The distribution of the number of observed irregularities during each storm phase (a)–(d) and the percentage of occurrences of irregularities (e)–(h) during each storm phase is calculated from the numbers of irregularities divided by the numbers of GPS arcs. The blue bars indicate the initial phase, green bars indicate the main phase and the yellow bars indicate the recovery phase. The upper segment of each panel (a)–(d) represents the distribution during post-sunset (18:00–24:00 LT) and the lower segment (e)–(h) during post-midnight (00:00–07:00 LT).