et al, 1995Balan et al, , 1997C.-H. Chen et al, 2017;Sridharan et al, 1993;Yadav et al, 2020) sets in before the PRE-driven fountain can transport plasma from the dip equator to the crest region. Therefore, it is important to understand how much time the EIA crest region takes to respond to the PRE and how does it compare to the response time during daytime.…”

Ionospheric Nighttime Enhancements in the Equatorial Region as Revealed by the Beidou Geostationary TEC Observations

“…However, there were some events of equatorial ionospheric nighttime enhancements without an accompanying PRE. The equatorial ionospheric nighttime enhancements were also suggested to be associated with downward plasma drift caused by the westward equatorial electric fields rather than the effect of PRE (e.g., Liu et al., 2013; Yadav et al., 2020). Thus, whether the PRE could provide a favorable condition to produce the different types of equatorial ionospheric nighttime enhancements is also unclear.…”

“…The downward plasma drift related to the westward equatorial electric field after ∼20:00 LT can push plasma from high altitudes at higher latitudes to low ionospheric altitudes over the magnetic equator (Fejer et al, 1979(Fejer et al, , 1991. As a result, the nighttime electron density enhancement that occurred in the off-equatorial region can be gradually close to the magnetic equator (Le et al, 2014;Liu et al, 2013;Yadav et al, 2020). The nighttime foF2 and TEC enhancements with equatorward plasma movement after ∼20:00 LT when the h'F2 had an obvious decline at the magnetic equator were also observed for Type 1 (Figures 3a & 5a-5d).…”

“…The characteristics of ionospheric nighttime enhancement in the equatorial region have also been investigated by various observations and simulations. Its features, including the occurrence rates, durations, amplitudes, seasonal variations, latitude variations, interhemispheric differences, among many others, were revealed (Balan et al, 1994(Balan et al, , 1995Balan & Rao, 1987;Jain et al, 2011;Jiang et al, 2016;Le et al, 2014;Sahu et al, 2020;Yadav et al, 2020). However, these studies mostly focused on the characteristics of ionospheric nighttime enhancement based on the single or several observed stations.…”

“…In comparison to the middle latitudes, ionospheric nighttime enhancement in the equatorial and low-latitudinal regions was attributed to various ionospheric dynamic processes (Abur-Robb, 1969;Balan et al, 1994Balan et al, , 1995Balan & Rao, 1987;Farelo et al, 2002;Jain et al, 2011;Jiang et al, 2016;Le et al, 2014;Liu et al, 2013;Pavlov & Pavlova, 2007;Yadav et al, 2020). Through observation and simulation studies, Liu et al (2013) and Le et al (2014) discovered that the reversal equatorial fountain effect associated with the westward equatorial electric field (downward plasma drift) tends to push down the ionosphere, causing an increase in electron density in the F2 layer during the nighttime.…”