Magnetic flux systems involved in the May 2024 solar energetic events from AR 13664 inferred through coronal dimmings

Abstract

Coronal dimmings are transient depletions of coronal plasma observed in extreme ultraviolet and soft X-rays, interpreted as low-corona signatures of coronal mass ejections (CMEs). Their evolution is closely linked to CME dynamics, flare reconnection, and large-scale coronal magnetic restructuring. In May 2024, active region (AR) 13664 produced 66 > M-class flares and multiple fast CMEs that caused the strongest geomagnetic storm since 2003. We analysed 16 on-disc dimmings observed in SDO/AIA 211Ådata between May 1-14, 2024. We extracted dimmings using logarithmic base-ratio thresholding, and derived magnetic properties from SDO/HMI line-of-sight magnetograms. We identified flare ribbons in AIA 1600Ådata via adaptive thresholding, and computed reconnection fluxes from radial magnetic field maps. We examined the magnetic flux systems involved in the eruptions using PFSS and NLFF extrapolations. We found correlations between dimming and flare ribbon properties stronger than previously reported. The dimming morphology evolved systematically, with southward expansion before May 9 and northward afterward, coinciding with a shift in flare ribbon locations between two mayor east-west polarity inversion lines (PILs). These changes imply the presence of two distinct magnetic domains. The PFSS extrapolations showed that southward (northward) dimmings are mainly strapping flux dimmings with magnetic field lines vaulting above the southern (northern) PIL. The final extent of the dimmings was then given by the exterior flux involved in the eruption via stretching and reconnection. One event revealed an extended quiet-Sun dimming likely triggered by EUV wave-induced field opening.

Figures (8)

• Figure 1: Overview of the evolution of the X2.2 flare and associated dimming on May 9, 2024, at 08:45 UT. (a) SDO/AIA 211 Å direct images of AR 13664 at the flare’s end time, with the dimming region detected up to that point outlined in cyan contours. (b) Corresponding logarithmic base-ratio image. (c) Timing map indicating the first detection time in minutes of a dimming pixel. (d) Minimum intensity map from logarithmic base-ratio data. The associated movie is available online.
• Figure 2: (a) Comparison of the magnetic dimming area ($A_\phi$) against the flare ribbon area ($A_{rbn}$). (b) Total unsigned magnetic dimming flux ($\phi$) compared to the reconnection magnetic flux ($\phi_{rbn}$). Blue markers represent dimmings from the May 2024 events (crosses mark X-class flares and dots M-class flares), while grey crosses correspond to dimming events from Dissauer2018b. The black (blue) regression lines were exclusively fitted to the grey (blue) data points.
• Figure 3: Timing maps of all coronal dimming events overlaid on HMI LOS magnetograms taken 15 min before the associated flares. The colour of the shading indicates the first detection time of the dimming region in minutes after the flare onset. The flare ribbons detected before the flare end time are shaded in orange. The number indicated in the upper left corner of each subplot is the event number on Table \ref{['table:dimming_properties']}.
• Figure 4: Overview of the coronal dimming and flare ribbon morphology for event no. 11 (X2.2 flare, May 9 08:45 UT; top row) and event no. 13 (X1.1 flare, 9 May 17:23 UT; bottom row). Left column: AIA 1600 Å images at the flare peak. The magnetic field regions $|B|\ge 250$ G are shaded in blue (positive) and red (negative). Middle column: AIA 211 Å images at the same time step as in the left column with detected instantaneous dimming contours (cyan). Right column: HMI magnetograms overlaid with the final dimming regions (cyan) and flare ribbons (orange). Red lines show the two main PILs, and the yellow line shows a separatrix layer extracted from squashing factor maps by Jarolim2024magnetic. The associated movie is available online.
• Figure 5: Magnetic field structures rooted in the coronal dimmings of event no. 11 overlaid on an HMI radial magnetogram taken on May 9 at 08:36 UT. Coronal dimming regions are shaded in blue and flare ribbons in orange. Black and yellow arrows point to the flare ribbons. (a) Subregions of dimmings contoured in purple, cyan, and green and denoted as D1-D3. (b) All magnetic field structures from the PFSS extrapolation from D1 in purple, D2 in cyan, and D3 in orange and red. Grey field lines show open flux near the AR. (c) and (d) Field lines sampled from NLFF extrapolations showing the flux rope (panel a, colour refers to local current density) and the field lines connected to the flare ribbons (panel d, cyan).