Evolution of the interacting coronal mass ejections that drove the great geomagnetic storm of 10 May 2024

Abstract

Context. The arrival of a series of coronal mass ejections (CMEs) at the Earth resulted in a great geomagnetic storm on 10 May 2024, the strongest storm in the last two decades. Aims. We investigated the kinematic and thermal evolution of the successive CMEs to understand their interaction en route to Earth. We attempted to find the dynamic, thermodynamic, and magnetic field signatures of CME-CME interactions. Our focus was to compare the thermal state of CMEs near the Sun and in their post-interaction phase at 1 AU. Methods. The 3D kinematics of six identified Earth-directed CMEs were determined using the graduated cylindrical shell (GCS) model. The flux rope internal state (FRIS) model was implemented to estimate the CMEs' polytropic index and temperature evolution from their measured kinematics. The thermal states of the interacting CMEs were examined using in situ observations from the Wind spacecraft at 1 AU. Result Our study determined the interaction heights of selected CMEs and confirmed their interactions that led to the formation of complex ejecta identified at 1 AU. The plasma, magnetic field, and thermal characteristics of magnetic ejecta (MEs) within the complex ejecta and other substructures, such as interaction regions within two MEs and double flux rope-like structures within a single ME, show possible signatures of CME-CME interaction in in situ observations. The FRIS-model-derived thermal states of individual CMEs reveal their diverse thermal evolution near the Sun, with all CMEs transitioning to an isothermal state at 6–9 R⊙ except for CME4, which was in an adiabatic state due to a lower expansion rate. The electrons of the complex ejecta at 1 AU are in a predominant heat-release state, while the ions show a bimodal distribution of thermal states. On comparing the characteristics of CMEs near the Sun and at 1 AU, we suggest that such a one-to-one comparison is difficult due to the CME-CME interactions significantly influencing the CMEs' post-interaction characteristics.