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.