Onset diagnostics for the 3-minute time-averaged data of STEREO-A / SEPT 85–105 keV electrons on the 2021-04-17 SEP event. The corresponding onset time plot is displayed in Fig. 1. The figure shows the CUSUM function (dark red curve) along with its arguments: z-standardized intensity (green), k (dashed black line), and h (dash- dotted black line). The figure also shows, just like the standard plot without diagnostics, the mean of the background (horizontal dashed red line) and the µ + nσ line, where n = 2 (horizontal dotted red line).