High‐altitude energy source(s) for stable auroral red arcs

Abstract

Stable auroral red (SAR) arcs have been viewed across the midlatitude night sky with interest since their discovery in 1956. This relatively late discovery (compared to the poleward aurora) is a direct consequence of the subvisual levels of the delicate and diffuse light that makes up the SAR arc. Except in rare instances when the emissions actually crossover the threshold to visible levels, optical instruments are required to register their presence and document their morphology and occurrence frequency. SAR arcs are seen as relatively featureless, slowly changing bands of red light that can extend across the entire night sky. Early observations from the ground and from satellites established the relationship between SAR arcs and magnetic disturbances in near‐Earth space involving the ring current (a population of high‐energy ions trapped in the Earth's magnetic field). The long‐lived, soft, red glow of SAR arcs reflects the slow energy loss from the ring current ions as they bounce back and forth in the Earth's confining field geometry, but the exact sequence of physical processes that feed a portion of the ring current energy to the SAR arc region is a matter of continuing debate. The midlatitude location of the SAR arc bands mirrors the high‐altitude location of the outer portion of the ring current torus, but exactly how this positioning is related to gradients or enhancements in the high‐altitude extension of the cold ionospheric plasma remained an open question. Our knowledge of SAR arcs and associated signatures now spans more than two full solar cycles. Detailed observations of the high‐altitude space environment overlying the SAR arc region have been made, indicating that heavy ions from the lower atmosphere, energized by the solar wind interaction with the magnetosphere, play an important role in the SAR arc process. Sophisticated models of the coupled high‐ and low‐altitude regions, involved in SAR arc formation, paint an interesting picture of the flow of energy between the atmosphere and near‐Earth space. This energy flow culminates in the SAR arc's glowing red bands, which act, in turn, as sensitive markers of the interaction regions and processes. This emerging view of the SAR arc process and the new observational information that provided the underpinnings of its development from a rich framework of past theoretical and observational work is the subject of the present review.