3.1. Diurnal variation of ion density
Figure 3 depicts the behavior of the ion density with local time. The diurnal variations of ion density are shown in three seasons: June solstice (May, June, July, and August: middle panel), Equinox (March, April, September, and October: bottom panel), December solstice (November, December, January, and February: top panel) during the solar minimum of the December 2019 to November 2020. The local time is binned by 1 hour, and the plot shows median ion density measured by F7/C2E1 (blue lines) and ICON (black lines). The ion density in the F region decreases at night and increases during the day, which is due to photoionization by solar EUV radiation and diurnal variations of the electromagnetic drifts (i.e. \(E\times B\) drifts) that transport plasma to a higher altitude during the day and to lower altitude at night (e.g., Schunk and Nagy, 1978; 2000, Anderson et al., 2004). The diurnal variation pattern of ion density is characterized by remarkable one peak values, occurring in the noon hours around 10:00–14:00 LT. The local time dependence of ion density measured by F7/C2E1 IVM is similar to that measured by ICON. The magnitude of ion density during noon hours has a maximum value in the December solstice and a minimum value in the June solstice. Also, the ion density of Equinoxes is greater than that of June solstice. The ionosphere plasma density is determined by the plasma production, loss, and transport processes. Plasma production and loss are affected by EUV and recombination, respectively. Since the recombination is affected by thermospheric neutral density and the hemispheric wind effect, various factors drive the seasonal variation in plasma density. The difference in ion density resulting from the hemispheric wind effect will be discussed in Fig. 4.
In Fig. 3, nighttime ion density observed by ICON IVM is slightly greater than F7/C2E1 IVM, but vice versa during daytime. During the data period from 2019 to 2020, the ICON altitude (550–600 km) is about 50 km higher than the F7/C2E1 altitude (500–550 km). An altitude difference of 50 km between F7/C2E1 altitude and ICON altitude may cause a difference in density between F7/C2E1 and ICON. The solar EUV radiation changing with solar zenith angle plays a significant role in ion production in the ionosphere during daytime. After the solar EUV radiation disappears during nighttime, recombination has an important role in ion density. A slight difference in height causes a difference in these two processes, resulting in a difference in ion density. Interestingly, the difference in density between the two satellites is maximized during 10–17 LT in December. The ICON densities do not vary significantly with the season, but F7/C2E1 densities increase significantly during the daytime only in the December solstice. If the velocity data of F7/C2E1 is fully calibrated in the future, it will be possible to identify the cause of this difference.
The ion density variation with local time is different depending on the season, and it also have a dependence on solar activity. In the diurnal variation of the ion density measured by F7/C2E1 and ICON during the low solar activity from December 2019 to November 2020, no increase in density due to pre-reversal enhancement (PRE) was observed. Xiong et al., (2016) reported the diurnal variation of electron density observed from Swarm A and C during moderate solar activity from April 2014 to April 2016. In Fig. 4(b) of Xiong et al. (2016), the electron density in the equinox measured by the Swarm increases rapidly near sunset (1700–1900 LT) because of the PRE effect. However, this characteristic is absent in the ion density measured by F7/C2E1 and ICON.
The seasonal and solar activity variations of evening PRE have been reported by several literatures (Fejer et al., 1981; 1991; 2008; Kil et al. 2007, 2009). The PRE magnitude of the vertical plasma drift tends to increase with solar flux (Fejer et al., 1991; 1999; Kil et al., 2011b). The magnitude has also a dependence on seasonal variation, reaching its maximum during the equinox and its minimum during the June solstice (e.g. Fesen et al., 2000). The PRE can uplift the height of the equatorial F region, making a favorable condition for EPB development. The reduced ion-neutral collision frequency at higher altitudes has a positive effect on the growth of Rayleigh-Taylor instability associated with EPB generation.
The solar activity dependence of the EPB occurrence probability was investigated based on the in-situ satellite measurements of DMSP during 1989–2004 (Huang et al., 2002; Gentile et al., 2006)d NOFS during − 2008–2014 (Huang et al., 2015) and CHAMP and GRACE during 2001–2009 (Xiong et al., 2010). More EPBs occur at a higher solar activity. The F7/C2 and ICON may observe relatively fewer EPBs during the solar minimum condition from December 2019 to November 2020, but can still provide opportunities to study the seeding effect from the lower altitude. We will introduce an example of the EPB observed through the simultaneous observations of ICON and F7/C2 in section 3.2.
Figure 4 presents the local time variation of the averaged ion density for three different seasons at the equatorial (± 5°) and low latitude (± 10° to ± 20°) regions to characterize the structure of EIA. Regardless of latitude and season variation, the patterns of ion density measured by both satellites are similar, but the densities at F7/C2E1 altitudes appear to be greater than or nearly identical to those of ICON. Coley et al. (2010) reported the ion density observed by C/NOFS IVM during the summer of 2008. The ion density range of C/NOFS at an altitude of 500–550 km is consistent with that of F7/C2E1 and ICON shown in this study. The difference in density between the two satellites was clearly observed during the daytime of the equatorial region, where the densities of Equinoxes and December solstice are greater than those of June solstice. The density of the ICON altitude increases rapidly before 1800 LT in the equatorial region of Equinox, which appears to be due to the PRE. However, this effect is not seen at the F7/C2E1 altitude, which may indicate a difference in altitude of the PRE effect.
We separated the data into the southern and northern hemispheres to compare the ion density distributions obtained from both satellites in low latitude regions. The low latitude F7/C2E1 densities between 1200-1400LT are roughly a similar distribution, but there is a clear difference in the southern hemisphere of the December solstice. Since the summer-to-winter winds during solstice affects the plasma motion and ionospheric morphology (Rishbeth et al., 2000; Lin et al., 2007), the F region neutral wind has a significant effect in the hemispheric asymmetry of the topside ionosphere. West et al. (1997) examined variations in ion composition associated with solar activity using DMSP F10 and concluded that F region neutral winds cause modulation of the F peak height, which is responsible for the hemispheric asymmetry of the topside ionosphere. Su et al. (1998) showed that the electron density obtained by the Hinotori satellite revealed an effect of ion drag on the meridional wind. Further, Kil et al. (2006) suggested that equatorial winds stand against downward plasma diffusion, resulting in hemispheric asymmetry of the topside ionosphere at an altitude of 840 km. The summer-to-winter winds are equatorward (poleward) at the summer (winter) hemisphere preventing (accelerating) the downward diffusion of the plasma at topside. The process results in stronger topside density at the summer hemisphere than the winter hemisphere. Kwak et al. (2019) presented the hemispheric asymmetry of EIA with solar cycle is associated with the fountain process and interhemispheric wind. In Fig. 4c, the density of the southern hemisphere (summer) is larger than those of the northern hemisphere (winter), which can be interpreted as the hemispheric wind effect during the December solstice. The summer and winter hemisphere asymmetry is more prominent during the December solstice than the June solstice, which is consistent to the annual asymmetry of the ionosphere (Millward et al., 1996; Rishbeth. and Müller-Wodarg, 2006). The more prominent annual asymmetry effect in the December solstice may cause differences in the density values of F7/C2E1 and ICON during the daytime (10–17 LT) in Fig. 3c. However, the hemispheric wind effect in the June solstice had less influence on the distribution of ion density (Fig. 3a, Fig. 4a).
The morphology of topside plasma density has been reported in several literatures, and it differs from the F peak height (Su et al., 1998; Kil et al., 2006; Liu et al., 2007). Figure 5 shows the distribution of averaged ion density on the topside F region as a function of magnetic latitude observed by (left) F7/C2E1 and (right) ICON for three seasons. The ion density measurements from F7/C2E1 and ICON are sorted into local time (1 h) by magnetic latitude (5◦) bins. The ion density at low latitude is higher during the Equinox and December solstice than during the June solstice. The rapid increase of the density of the ICON altitude before 1800 LT in the equatorial region of Equinox, which appears to be due to the PRE, is also confirmed. The topside ion density of ICON altitude and F7/C2E1 altitude did not show off-equatorial maxima in both hemispheres by the equatorial plasma fountain effect. The F7/C2E1 density has a similar distribution to the ICON density, but its amplitude appears to be stronger. Lee et al. (2011) investigated the F region electron density obtained from FORMOSAT-3/COSMIC in 2007 and showed that although the EIA peaks in both hemispheres were observed due to the fountain effect at the F peak height, the EIA structures of the southern and northern hemispheres at the 500 km altitude could not be clearly distinguished under solar minimum condition.
3.2. Simultaneous observations of equatorial plasma bubble (EPB)
In this section, we examine the EPB through the simultaneous observations of ICON and F7/C2 on 18 October 2020. The ICON altitude and F7/C2 altitude on those days were near 600 km and 550 km, respectively. Figure 6 presents a series of four consecutive ion density measurements derived from ICON and F7/C2 on 18 October 2020, showing the spatial characteristics and time evolution of EPBs. When ICON passed around Taiwan at sunset, the three F7/C2 satellites (named hereafter F7/C2E1-F7/C2E3) also performed simultaneous observations around EPBs. Figure 6a shows the longitude-latitude trajectory of ICON (blue), F7/C2E1 (magenta), F7/C2E2 (cyan), F7/C2E3 (yellow) around 2100 LT, and F7/C2E4 (green) around 2200 LT, respectively. The black dashed line represents the magnetic equator. In Figs. 6b-6e, the black curve is ion density measured by the IVM onboard each satellite. In order to calculate the smooth background density (red curve), we first applied the function of a Savitzky-Golay low-pass filter (order = 3, window size = 51 data points). Then, the smoothing process was repeated for the low-pass filter values using 50 data points running average to remove small-scale fluctuations (e.g., Kil et al. 2011, Choi et al., 2017, Smith and Heelis, 2018, Smith et al., 2018).
ICON detected an ion density depletion at sunset (Fig. 6b). The trajectory of ICON is located at low latitude in the evening sector, and its altitude is 550 km. A series of bubbles are detected in the longitude region 105°-130°E. The EPBs morphology observed by (b) ICON is almost identical to that of (c) F7/C2E1. The (d) F7/C2E2 observed the EPBs at the longitude at which ICON observed the bubble. The EPBs are developed at the magnetic equator and expand to lower latitudes along the magnetic field lines. From the comparison between (b) ICON and (d) F7/C2E2, it can be confirmed that the bubble observed near the magnetic equator extends around Taiwan. Around 2100 LT, F7/C2E3 observed the EPBs in the opposite hemisphere, which means that the bubble expands along the magnetic field lines into the opposite hemisphere.
Plasma bubbles have been observed using an all-sky imager by National Cheng Kung University at Tainan Astronomical and Educational Area (23.1°N, 120.4°E) in Taiwan (Rajesh et al., 2017). On 18 October 2020, the all-sky imager observations were restricted by cloudy weather, but plasma bubbles were detected in some images. Figure 7 presents the coincident observations of plasma bubble over Taiwan (a) 2230 LT and (b) 2236 LT on the night of October 18, 2020. The airglow images in Fig. 7 are unwrapped into geographic coordinates, assuming an emission height of 250 km (Rajesh et al., 2017). The F7/C2E3 orbit tracks are mapped to low latitudes at an altitude of 250 km along magnetic field lines. The F7/C2E3 orbit and ion density are shown with green and red curves in Fig. 7, respectively. A different series of plasma bubbles were detected in comparison to the previous orbit in the longitude 110°-130°E. OI 630.0-nm airglow images show plasma bubbles as band-like airglow depletions (blue dashed box) that are elongated in the north-south direction. The dark bands of airglow images correspond to plasma bubbles observed in the F7/C2E3 orbit.