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 Bs associated with an interplanetary fast-forward shock-preceded sheath (Bs of 25 nT), followed by a magnetic cloud (MC) (Bs 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-Alfv & eacute;nic solar wind interval for similar to 2.1 hr. Ionospheric Joule heating accounted for a significant part (similar to 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-Alfv & eacute;nic 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 similar to 470 km altitude and altitude-integrated ionospheric total electron content are found to increase by similar to 60% and similar to 80%, with similar to 33% and similar to 67% increases in their latitudinal extents compared to their quiet-time values, respectively. A fast interplanetary coronal mass ejection (ICME) and its upstream sheath caused severe disturbances in the Earth's magnetosphere during 23-24 April 2023. The sheath anti-sunward of the fast ICME shock was composed of high-density plasmas and intense magnetic fields. This was followed by a plasma density rarefaction and intense magnetic fields with a field rotation (known as a magnetic cloud). This complex interplanetary structure resulted in a double-peak geomagnetic storm, and several intense auroral substorms. Solar wind kinetic energy transferred into the magnetosphere during the geomagnetic storm caused large Joule heating in the auroral ionosphere in both dayside and nightside of Earth. Compression of the magnetosphere by the shock/sheath caused losses of relativistic-energy electrons from the outer radiation belt at the beginning of the magnetospheric event. The equatorial ionospheric anomaly structure, characterized by a low plasma region on the geomagnetic equator and plasma enhancements on both sides (similar to +/- 10 degrees) of the equator, was significantly altered during the magnetic storm. In particular, the plasma density crests were more intense and expanded in hemispherical distribution. These variations are attributed to the prompt penetration electric fields to the equatorial ionosphere, which in turn modulated the equatorial ionospheric dynamics. These results should be important for prediction and modeling of geomagnetic storms and their impacts. The April 2023 double-peak geomagnetic storm is a classic event with a sub-Alfv & eacute;nic region located in the middle of a magnetic cloud Relativistic electrons displayed classic flux variations and dayside/nightside Joule heating was the dominant storm energy dissipation The near-equatorial ionospheric plasma responded to a prompt penetration electric field