Quasi-planar ICME Sheath: A Cause of the First Two-Step Extreme Geomagnetic Storm of the 25th Solar Cycle Observed on 23 April 2023.

Interplanetary Coronal Mass Ejections (ICMEs) are prominent drivers of space weather disturbances. The reported studies suggested that the planar ICME sheath and planar magnetic clouds (MCs) cause extreme geomagnetic storms. Here, we investigated the first two-step extreme geomagnetic storm (Sym-H∼-231 nT) of 25th solar cycle. Our analysis demonstrates the transformation of ICME sheath into quasi-planar magnetic structures (PMS). The study corroborates our earlier reported finding that the less adiabatic expansion in possible quasi-PMS transformed ICME enhanced the strength of the southward magnetic field component. It contributes to the efficient transfer of plasma and energy in the Earth’s magnetosphere to cause the observed extreme storm. We found that the magnetosphere stand-off distance reduced to <6.6RE (where RE stands for the Earth’s radius), and it impacts geosynchronous satellites moving close to 6RE.