Statistical study of subauroral arc detachment at Athabasca, Canada: new
insights on STEVE
Abstract
We present the first comparative statistical study of subauroral arc
detachment from the main auroral oval at Athabasca (magnetic latitude =
61.5°N), Canada, for three different types of subauroral arcs: pure red
arc, red arc with simultaneous emission in green-line (red+green arc),
and STEVE (strong thermal emission velocity enhancement). Based on
15-years (2006-2020) of all-sky imaging observations, this study not
only uncovers the occurrence characteristics of different arcs but also
provides important insights into the specific geomagnetic conditions
under which STEVE develops. Red arc was the most common subauroral arc
(139 events), followed by red+green arc (42 events), and STEVE (26
events) was a rare phenomenon. The detachment rate of red and red+green
arcs exhibits dependence on both the solar flux and geomagnetic
activity. The detachment rate of STEVE was higher during premidnight,
whereas red and red+green arcs were higher around the midnight sector.
The geomagnetic activity was relatively higher for STEVE, the decrease
in the AL index and local X-component magnetic variations were
~2-3 times higher for STEVE as compared to other arcs.
STEVE shows a strong association with asymmetric ring current in terms
of prominent bay-like enhancement in ASY-H index prior to the STEVE
detachment. Such bay-like enhancement was ~4 times
higher for STEVE as compared to other arcs. STEVE events were
accompanied by dispersionless injection for both electron and proton
flux at the geosynchronous orbit. These results unambiguously suggest
that STEVE develops after the substorm associated energy injection and
significant intensification of asymmetric ring current.