# Variations in the Equatorial Ionospheric F Region Current during the 2022 Tonga Volcanic Eruption
## 4 Discussion
Since panels a-h are during storm recovery phase, one can conclude that the obvious longitudinal wave structure in panels g and h might be related to the volcanic eruption, rather than to the recovery phase of the magnetic storm. In addition, as simulated by [PERSON] et al. (2005) [PERSON] et al. (2005) and explained by [PERSON] et al. (2022) [PERSON] et al. (2022), magnetic storms mainly cause westward disturbance winds at middle and low latitudes in the nighttime. These westward disturbance winds differ from the polarity-switching characteristics of zonal winds in both the E and F regions (see Figures 3 and 5g,h), as shown in the present work. Thus, we speculate that the volcanic eruption might generate atmospheric waves that propagate into the upper atmosphere and affect the F-region ionospheric current system. As known to us all, the Lamb waves propagate in the sound speed of typically 300-350 m/s and are mostly confined to the troposphere. Owing to the decreasing density, the amplitudes of Lamb waves increase exponentially with height. Then, the energy of Lamb wave can leak into the upper atmosphere when their horizontal phase speed of \(\sim\)300 m/s are resonant with the atmosphere [PERSON] et al. (2022); [PERSON] et al. (2022). Thus, one possibility is that the wave signals from the lower atmosphere might be transmitted to ionospheric F region heights through direct propagation of waves. Another possibility is that the E region or lower F region neutral wind is affected directly by the low atmosphere wave, and through the air viscosity process, the neutral wind in the top ionosphere can be affected. Figure 5: The magnetic zonal wind as observed by ICON is displayed as a function of the altitude and geographic longitude. Panels **(a–f)** are before the arrival of the volcanic eruption at Swarm longitudes, and panels **(g,h)** are after the arrival of the eruption at Swarm longitudes. The vertical dashed lines denote the longitude of Swarm A. Longitudinal wave structure with reversed polarities are as indicated by the solid and dashed white curves in panels **(g,h)**. The Lamb wave during Tonga volcanic eruption in the ionosphere has been reported in a lot of studies [23; 24; 25; 26; 27]. For instance, [PERSON] et al. (2022) [27] disclosed that the wave disturbances, in a speed close to sound wave, from surface to upper thermosphere can be observed by a comprehensive set of satellites. The Lamb waves in the ionosphere could drive the corresponding large-scale travelling ionospheric disturbances through ion-neutral collision [5]. The Lamb wave related to the volcanic eruption can cause TID in conjugate hemispheres [25; 26]. Another factor that might affect the FRC densities analyzed herein is the prompt penetration electric field (PPEF). As shown in Figure 1, the merging electric field changed quickly at 14:00 UT, exhibiting a decrease of 4 mV/m; this change corresponded to the westward enhancement of the FRC. However, the westward enhancement of the FRC was most significant near 17:00 UT, but the corresponding merging electric field measured at this time was not as significantly variable as that measured at 14:00 UT. When examining the other UT periods, such as the 00:00 UT-04:00 UT period, Em also showed frequent transient variations, but the corresponding FRC changes were not as significant, indicating that the PPEF might not have been the dominant factor inducing the FRC polarity changes observed during this period.