Comparative analysis of two episodes of strongly geoeffective coronal mass ejection events in November and December 2023

Context. In autumn 2023 a series of close-in-time eruptive events were observed remotely and measured in situ. For that period, we studied a set of analogous events on the Sun, where several coronal mass ejections (CMEs) were launched partly from the same (active) regions close to a coronal hole. The two episodes of events are separated by a full solar rotation covering the period October 31–November 3 and November 27–28, 2023. Aims. The two episodes of eruptive events are related to strong geomagnetic storms occurring on November 4–5 and December 1–2, 2023. We point out the complexity for each set of events; our aim is to understand how the global magnetic field configuration, solar wind conditions, and interaction between the structures relate to these geomagnetic effects. Methods. We used the graduated cylindrical shell (GCS) 3D reconstruction method to derive the direction of motion and speed of each CME. The GCS results served as input for the drag-based model with enhanced latitudinal information (3D DBM), facilitating the assessment of its connection to in situ measurements. This approach significantly aids in the integrated interpretation of in situ signatures and solar surface structures. Results. The first episode caused visible stable auroral red (SAR) arcs, with a Dst index that dropped in three steps down to −163 nT on November 5, 2023. Close in time, two CME-related shocks arrived, separated by a sector boundary crossing (SBC), and followed by a short-duration flux rope-like structure. For the second episode, auroral lights were observed related to a two-step drop in the Dst index down to −108 nT on December 1, 2023. A shock from a CME within the magnetic structure of another CME ahead was identified, again combined with a SBC. Additionally, a clear flux rope structure from the shock producing CME was detected. In both events, we observed distinct short-term variations in the magnetic field (“ripples”) together with fluctuations in density and temperature that followed the SBC. Conclusions. The study presents a comparative analysis of two episodes of multiple eruptive events in November and December 2023. In addition to the interacting CME structures, we highlight modulation effects in the geomagnetic impact due to magnetic structures that are related to the SBC. These most likely contributed to the stronger geomagnetic impact and production of SAR arcs for the November 4–5, 2023, event. At the Sun, we found the orientation of the heliospheric current sheet to be highly tilted, which might have caused additional effects, due to the CMEs interacting with it.