Viewing directions of NMs in the GSE coordinates at around the onset of GLE73 at 15:50 UT on 28 October 2021. Geomagnetic conditions were slightly disturbed (Kp = 1.0). The colour lines, acronyms, and numbers depict the asymptotic directions, NM stations, and rigidities in the range 1–5 GV, respectively. The line of equal pitch angles relative to the anisotropy axis is plotted for 30° for the sunward direction (solid black line) and for 150° for the anti-sunward direction (dashed black line).