Solar proton energy spectra observed during the evolution of the solar energetic particle event dated October 28, 2021. The dot-dashed line indicates the onset of the event (15:35–16:35 UT). The dotted line corresponds to the peak of the event observed on ground (17:30–18:20 UT) and the dashed line represents the peak of the event in space (20:35–22:35 UT). Low-energy EPD/HET data below 100 MeV and NM observations above 700 MeV have been interpolated between 100 MeV and 700 MeV according to Grimani et al. (2013, red lines).