3.2.5 ASY-H index
Figure 9 shows the superposed epoch analysis for the ASY-H index. In general, the ASY-H index began to increase gradually ~1 hour before the arc detachment for all arcs. For STEVE, the ASY-H index showed an abrupt enhancement ~30 mins before the detachment, exhibiting a prominent bay-like structure. The ASY-H index shows an enhancement of 20 nT for STEVE and 5 nT for red and red+green arc. The detachment of arcs occurred immediately after the peak in ASY-H index for all arcs. The maximum magnitude of ASY-H index was ~45 nT for STEVE, 25 nT for red and red+green arc. Thus, bay-like enhancement in the ASY-H index is significantly prominent for STEVE as compared to red and red+green arc. Although the magnitude of ASY-H index for red arc and red+green is same, the bay-like feature of ASY-H appeared prominently for red arc in comparison to red+green arc. The prominent bay-like ASY-H increase for STEVE probably indicates the connection of asymmetric ring current to the generation of STEVE during the substorm expansion phase.