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.