3.2.4 SYM-H index
Figure 8 shows the superposed epoch analysis for the SYM-H index. Takagi et al. (2018) reported no clear change in the SYM-H index before or after the detachment of SAR arcs. Similarly, the SYM-H index shows no change before and after the detachment of both red and red+green arcs. Unlike other arcs, the SYM-H index exhibits slight variations before and after the detachment of STEVE. The SYM-H index shows the hump of slight enhancement just prior to the detachment of STEVE. The hump is followed by a gradual decrease with the minimum values occurred 1 hr after the detachment of STEVE. The minimum value of the SYM-H index remains near -15 nT for red arcs, -20 nT for red+green arcs, and -25 nT for STEVE events. This indicates that STEVE likely to occur during relatively geomagnetically disturbed periods with more developed ring current as compared to red and red+green arcs.