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.