Solar energetic electron event on 9 October, 2021. The right panel shows a view of the heliographic equatorial plane from the north, in the Stonyhurst coordinate system, and the spacecraft constellation on 9 October 2021 at 06:30 UT. The different coloured squares indicate the location of all observing spacecraft, namely, L1 (Earth, green), STEREO-A (red), Solar Orbiter (blue), and PSP (purple). The nominal Parker spiral (curved line) from the Sun to each spacecraft is shown for each observer. The small back arrow and the dashed black line indicate the flare location and the reference Parker spiral. The large black arrows help locate the in situ omnidirectional electron recordings at Solar Orbiter and STEREO-A, which are presented in the two rectangular panels on the left. Note that Solar Orbiter/STEP and STEREO-A/HET data are not omnidirectional. STEP covers roughly the same field of view as EPT’s ‘Sun’ telescope, e.g., looking along the nominal Parker spiral towards the Sun, while HET is pointing almost perpendicular to the nominal Parker spiral direction in the ecliptic plane (see, Sect. 4.4.1). The grey shaded region indicates the extent in which energetic particles were observed.