1 Introduction
The subauroral ionosphere is the region where magnetic field changes
from stretched to more dipolar field like topology. The nightside
poleward part of the subauroral ionosphere is affected by electron
precipitation from the plasma sheet, whereas the equatorward part is
maintained by plasma from the plasmasphere. The ring current is the
energy source for the magnetospheric electron heat flux in the
subauroral region (e.g. Hoch, 1973; Rees and Roble, 1975; Kozyra et al.,
1987). During geomagnetically disturbed intervals, the heated electrons,
generated by the interaction between plasmasphere and ring current, fall
into the ionosphere at subauroral latitudes along magnetic field lines.
Further, the strong poleward electric field located equatorward of the
auroral oval produces latitudinally narrow regions of duskward plasma
flow at subauroral latitudes known as polarization jet (Galperin et al.,
1974) or SAID (subauroral ion drifts) (Smiddy et al., 1977; Spiro et
al., 1979). Thus, the subauroral ionosphere experiences a number of
highly dynamic processes related to convection electric field, coupling
of trapped energetic particles, and thermal plasma, causing the
formation of various types of optical features in this region (e.g.,
Frey, 2007, for a review). One of the most fascinating optical features
observed at the subauroral ionosphere is the ‘subauroral arc’. Unlike
typical auroral arcs, the subauroral arcs may not be always caused by
direct excitation from energetic particle precipitation (e.g.,
Gallardo-Lacourt et al. 2021, for a review).
Stable auroral red (SAR) arcs are most common type of subauroral arc and
have been studied extensively since their discovery in the 1950s
(Barbier, 1958). The red-line emissions of SAR arcs are caused by the
interaction of the inner edge of the ring current with the contracted
plasmasphere. The Coulomb collision is considered as a main mechanism
through which energy is transferred from ring current ions (several tens
of keV) to plasmaspheric electrons (energies less than 1 eV) (Cole,
1965; Kozyra et al., 1987). Energized thermal electrons can transport
energy down to the ionosphere via heat conduction or as a low-energy
electron flux (e.g., Cole, 1965), exciting atomic oxygen to the
O(1D) state. SAR arcs with weak green-line (557.7 nm)
emission have been reported by past studies (e.g., Mendillo et al.,
2016; Inaba et al., 2021). A newly recognized subauroral optical
structure, known as STEVE (strong thermal emission velocity
enhancement), has also grasped the attention of the space physics
community in recent years (e.g., MacDonald et al., 2018). STEVE is a
visible purple color arc, often occasionally accompanied by green ray
structure at the lower altitudes, known as picket fence.
Similar to SAR arc, STEVE is shown to be associated with enhancement in
temperature and westward ion drift
(MacDonald
et al., 2018). Both SAR arcs and STEVE have been reported to occur in
the recovery phase of substorms (Takagi et al., 2018; Gallardo‐Lacourt
et al., 2018). They are generated at or just inside the plasmapause
(Cornwall et al., 1971; Kozyra et al., 1997; Chu et al., 2019). Although
STEVE shows some similarities to SAR arcs in terms of their location and
occurrence, the two phenomena differ markedly from each other. While SAR
arcs are subvisual, spectrally pure red-line emission with little or no
green emission, and long-lived (10 hours or longer) (e.g. Nagy et al.,
1970), purple STEVE is visible to the naked eye, latitudinally narrow,
and short-lived (~around one-hour) (Gallardo‐Lacourt et
al., 2018). Unlike that of monochromatic SAR arcs (only 630.0 nm), STEVE
spectrum consists of a continuum spectrum spanning between
~400 and 730 nm (Gillies et al., 2019).
Recent studies have shown that common feature of both SAR arcs and STEVE
is equatorward arc detachment from the main auroral oval (e.g., Shiokawa
et al., 1999; 2009; 2017; Takagi et al., 2018; Gallardo‐Lacourt et al.,
2018; Yadav et al. 2021a). Using an all-sky imager (ASI) at Athabasca,
Takagi et al. (2018) and Yadav et al. (2021) reported that, initially,
SAR arcs and STEVE arcs appeared very close to the main auroral oval. As
time progressed, this arc separated itself from the oval and moved
equatorward when the main oval returned to higher latitudes. This
feature is referred as the “detachment of arc from the oval”. Anger et
al. (1978) and Moshupi et al. (1979) first used the term “detached
arcs” to describe the arc-like auroral features equatorward of the
auroral oval observed by the ISIS 2 satellite scanning photometer. The
ISIS-2 detached arcs displayed emissions in 391.4 nm (\(N_{2}^{+}\)) and
557.7 nm with no emission enhancement in 630.0 nm and occurred in the
afternoon sector, hence are referred as “afternoon detached arcs”. The
detached arcs in the midnight sector at the subauroral latitudes with
emission in both red- and green line (red+green arc) have been shown to
be associated with the low-energy particle precipitation from the plasma
sheet (Yadav et al., 2021b; under communication). Detached proton arcs
have also been observed in the subauroral region (Sakaguchi et al.,
2008; Ozaki et al., 2021; Zhou et al., 2021; and reference therein). The
proton arcs have been shown to be formed by the precipitation of
high‐energy ions caused by the interactions of electromagnetic ion
cyclotron (EMIC) waves with ring current ions (e.g., Sakaguchi et al.,
2008). On account of the generation mechanism of subauroral proton arcs,
we have not considered protons arcs in the present study.
In the past, individual statistical study of SAR arc and STEVE have been
carried out. For example, Gallardo‐Lacourt et al. (2018) performed a
statistical analysis for 28 STEVE events using Time History of Events
and Macroscale Interactions during Substorms (THEMIS) all-sky imager and
the Redline Emission Geospace Observatory (REGO) database that is, they
performed the statistical analysis of STEVE using multiple sites. Based
on 11 years of ASI observations at Athabasca, Takagi et al. (2018)
performed a statistical analysis of detached SAR arc. Note that in the
study of Takagi et al. (2018), weak 557.7-nm emission also accompanied
the SAR arc for some cases. A statistical study of subauroral arc with
simultaneous emission in red and green-line, hereafter referred as
red+green arc, remains unexplored. A detailed comparative statistical
study of geomagnetic conditions for different subauroral arcs at a
single station is also missing from the literature. Here, we present a
comparative statistical analysis on the detachment of red arcs,
red+green arcs, and STEVE with reference to solar flux, geomagnetic
activity, and magnetic local time (MLT) distribution. Such a comparative
study will not only enable disentangling the mysteries associated with
STEVE, but also shed light on the specific geomagnetic conditions under
which the arcs detached as red arc, red+green arc, and STEVE.