Shuai Fu, Gang Li, J. Tacza, Tao Chen, J. F. Peng
On 8 May 2024, the solar active region 13664 produced multiple powerful coronal mass ejections that impacted Earth around 17:00 UT on 10 May, triggering the most intense geomagnetic storm in the past two decades. Concurrently, a significant Forbush decrease (FD) in galactic cosmic rays (GCRs) and the 74th ground level enhancement (GLE74) were detected by ground-based neutron monitors. This study reports variations in the near-surface vertical atmospheric electric field (AEF, Ez) during these solar-induced disturbances, using fair-weather data from high-altitude Gar station (4259 m a.s.l.). AEF deviation was calculated relative to the diurnal background. On 11 May, the deviation reached approximately +0.02 kV/m at 00:00 UT when the FD minimum occurred, but turned negative around 02:00–04:00 UT (GLE74 phase), dropping to −0.04 kV/m near the GLE peak (∼03:00 UT). It then entered a sustained positive phase, peaking at +0.13 kV/m and lasting until late 14 May, when the geomagnetic activity returned to its pre-event level. We attribute the long-lasting AEF enhancement to the sustained high-speed solar wind (exceeding 700 km/s for 66 h) and the strongly compressed magnetopause (down to 5 Re). The differing AEF responses to FDs and GLEs can be explained by Ohm's law (Jz=σEz), assuming a constant air-Earth current density (Jz). During FDs, reduced GCR-induced ionization suppresses atmospheric conductivity (σ), leading to an enhanced Ez; in contrast, enhanced ionization during GLEs elevates σ at the ground level, thereby reducing Ez. However, a global atmospheric electricity model is still needed to further reveal the underlying mechanisms.