# Variations in the Equatorial Ionospheric F Region Current during the 2022 Tonga Volcanic Eruption
## 3 Results  
There was a geomagnetic storm occurring before the volcanic eruption. In order to distinguish the effects of magnetic storms, reconnection electric fields and volcanic eruption on FRC, we firstly show the time evolution of reconnection electric field and geomagnetic activity index. Figure 1 shows the temporal variation in the solar-wind merging electric field (Em) [19] and symmetric-H-component (SYM-H) index measured between 14 and 15 January 2022. The merging electric field is a geo-effective solar wind electric field parameter that can measure the input of solar wind energy to the magnetosphere. The main phase of the storm analyzed herein began at approximately 14:00 UT on 14 January, at which time Em gradually intensified. The SYM-H index fell to the minimum value of \(-100\) nT at approximately 22:25 UT on 14 January; at this time, Em attained its peak value. During the storm recovery phase, Em fluctuated from 22:25 UT on 14 January until the end of the day on 15 January. The Tonga volcanic eruption started around \(-20.54^{\circ}\) GLat, \(-175.38^{\circ}\) GLon at 04:15 UT on 15 January, as indicated by the vertical red dashed line in Figure 1. Approximately 8 h after the eruption, the Swarm satellites detected intermittent upward and downward FRC signals at the magnetic equator during the daytime (around 11 MLT), as indicated by the upward and downward arrows in the figure; these fluctuations are the main interest considered in the present work. The top panel in Figure 2 shows the daytime (around 11 MLT) FRC distribution around the magnetic equator (\(\pm 1^{\circ}\) MLat) derived along 16 consecutive orbits on 15 January 2022. The FRC is normalized by the peak current density throughout the day (the maximum FRC is 258.2 nA/m\({}^{2}\)). The vertical dashed line indicates the zero current. The radially upward (positive) FRC shifts are shown to the right of the dashed line, while the radially downward (negative) current shifts are shown to the left. The figure shows that a large FRC occurred above the magnetic equator after 12:00 UT, reflecting several polarity reversals. We averaged the FRC in each cross-equator orbit segment within \(\pm 1^{\circ}\) MLat to represent the FRC in the magnetic-equator region. In order to separate the effects of magnetic storm and volcano eruption, the latitudinal averaged FRC on 13-15 January is shown together in the bottom panel of Figure 2 as a function of UT. In this figure, positive values denote the radially upward FRC, while negative value denotes the radially downward FRC. The red arrows and vertical dashed lines denote the radially upward FRC on 15 January 2022, while blue arrows and vertical dashed lines denote the radially downward FRC, following the same format as that used in Figure 1. The figure shows that strongly positive FRCs were identified at 12:36 UT and 15:30 UT, while strongly negative FRCsemerged at 14:06 UT and 17:12 UT. The FRC magnitudes in the subsequent orbits monitored after 12:36 UT were obviously larger than those measured before the volcanic eruption effect took place (at 04:15 UT) and propagated until daytime (approximately 11 MLT), approximately 10 h after the eruption. Figure 1: Temporal variations in Em and the SYM-H index from 14–15 January 2022. The vertical red dashed line denotes the onset of the Tonga volcanic eruption. The red arrows denote radially upward FRC variations, and while the blue arrows denote radially downward FRC fluctuations. Figure 2: **Top panel:** Latitudinal variations in the normalized FRC within \(\pm 1^{\circ}\) MLat observed by dual Swarm A and C satellites on 15 January 2022. The vertical dashed lines indicate the zero current,  
while the positive (negative) FRC incidents deviate to the right (left). The x-axis range is from \(-1\) to \(1\). **Bottom panel**: Temporal variations in the latitudinal averaged FRC during 13-15 January. Black curve is on 13 January, red curve is on 14 January, and blue curve is on 15 January.