Figure 2: Typical NS keogram for red arc (Event 1), red+green
arc (Event 2), and STEVE (Event 3).
For arc detachment statistics, first, the feature of arc detachment was
identified from NS keograms by visual inspection. Then, we checked
all-sky images to ensure that the features picked up from the keograms
were east-west elongated arcs detaching from the main auroral oval. Only
those arcs are considered which persisted at least 30 mins after their
detachment from the main auroral oval. STEVE consists of continuum
spectrum from 400-730 nm (Gilles et al., 2019). Therefore, although
Figure 1 and 2 shows the STEVE emissions only in red- and green-line,
the presence of STEVE is identified by using the images at all six
filters including the “nominal background”, except for the OH band, as
described in Yadav et al. (2021a). The primary objective of this study
is to examine the detachment rate. So, cases in which detachment was not
observed were not considered. Since STEVE is a rare event, we have
considered STEVE events even when the detachment was ambiguous.
Given the fact that the ASI measurements have limitation as observations
cannot be made if the sky is cloudy, only those nights are considered
for which at least 4-hour observation was present. After this
restriction, the number of nights available to us for identifying the
arc detachment were reduced to 948. In 948 nights of observations and
6376 hours of data, we identified 139 red arcs, 42 red+green arc, and 26
STEVE events. Note that Takagi et al. (2018) analyzed the total
occurrence (from the detachment until the arc remained detectable in the
ASI images) of SAR (red) arcs, whereas the present study is focused
solely on the detachment rate of red arcs, red+green arcs, and STEVE.
STEVE does not invariably display the feature of detachment from the
main auroral oval. Occasionally, STEVE is found to emerge at the
equatorward boundary of diffuse emissions that had already expanded
equatorward from the main auroral oval. Out of 26 STEVE events, the
detachment from the main auroral oval was observed for 13 events. For 4
events, the feature of detachment was not clear either because of the
absence of images or presence of clouds. For the remaining 8 events,
STEVE developed at the equatorward boundary of the intense diffuse
emission that expanded equatorward from the main auroral oval. Two
examples of such events are shown in supplementary Figure S1.