Figure 1: Typical all-sky images of red arc on September 17,
2017 (arc in 630.0 nm with no corresponding emission in 557.7 nm; upper
panel A), red+green arc on May 14, 2015 (emission in both 630.0 nm and
557.7 nm; middle panel B), and STEVE on March 11, 2010 (bright arc in
630.0 nm and picket fence in 557.7 nm; bottom panel C).
Figure 2 shows a typical example of NS keogram for red-line (630.0 nm)
and green-line (557.7 nm) for red arc (Event 1; upper panel A),
red+green arc (Event 2; middle panel B), and STEVE (Event 3; bottom
panel C). Before the detachment of all arcs, the high-latitude aurora
expands equatorwards. When the oval aurora returned to higher latitudes,
an arc detached itself from the main auroral oval and moved further
equatorward. This phenomenon is referred as detachment of arc from the
main auroral oval as described by Takagi et al. (2018) for SAR arc and
Yadav et al. (2021a) for STEVE. For red arc (Event 1), this detachment
was observed at ~0740 UT in the red-line keogram.
Although the equatorward extension of oval aurora is observed in the
green-line keogram, arc detachment did not occur. For red+green arc
(Event 2), the detachment of arc is observed in both the red-line and
green-line keogram at ~0645 UT. A narrow and bright arc
structure detaching itself from the main auroral oval at
~0615 UT is STEVE in the red-line (Event 3). By
contrast, red arc appeared diffuse and wide (Event 1). For STEVE (Event
3), although 557.7 nm image shows the picket fence structure, the
detachment of arc at ~0615 UT is not observed, like in
the red-line.