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

Abstract

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 INTRODUCTIONGlowing gases in the Earth's upper atmosphere provide dynamic tracers of the pathways energy takes from high to low altitudes as the Earth interacts with the near-space environment. The behaviors of these glowing regions in space and time offer definite clues to the high-altitude energy source, to the energy transport pathways, and to the manner in which the atmosphere adjusts to the additional energy influx. In general, atmospheric emissions are the end result of a series of physical processes that connect the Earth's atmosphere to the external near-space environment. The understanding of these natural systems has undergone major advances over the last decade, as measurements, by spacecraft within and above the emission regions and observations from ground-based optical and radar instruments, provide a fertile proving ground for theoretical models. In this review, recent developments in our understanding of the stable auroral red (SAR) arc system are described. Over the last decade, spacecraft missions and accumulating long-term ground-based observations of SAR arcs have led to revised views on their formation and variability. SAR arcs are glowing red emissions that appear at high altitudes in the midlatitude night sky, extending in diffuse, sometimes continuous, bands from the dusk to the dawn terminators. They appear simultaneously in both the northern and southern hemispheres. The dayside portion of the SAR arcs is difficult to detect above the much stronger background dayglow emissions. SAR arcs occur on approximately 10-12% of nights throughout the solar cycle, with the preponderance being near the period of maximum solar activity. However, the

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“…They were reported as being observed in all directions, including the South, and to be red (Yau et al 1995) which makes them unfavorable candidates for classical aurorae. By their features these could have been stable auroral red (SAR) arcs (Zhang 1985) which in modern times are seen at mid-latitudes mainly during the recovery phase of geomagnetic storms (Kozyra et al 1997). These arcs are mainly driven by the ring current and differ from normal auroral phenomena.…”

An average image of proton plasma pressure and of current systems in the equatorial plane derived from AMPTE/CCE‐CHEM measurements

“…Furthermore, the total energy, contained in the ring current particles, which is of the order of 1023 erg, is sufficiently large to cause appreciable atmosphere ionization or heating if released in a short time. Many effects, like subauroral electron temperature enhancements [Brace et al, 1967], stable auroral red arcs (SAR) [Kozyra et al, 1987[Kozyra et al, , 1997 and energetic neutral and ion precipitating fluxes [Dessler et al, 1961], are observed at midlatitudes as consequences of this energy release. Moreover, the ring current can also act as an energy source contributing to plasma instabilities and wave growth.…”

Ring Current Energy Input and Decay

“…The upper atmosphere also provides evidence for a partial ring current during storms, with Craven et al (1982) (and references therein) showing that stable auroral red (SAR) arcs are primarily limited to the dusk-to-midnight sector. In their review of SAR arcs, Kozyra et al (1997a) concluded that the Coulomb interaction between the thermal electrons and the ring current is responsible for the magnetospheric heat flux that produces SAR arcs. From simulations, Liemohn et al (2000) showed that the local time asymmetry of the partial ring current matches the SAR arc observations.…”