# Variations in the Equatorial Ionospheric F Region Current during the 2022 Tonga Volcanic Eruption
## 4 Discussion
This could result in the well-known  
Figure 3: Panels **(a,b,e,f)**: The ICON orbit track segment is displayed as a blue curve as a function of the geographic latitude and longitude. The iso-distance lines surrounding the Tonga volcano eruption epicenter are marked at 2000-km intervals in green. The black dashed curves mark the magnetic equator. The red vertical dashed line shows the longitude of the upward FRC observed by the Swarm constellation, while the blue vertical dashed line shows the longitude of the downward FRC observed by the Swarm satellites in the daytime. Panels **(c,d,g,h)**: The magnetic zonal wind is displayed as a function of the altitude and geographic longitude. The red arrows and vertical dashed lines denote the longitude of the radially upward FRC, while the blue arrows and vertical dashed lines denote the radially downward FRC. equatorial ionosphere anomaly (EIA), which is characterized by two electron density enhancement bands at higher magnetic latitude across the magnetic equator. On the contrary, when the E region westward electric field at 15:30 UT mapped into F-region, the plasma moved downward, preventing the EIA formation. For further verification of this speculation, Figure 4 shows the latitudinal variation of electron density between 12:00 and 16:00 UT observed by Swarm C at around 470 km altitude (blue curve, Swarm A has no data available). Compared with the previous orbit around 12:36 UT, the prominent EIA appeared on both sides of the magnetic equator around 14:06 UT due to the strong eastward dynamo electric field. Around 15:36 UT, the EIA disappeared due to the westward dynamo electric field. The Ne pattern during the eruption period is further compared with that of the quiet period on 13 January 2022. Figure 4 illustrates the latitudinal variation of Ne between 13:00 UT and 16:00 UT on 13 January 2022 (black curve). It can be seen that a more prominent EIA structure emerging around 14:00 UT on 15 January 2022 when compared to that on 13 January 2022. This indicates a stronger eastward electric field occurring on 15 January 2022 than that of the quiet day. In contrast, the Ne is generally weaker around 15:36 UT on 15 January 2022 than that of the quiet day, consistent with the changed dynamo electric field as outlined above. These new findings of E-F wind polarity switch and impact on the ionospheric current and electron density agree with the timing of the volcano-induced atmospheric Lamb waves as they swept across the American sector. Because this event occurred during the storm-recovery phase, one might argue that the storm disturbance wind might affect the ionosphere current system. To check whether the zonal wind change is related to magnetic storm recovery phase or not, Figure 5 shows the longitudinal and latitudinal distribution of the magnetic zonal wind before and after the arrival of the volcanic eruption around Swarm longitudes. All subfigures are during storm recovery phase. It can be seen that after the arrival of the volcanic eruption at the Swarm longitude (panels g-h), the longitudinal distribution of the magnetic zonal wind at  
Figure 4: The magnetic latitudinal variation of ionospheric F region electron density in the daytime (11 MLT) observed by Swarm C between 12:00 UT and 16:00 UT on 15 January 2022 (blue curve). For comparison, Ne between 13:00 UT and 16:00 UT on 13 January 2022 are also shown in black curve. The UT and GLon of the satellite passing through the magnetic equator are listed on the right axis. fixed height shows obvious peaks (eastward) and troughs (westward). The eastward or westward peak wind velocities at different altitudes are chosen to display the longitudinal wave structure, as indicated by white solid line or dashed line. But before the arrival of the volcanic eruption (panels a-f), no such longitudinal wave structure can be detected. The large-scale westward wind can be found within 60-120\({}^{\circ}\) GLon above 200 km altitude.