3.2.1 AL index
Figure 5 shows the results of superposed epoch analysis of AL index from 3 hr before to 3 hr after the detachment of (a) red arcs, (b) red+green arc, and (c) STEVE. The vertical dashed line at zero hour represents the time of detachment of arcs as detected in the optical data. The average values defined every 1-min for red arc, red+green arc, and STEVE are shown by red, green, and purple curve, respectively. The grey lines show the standard deviation. The AL index began to decrease ~1 hr prior to the detachment of all arcs. Compared to the values 1-hr prior to the detachment, the AL-index exhibits a decrease of ~ 250 nT for STEVE, ~130 nT for red and red+green arc. The detachment of arcs occurred towards the end of expansion phase and beginning of recovery phase. Unlike red and red+green arc, the AL index for STEVE shows the presence of small perturbations around the end of expansion phase. In order to investigate this aspect in detail, we made the superposed epoch analysis using X-component magnetogram at FSMI, enable focusing on the detachment of arc with respect to auroral-zone substorm activity in the longitude of Athabasca ASI.