Representative example of the solar associations for the 28 October 2021 > 100 MeV SEP event (No 137) corresponding to GLE73. Panel (a): ERNE proton counts at > 100 MeV. Panel (b): Soft X-ray flux (1–8 Å, GOES, blue) and high-energy flare emissions, including hard X-rays (Solar Orbiter/STIX, 4–10 keV, purple) and γ-rays (Fermi/LAT, > 100 MeV flux, pink). Panel (c): Radio dynamic spectrum (e.g. Wind/WAVES) with over-plotted CME height–time profile from coronagraph observations (SOHO/LASCO). Panel (d): Ground-level neutron monitor data showing relativistic particle increases.