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.