Solar Flare‐Induced Prenoon‐Postnoon Asymmetry of Ionospheric Electron Density

Abstract

Previous studies have shown that total electron content (TEC) in high‐latitude ionospheres exhibits a prenoon‐postnoon asymmetrical distribution (e.g., Förster et al., 2017, https://doi.org/10.1002/9781119216346.ch10; Wang et al., 2022, https://doi.org/10.1029/2022ja030292). It remains unclear whether the solar flare intensifies or weakens this asymmetry and whether relevant electrodynamic processes are affected. Here, we investigated the prenoon‐postnoon asymmetrical distribution of ionospheric TEC and electron density (Ne) during the X9.3 solar flare on 6 September 2017, utilizing Global Navigation Satellite Systems (GNSS) and an ionosphere‐thermosphere coupled model. After eliminating local time effects, we found that the flare intensified the prenoon‐postnoon asymmetry at high latitudes, with a TEC increase of 2.6 TECU at 14 LT and 2 TECU at 10 LT near 60° latitude. Additionally, Ne enhancement around the F₂ region electron density peak height (hmF₂) showed the strongest asymmetry, with prenoon‐postnoon ratio (PPR) reaching 14.2%. Analysis of the F‐region ion continuity equation indicates that chemical processes played a primary role in the asymmetry of Ne change rate, with significantly increased electron temperature amplifying the asymmetry in ionizing efficiency. Moreover, this asymmetry triggered changes in the electrodynamics of the polar ionosphere, increasing ionospheric conductivity and field‐aligned currents more in the postnoon sector than in the prenoon sector. These findings enhance our understanding of solar flare impacts on Earth's magnetosphere‐ionosphere‐thermosphere system, improving our ability to diagnose rapid changes in geospace weather.