# Variations in the Equatorial Ionospheric F Region Current during the 2022 Tonga Volcanic Eruption
## 4 Discussion  
One of the possible reasons for the strong FRC density identified in this work might have been the F-region magnetic zonal winds. Note that we are using the magnetic zonal wind in the present work. Figure 3 shows the ICON tracks and altitudinal distribution of the MIGHT1 wind observations obtained during four subsequent orbits from 11:30 to 17:30 UT. The Tonga volcanic eruption occurred around \(-20.54^{\circ}\) GLat, \(-175.38^{\circ}\) GLon at 04:15 UT. The estimated eruption wave-front location is shown in green curves. The ICON and Swarm orbit tracks are also shown on the map. Although the ICON satellite flies at a higher latitude than the Swarm satellites, both the ICON and Swarm satellites are located at nearly the same great-circle distances from the eruption center, that is, nearly 17,000 km in Figure 3a, 13-15,000 km in Figure 3b, 11-13,000 km in Figure 3c, and 9-11,000 km in Figure 3d (see the green curves in the figure). In addition, the altitude map of ICON-monitored winds covers the longitudinal coverages of the Swarm satellites in the daytime (around 12 MLT). ICON cannot measure neutral wind above 300 km altitude. However, the F region neutral wind pattern might not change too much between 300 and 400 km altitudes due to the air viscosity [13]. A strong eastward wind above 250 km altitude approaching \(-30^{\circ}\) GLon around 11-12 MLT (Swarm A longitude) can be seen in Figure 3c, followed by a strong westward wind above 250 km altitude, as shown in Figure 3d. Another pair of strong westward and eastward F-region neutral winds above 250 km altitude can be found in the further-westward longitudinal zone shown in Figure 3d,g. Such a wave-like disturbance zonal wind structure might have been caused by the Lamb wave associated with the volcanic eruption [20]. As shown in Figure 3c,d,g,h the F-region neutral wind (above 250 km altitude) in the longitudinal zone of the Swarm constellation (shown with the vertical red or blue dashed line) changed polarity from eastward to westward, and vice versa, three times. This finding was consistent with the polarity change observed in the FRC by the Swarm satellites; i.e., the eastward wind induced an upward-oriented FRC, as indicated by the red dashed line and upward arrow, while the westward wind caused a downward-oriented FRC, as indicated by the blue dashed line and downward arrow. We conducted a cross-correlation analysis on the FRC densities recorded in these four orbits and the corresponding F-layer zonal wind velocities. It indicates that a good correlation was found between these two parameters (R = 0.997). The FRC tended to intensify with strengthened wind, further indicating that these two physical processes share a close internal relationship. In addition to the F-region wind changes, the E-region wind at 100-150 km also shows polarity reversals, as Figure 3d,g reflect. Around 14:06 UT (Figure 3d), the zonal wind at Swarm A longitude (see vertical blue dashed line) was strongly westward at altitude below 120 km and eastward at 120-150 km. However, around 15:30 UT (Figure 3g), the zonal wind at Swarm A changed to be weakly eastward below 120 km and strongly westward at 120-150 km (see vertical red dashed line). Both the Hall region westward (eastward) wind below 120 km and the Pedersen region eastward (westward) wind above 120 km could contribute to the generation of the eastward (westward) electric field in the E layer [21]. These reversals induced an intense eastward electrojet around 14:06 UT and a subsequent intense westward counter-electrojet around 15:30 UT observed by Swarm, as was reported by, [PERSON] et al. (2022) [3] and [PERSON] et al. (2022) [4]. When the E layer eastward electric field at 14:06 UT was projected onto the F layer along the magnetic line, it could drive the equatorial plasma vertically upward. Subsequently the plasma at higher altitudes diffused along the field line toward higher latitudes due to the pressure gradient and gravity forces.