Authors: Ji-Hyeon Yoo (Korea Astronomy and Space Science Institute, Chungbuk National University), Ryun-Young Kwon (Korea Astronomy and Space Science Institute), Dae-Young Lee (Chungbuk National University)

The onset of the solar energetic particle (SEP) event on 2022 June 13 was observed to be delayed by about 10 hours relative to the time of the associated solar event—an M3.4 class flare accompanied by a halo coronal mass ejection (CME). In this study, we have examined the cause of the onset time delay of this SEP event and its observational characteristics. We confirmed the absence of an Extreme Ultraviolet (EUV) wave from Solar Dynamics Observatory (SDO) and Solar TErrestrial Relations Observatory-Ahead (STEREO-A). In contrast, coronagraph images of Solar and Heliospheric Observatory (SOHO) and STEREO-A revealed a distinct CME-driven shock, appearing as a halo front. Based on these observations, we suggest that the CME-driven shock capable of accelerating particles developed only after the CME had propagated to a further distance. Furthermore, by deriving the shock compression ratio based on a three-dimensional ellipsoid model, we identified that the strong shock became magnetically connected to the Parker spiral lines of STEREO-A and Lagrangian point 1 (L1) in a higher corona region. These findings likely serve as evidence explaining the delayed onset of the SEP event. Additionally, notable differences were found in the in-situ solar wind observations at 1 AU, with STEREO-A and L1 separated by ~28 degrees in heliolongitude. Compared to the observations at L1, STEREO-A showed clearer increases in energetic proton and electron fluxes, as well as much larger variations in solar wind speed, density, and magnetic field strength during the shock passage. It implies that STEREO-A encountered the shock’s leading front, while the L1 point intersected its flank. This interpretation is further supported by the clear detection of energetic storm particles (ESPs) at STEREO-A, where particle acceleration likely occurred in a region with higher Mach numbers. Through this study, we provide insights into the origin of substantially delayed SEP events and present an approach for their detailed analysis.