3.2 Superposed epoch analysis
In order to understand the specific geomagnetic activity during which
subauroral arcs detached as red arc, red+green arc, and STEVE, we
perform the superposed epoch analysis for different geomagnetic activity
indices from 3 hr before to 3 hr after the detachment event. Note that
the superposed epoch analysis of AL-index and SYM-H index for STEVE
(Gallardo-Lacourt et al., 2018) and SAR arc (Takagi et al., 2018) has
been already reported. These studies highlighted that both STEVE and SAR
arc occurred at the end of expansion phase or just at the beginning of
recovery phase. In the following subsections, we compare the results of
superposed epoch analysis of different geomagnetic activity indices for
three different types of arcs.