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.