The April 2023 SYM-H = −233 nT Geomagnetic Storm: A Classical Event

The 23–24 April 2023 double-peak (SYM-H intensities of −179 and −233 nT) intense geomagnetic storm was caused by interplanetary magnetic field southward component B_s associated with an interplanetary fast-forward shock-preceded sheath (B_s of 25 nT), followed by a magnetic cloud (MC) (B_s of 33 nT), respectively. These interplanetary structures were led by a coronal mass ejection erupted from the Sun in association with an M1.7 X-ray flare. At the center of the MC, the plasma density exhibited an order of magnitude decrease, leading to a sub-Alfvénic solar wind interval for ∼2.1 hr. Ionospheric Joule heating accounted for a significant part (∼81%) of the magnetospheric energy dissipation during the storm main phase. Equal amount of Joule heating in the dayside and nightside ionosphere is consistent with the observed intense and global-scale DP2 (disturbance polar) currents during the storm main phase. The sub-Alfvénic solar wind is associated with disappearance of substorms, a sharp decrease in Joule heating dissipation, and reduction in electromagnetic ion cyclotron wave amplitude. The shock/sheath compression of the magnetosphere led to relativistic electron flux losses in the outer radiation belt between L* = 3.5 and 5.5. Relativistic electron flux enhancements were detected in the lower L* ≤ 3.5 region during the storm main and recovery phases. Equatorial ionospheric plasma anomaly structures are found to be modulated by the prompt penetration electric fields. Around the anomaly crests, plasma density at ∼470 km altitude and altitude-integrated ionospheric total electron content are found to increase by ∼60% and ∼80%, with ∼33% and ∼67% increases in their latitudinal extents compared to their quiet-time values, respectively.