Magnetic interaction analysis of multiple interplanetary coronal mass ejections that led to a historic geomagnetic storm in May 2024

Interplanetary coronal mass ejections (ICMEs) are large-scale eruptive phenomena capable of shedding a huge amount of solar magnetic helicity and energy and can potentially drive strong geomagnetic storms. They complexly evolve while preceded and followed by other large-scale structures, such as ICMEs. Magnetic interaction among multiple ICMEs may result in intense and long-lived geomagnetic storms. Our aim is to understand the reason for the substantial changes in the geoeffectivity of two meso-scale separated counterparts of a complex solar wind structure by investigating their magnetic content, helicity, and energy as well as their magnetic interaction among multiple ICMEs. We utilized the in situ observations of solar wind from the Wind and the Solar Terrestrial Relations Observatory-A (STA) spacecraft during the strongest geomagnetic storm period in past two decades on May 10-11, 2024. We performed heliospheric imaging analysis to locate the solar sources, investigated the interplanetary propagation and Earth-arrival of the driver, and performed a time-frequency domain analysis of the in situ magnetic field vectors in injection and inertial ranges of magnetohydrodynamic (MHD) turbulence to quantify the driver's magnetic content at two counterparts. Our investigation confirms complex interactions among five ICMEs resulting in distinct counterparts within a coalescing large-scale structure. These counterparts possess substantially different magnetic content. We conclude that the STA-observed complex counterpart resulted from the interaction among common-origin ICMEs favorably orientated for magnetic reconnection, had a 1.6 and 2.8 times higher total magnetic energy and helicity, respectively, than the Wind-observed counterpart that included a left-handed filament-origin ICME. The left-handed ICME non-favorably oriented for magnetic reconnection with the surrounding right-handed common-origin ICMEs at Wind. Therefore, despite belonging to a common solar wind structure, two medium-separated counterparts had the potential to lead to different geoeffectivity. This ultimately challenges space weather predictions based on early observations.