Hosub Song

1 Introduction Geomagnetic storms induce significant changes in the ionosphere, largely driven by three major factors. First, Prompt Penetration Electric Fields rapidly alter the electric field in the ionosphere, causing disturbances in electron density by enabling electric fields from the magnetosphere to penetrate to low latitudes (Astafyeva et al., 2016; Klimenko & Klimenko, 2012; Tsurutani et al., 2008). This leads to immediate and significant changes in the distribution of ionospheric plasma.

1 Introduction Equatorial plasma Irregularities (EPIs) signifies electron density irregularities in the nightside equatorial and low-latitude ionospheric F-region (Aa et al., 2020; Burke et al., 2004; F. Huang et al., 2021; Kil, 2015; Kuai et al., 2022; Park et al., 2008; Park, Mende, et al., 2022; Xiong et al., 2010).

2.2 Data Processing Methods The wavefront of the nighttime MSTID is known to be tilted with respect to the magnetic meridian (e.g., Otsuka et al., 2004), which conforms to the backward-C shape virtually straddling the dip equator. Thanks to the formation flight of Swarm A and C, we can investigate whether the topside disturbances observed by Swarm exhibit similar directivity.