Daikou Shiota

In early May 2024, active regions (ARs) 13663 and 13664 appeared on the solar disk and produced 17 X-class flares. The associated coronal mass ejections caused geomagnetic disturbances, named the Gannon Storm, resulting in significant societal impacts. To investigate why these regions generated successive X-class flares, we conducted detailed analyses of nine X-class flares that occurred near the solar disk center using data from the Solar Dynamics Observatory and post-event analysis based on the κ-scheme, a flare prediction scheme utilizing nonlinear force-free field modeling. Comparative analysis was also performed with ARs 12192 and 12673, which produced multiple X-class flares during the previous solar cycle 24. Our analysis showed that AR 13664 maintained flare-favorable conditions through the emergence of multiple twisted magnetic flux systems and sustained flux injection over 11 days, which continuously increased the releasable magnetic energy (Er) and preserved small critical radii (rc) conducive flare onset reconnection. AR 13663 also produced multiple flares due to strong magnetic shear and a rapid flux injection episode lasting 2.5 days, which led to the rapid growth of flare-triggering magnetic structures. All nine flares originated under the conditions of rc < 1.1 [Mm] and Er > 1.9x1030 [erg], which we propose as new threshold values for X-class flare prediction using the κ-scheme. Additionally, some flares were triggered by small-scale reconnection outside strong shear regions, propagating along the magnetic polarity inversion line like a fuse and activating larger energy release. These findings highlight the importance of monitoring both strong and weak shear regions for effective flare forecasting. This study demonstrates the potential to contribute to the development of a preemptive and stepwise warning system for large flares based on the κ-scheme, by enabling future statistical analyses of a broader set of events—including M-class flares—to derive threshold values corresponding to flare magnitudes.

In gradual Solar Energetic Particle (SEP) events, shock waves driven by coronal mass ejections (CMEs) play a major role in accelerating particles, and the energetic particle flux enhances substantially when the shock front passes by the observer. Such enhancements are historically referred to as Energetic Storm Particle (ESP) events, but it remains unclear why ESP time profiles vary significantly from event to event. In some cases, energetic protons are not even clearly associated with shocks. Here, we report an unusual, short-duration proton event detected on 5 June 2011 in the compressed sheath region bounded by an interplanetary shock and the leading edge of the interplanetary CME (or ICME) that was driving the shock. While < 10 MeV protons were detected already at the shock front, the higher-energy (> 30 MeV) protons were detected about four hours after the shock arrival, apparently correlated with a turbulent magnetic cavity embedded in the ICME sheath region.