On some features of the solar proton event on 2021 October 28 - GLE73

In addition to several recent articles devoted to the rare event of ground-level enhancement of the solar relativistic proton flux observed on 2021 October 28 - GLE73, we study the 10-100 MeV solar energetic particle (SEP) component of this event. Based on the Geostationary Operational Environmental Satellite data for 26 GLEs recorded since 1986, we have formed a scatter plot displaying the ratio of the peak fluxes of the $\gt $10 MeV ($J_{10}$) and $\gt $100 MeV ($J_{100}$) protons and their energy spectra. Two extreme characteristics of the prompt component of the SEP-GLE73 event were revealed: (1) very small $J_{10}$ and $J_{100}$ proton fluxes and (2) a very hard energetic spectrum in the 10-100 MeV range. There are only two events with these characteristics similar to SEP-GLE73, namely GLE40 (1989 July 25) and GLE46 (1989 November 15). A correspondence was demonstrated between the hard frequency spectrum of microwave radio bursts of initiating flares and the hard SEP energy spectrum of these two and other GLEs. These results suggest that the flare magnetic reconnection both in the impulsive and post-eruption phases plays an important role in the acceleration of the SEP-GLE protons.