# Variations in the Equatorial Ionospheric F Region Current during the 2022 Tonga Volcanic Eruption
## 1 Introduction  
There was an intense volcanic eruption event that occurred in Tonga on 15 January 2022 (20.54\({}^{\circ}\)S, 175.38\({}^{\circ}\)W). According to some data, this huge volcanic eruption released energy equivalent to 200 megatons of TNT and created strong tsunami waves [1]. It was the most powerful volcanic explosion in more than 30 years. Recently, a number of studies have reported the impacts of volcanic eruption-related waves on the equatorial electrojet and total electron content; these impacts are all related to changes in the E-region dynamo wind (below 150 km) [2; 3; 4; 5]. The disturbed F region neutral zonal wind (above 400 km) during the volcanic eruption might modulate the ionospheric radial current [6], termed as F region current (FRC) in the present work. However, there is still a lack of work on the changes of ionospheric F region current in response to the volcanic eruption. Ionospheric currents play important roles in the energy transfer process in the geo-space system, which could damage electrical power grids. The neutral wind is oriented westward in the daytime while directed eastward in the nighttime around the magnetic equator. As a consequence, the daytime wind dynamo electric field is directed downward, while the nighttime dynamo electric field is directed upward. This is opposite to the orientation of the zonal plasma drift electric field. Both electric fields contribute to the development of ionospheric F region current at the dip equator. The equatorial FRC can be expressed as \(\sigma_{p}(E_{z}-U_{y}B_{x})\)[6], where \(\sigma_{p}\) is the Pedersen conductivity in the F region, \(U_{y}\) is the magnetic zonal wind in the F region (positive value means eastward direction), \(B_{x}\) is the geomagnetic field in the northward direction, and \(E_{z}\) represents the contribution of zonal plasma drift in the F region (positive value means downward direction). The first term, \(\sigma_{p}E_{z}\), represents the effect from polarization electric field, and the second term,\(\sigma_{p}U_{y}B_{x}\), is the wind dynamo current. The relative contributions of these two electric fields to the F-layer FRC were comprehensively studied and it showed that the radial current (i.e., FRC) was mainly controlled by the neutral wind dynamo electric field [7]. Using observations from Challenging Minisatellite Payload (CHAMP) satellite, [PERSON] and [PERSON] (2006) [8] showed that the F-layer dynamo current around the magnetic equator was driven by the thermospheric wind. The FRC exhibits obvious variations with geographical longitude (GLon) [9]. Using CHAMP observations, [PERSON] et al. (2010) [10] revealed that at the equinox and June solstice, the daytime FRC exhibited a wave-4 longitudinal structure that was not prominent in December solstice. The wave-4 longitudinal structure of duskside FRC was much reduced in all seasons, which was attributed to the reduced E-F layer coupling efficiency. Using observations from the Swarm satellites, [PERSON] et al. (2019) [11] further revealed that the FRC had an obvious hemispheric asymmetry between the eastern and western hemispheres. The western-hemispheric daytime FRC was generally weaker than that of the eastern hemisphere, whereas the situation was reversed at night. The authors deduced that the mean altitude of the dynamo region was higher in the evening than around noon. [PERSON] et al. (2022) [12] showed that the low atmospheric migrating tides could also contribute to the eastern-western hemispheric asymmetry of the daytime FRC by using thermosphere-ionosphere electrolytic general circulation model (TIE-GCM) simulations. [PERSON] et al. (2022) [13] investigated the FRC under different magnetic strengths and solar activities by using CHAMP observations and TIE-GCM simulations. They found that the daytime and nighttime FRCs increased significantly with decreasing magnetic strengths and increasing solar activities due to the ionospheric conductivity increasing in response to these changes.