Figure 2 shows the day-to-day variation of the peak PRE (magenta line) with (a, b) electron density, (c, d) eastward current density, and (e, f) eastward neutral wind at the time when the PRE reached a peak (around 1830–1900 LT) during (top) March–April and (bottom) September–October of the year 2012 over (10° N, 100° E; GAIA co-ordinates) near Chumphon in Thailand. The horizontal axis depicts the day number of the year. The day number 61–91 corresponds to March and day number 92–121 corresponds to April. The electron density peak (F2 peak) exists at an altitude between 350 and 550 km, and varies day by day. The F2 peak is elevated to higher altitudes by PRE around evening terminator. The electron density in the E-region is quite smaller than in the F-region because the E-region plasma disappears due to rapid recombination soon after the solar extreme ultraviolet (EUV) radiation ceases. The rapid recombination of the plasma also makes a steep gradient of the electron density at the bottomside of the F-layer. Day-to-day variation in the altitude of bottomside F-layer is also seen. From comparison with the peak PRE, it is found that the F2 peak and bottomside altitudes ascend and the electron density at the F2 peak altitude decreases when the peak PRE is intense (Fig. 2a, b). In Fig. 2c, d, eastward electric current is shown. The eastward current density is enhanced at an altitude of 110–120 km. This intense current density corresponds to EEJ. The intensity of EEJ varies day by day, and its day-to-day variation ranges between 0.05 and 0.12 Am−2 approximately. Intensification in the eastward current density coincides with enhancement of the peak in PRE, indicating positive correlation between peak PRE and current density on most of the days except for day numbers 68, 275, 284, 294 and 301. As shown in Fig. 2e, f, the neutral wind is eastward in the F-region and westward in the altitude range between 120 and 200 km. Day-to-day variation of the eastward wind in the F-region ranges from 50 to 100 ms−1, and displays distinct positive correlation with peak PRE. When the PRE is weak, the neutral wind weakens and correlation between the peak PRE and E-region neutral wind is not discernible. On few days, the positive correlation is not clearly seen (such as day numbers 69, 83, 88, 115, 248, 254, 266–271 and 281) which may lead to weak positive correlation.
We have investigated quantitatively correlation of day-to-day variation of the PRE with that of the electron density, eastward current density and eastward neutral wind at the time when the peak PRE occurs. Figure 3 displays the Pearson correlation coefficient of the peak PRE with (a) electron density, (b) eastward current density, and (c) eastward neutral wind during the equinoctial months of March, April, September and October at the altitudes of 100–600 km for the years 2011 (red line), 2012 (blue line) and 2013 (green line). In Fig. 3a, it is observed that the electron density in the E-region (at altitudes of 100–150 km) shows negative correlation with the peak PRE, and the correlation coefficients are approximately − 0.4 for 2011, − 0.2 to − 0.6 for 2012 and − 0.3 to − 0.4 for 2013. The electron density in the F-region (at altitudes of 300–450 km) also show negative correlation with the peak PRE with maximum negative cross-correlation coefficients of approximately − 0.5 for 2011 and 2013 and − 0.6 for 2012. The eastward current density correlation coefficient ranges between − 0.3 and 0.4 approximately in the E-region (100–150 km) for all the years (2011–2013) and positive correlation (0 to 0.6) above 250 km altitude (Fig. 3b). The cross-correlation coefficient shows two positive peaks. One is seen around 150–160 km altitudes, and the correlation coefficients are approximately 0.6 during 2011 and 2012, and 0.2 in 2013. The other is around 350–450 km altitudes, and the cross-correlation coefficients are approximately 0.6 for 2011 and 2012, and 0.4 for 2013. In the F-region, the Pederson current flows in the same direction as the electric field (leading to the positive correlation). Thus, increase in eastward electric field intensifies the Pederson current. The day-to-day variation of current density may be primarily controlled by the electric field. However, the less correlation coefficient values in both E- and F-region in 2013 is not known. As shown in Fig. 3c, the cross-correlation coefficients between the peak PRE and eastward neutral wind are between − 0.2 and 0.2 below 150 km altitude for all the years (2011–2013). Above 300 km altitude, the cross-correlation coefficient is approximately 0.4 and does not show altitude dependence (nearly constant). This could be because the neutral wind does not change largely with altitude due to high viscosity in the F-region (Yerg 1955).
Figure 4 portrays day-to-day variations of peak PRE with (a, b) electron density, (c, d) eastward current density, and (e, f) eastward neutral wind at an altitudes at (top) 1700 LT and (bottom) 1500 LT in an altitude range of 100–600 km (left Y-axis; contour plot) over (10° N, 100° E) near Chumphon during the equinoctial months of March and April 2012. The electron density in the F-region is lower at 1500 LT than 1700 LT. The negative correlation between the peak PRE and electron density, which can be seen at the time of PRE, is not seen at 1500 LT. Regarding the day-to-day variation of eastward current density around 110 km, that is EEJ, intensity of EEJ enhances intermittently (Fig. 4c, d). Enhancement of EEJ intensity coincide well with increase of peak PRE. It is noted that the magnitude of the current density for EEJ increases up to 2.0 Am−2 at 1500 LT and up to 0.7 Am−2 at 1700 LT (Fig. 4c). The current density of EEJ decreases with time because the electron density in the E-region decreases rapidly at sunset. Figure 4e, f displays the day-to-day variation of eastward neutral wind at altitudes of 100–600 km and peak PRE over (10° N, 100° E). Correlation between the neutral wind and peak PRE at 1500 and 1700 LT is not discernible although the correlation is significant at the time of the peak PRE.
Figure 5 represents the cross-correlation coefficient of the peak PRE with (a) electron density, (b) eastward current density and (c) eastward neutral wind at altitudes of 110 km, 250 km, 300 km and 350 km at different local time (1100 LT, 1300 LT, 1500 LT, 1700 LT and 1900 LT) over the magnetic equator during the equinoctial months of 2011–2013. In case of plasma density, the correlation coefficient goes on decreasing from 1100 LT (around 0.3) to 1900 LT (ranging from − 0.4 to − 0.6 for different altitudes). The cross-correlation coefficients with eastward current density increases with local time from approximately 0.2–0.35 at 1100 LT to 0.4 at 1700 LT. The maximum correlation coefficient value of 0.4 is obtained at 1700 LT for all the altitudes 110 km, 250 km, 300 km and 350 km. Therefore, it is observed that the pre-sunset (at 1700 LT) eastward current density can predict the generation of PRE in GAIA as the peak PRE is observed around 1830 to 1900 LT mostly. In case of neutral wind, the maximum correlation is seen in the F-region (250, 300 and 350 km) with maximum correlation coefficient of 0.5 at 1900 LT.
Figure 6 displays the scatter plot showing the correlation of eastward current density at 1700 LT versus the peak PRE (during the equinoctial months of 2011–2013) at the altitude of 110 km over (10° N, 100° E; GAIA co-ordinates) near Chumphon in Thailand. It is found that the peak PRE and eastward current density depicts a positive correlation with correlation coefficient (R) of 0.407.