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.