Longitudinal spacecraft constellation and magnetic connectivity at 16:00 UT on 17 April 2021 (left) together with multi-spacecraft SEP observations (right). The upper panel shows ∼1 MeV electron intensities and the lower panel ∼25 MeV proton intensities (respectively ions) observed by the spacecraft, indicated by the same colors in the left figure. The black arrow marks the longitude of the associated flare (identified using Solar Orbiter STIX measurements as described in Sect. 4.1), and the dashed black spiral denotes the nominal magnetic field line connecting to this location. The observers’ configuration plot (left panel) was produced using the Solar MAgnetic Connection HAUS (Solar-MACH; Gieseler et al. 2023) tool.