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.