A Study of the Possible Mechanism of the Ground Level Enhancement on 28 October 2021

We have analyzed the particle acceleration processes and energy spectra for the ground level enhancement (GLE) event on 28 October 2021 using the in situ data registered by ground-based and space-borne instruments. It is found that the onset of the soft X-ray flare component (1 – 8 Å) lies close to the onset of the GLE (≥1 GeV) and high-energy (>500 MeV) proton fluxes, indicating that the GLE event might have been initiated by the flare acceleration process. It is observed that the coronal shock traced in m-Type II burst that operates over the flare acceleration phase plays a role on the impulsive phase of high energy particles. However, the coronal shock traced in DH-Type II burst that operates over the coronal mass ejection (CME) propagation phase plays a more important role in the reacceleration of the protons of low energy (≤80 MeV) than those of high energy (>80 MeV), thus supporting previous suggestions that the CME shock prolongs the temporal evolution of low-energy proton fluxes. The finding is supported by the analyses of proton energy spectra. For instance, the peak energy spectrum after the flare follows a double power-law, exhibiting that the break energy appeared at ∼80 MeV whilst the peak spectrum of the low-energy proton fluxes is harder than that of the high-energy proton fluxes.