Example of |E| exceedance data showing time-dependent variation and short-term clustering. (a) Annual relative probability (C_i/N) of |E| exceedances where C_i is the number of elements in each year’s bin and N is the total number of elements (blue bars), compared to the monthly mean total sunspot number (red line) between 2000 and 2018. (b) Time series of exceedances from 11 October 2016 to 07 November 2016; the horizontal blue dashed line corresponds to the threshold. Both plots correspond to |E| data at the latitude of station Muonio (68.02°N) and central longitude in Sweden (20.52°E) at the station latitude, which exceeds the 99th percentile threshold of 0.12 V/km.