5 Conclusions
While individual statistics of SAR arc and STEVE have been reported in previous studies, here we present for the first time comparative statistical study of three types of subauroral arc from the same station—an aspect not yet addressed. We have used 15-years (2006-2020; covering two solar minimum) of ASI observation at Athabasca to perform a comparative statistical study of red arcs, red+green arcs, and STEVE. The most commonly observed arc was red arc (139 events), followed by red+green arc (42 events) and STEVE (26 events), an extremely rare phenomenon.
We found that the detachment rate of red arcs was high for the solar minimum and high geomagnetic activity years, displaying dependence on both solar flux and geomagnetic activity. The detachment rate of red+green arc was higher for moderate geomagnetic activity years and had minimal or no occurrence for low geomagnetic activity. The detachment rate of STEVE also maximized in solar minimum and high-geomagnetic activity year (2019). Given that auroral oval expands equatorward with increasing geomagnetic activity, the occurrence of all subauroral arcs was low during peak geomagnetic activity year (2016-2017), indicating that occurrence rate of arcs has a dependence on the location of observing station. The detachment rate of STEVE was higher in the premidnight sector, whereas red arcs and red+green arcs exhibit higher detachment rate around the midnight sector.
In order to provide insights on the specific geomagnetic conditions during STEVE, we performed a comparative superposed epoch analysis for various geomagnetic activity indices for red arcs, red+green arcs, and STEVE. Results reveal that STEVE occurred during relatively higher geomagnetic activity, the substorm activity was ~2-3 times higher for STEVE as compared to other arcs. The ASY-H index shows the prominent bay-like enhancement with a peak just prior to the detachment of STEVE, the bay-like enhancement was ~4 times higher for STEVE as compared to other arcs.
Majority of STEVE events were accompanied by the feature of dispersionless injection for both electron and proton flux at the geosynchronous orbit ~30 mins prior to the STEVE detachment. The feature of dispersionless injection for particle flux at the geosynchronous orbit was also observed for red arcs and red+green arcs. However, the geomagnetic activity indices show peculiar features for STEVE and not for other arcs. For example, i) both ASY-H index and X-component magnetogram at Boulder showed the presence of prominent positive bay-like enhancement before the detachment of STEVE, ii) the SYM-H index and X-component magnetogram at Athabasca showed small enhancement ~30 mins prior to the STEVE detachment, and iii) the X-component magnetogram at Fort Smith and AL index showed the presence of additional activities during the time of STEVE detachment. The development of STEVE occurred at the end of substorm expansion phase and significant intensification of asymmetric ring current in the dusk sector.