Fig. 4
From: Geomagnetic storms of cycle 24 and their solar sources
Top panel is the yearly SSNs (red line) with the 13-month smoothed monthly SSNs (black line). The second, third, and forth panels are the yearly occurrence rates of |E d–d| > 5 mV/m, of E d–d > 5 mV/m, and of E d–d < −5 mV/m in the 1-year data. The bottom panel is yearly occurrence ratios of E d–d < −5 mV/m to |E d–d| > 5 mV/m