Coronal dimmings as diagnostics of the May 2024 solar energetic events

Coronal dimmings are regions of transiently reduced extreme ultraviolet (EUV) and soft X-ray (SXR) emission caused by plasma evacuation during the liftoff of coronal mass ejections (CMEs). As such, they serve as powerful diagnostics of CME initiation and early evolution. In May 2024, active region (AR) 13664 was among the most flare productive regions in recent decades, producing 54 M-class and 12 X-class flares. The rapid sequence of Earth-directed CMEs from AR13664 triggered the most intense geomagnetic storm in two decades. We present a two-part analysis of the coronal dimmings from AR 13664 associated with the May 2024 storms.

First, we systematically identify and analyse 22 dimming events (16 on-disc and 6 off-limb) and their characteristic parameters using SDO/AIA observations. We find that the dimming area, growth rate, and magnetic flux strongly correlate with GOES flare peak flux, fluence, and flare reconnection flux. These correlations are stronger than those found in previous statistical studies, highlighting the tight coupling between flares and dimmings. However, we find no correlation between dimming properties and CME maximum speed derived from SOHO/LASCO coronagraph measurements, suggesting that dimmings are more closely linked to the early-stage CME evolution rather than their later propagation. 

Second, we investigate the morphology and spatial evolution of the 16 on-disc dimmings in relation to flare ribbon location and coronal magnetic field structures. We employ high resolution PFSS and NLFF extrapolations alongside the dimming morphology to identify which magnetic structures are involved in the eruptions and how they participate in them. By considering the dimming expansion direction and the flare ribbon location, we identify two distinct magnetic domains associated with different polarity inversion lines. We also relate the dimming expansion, together with the orientation of the flare ribbons along the PILs, to the different geoeffectiveness of the associated CMEs.

These results underscore the extensive potential of coronal dimmings to characterise solar eruptions and understand the physical processes behind them.