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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SAR arc statistics provide information on the seasonal and solar cycle variations in the subauroral region electron temperature peak and associated magnetospheric energy source. There are two sources of long‐term (solar cycle and seasonal) variability in the magnitude of the subauroral region electron temperature peak and associated SAR arc emission intensity: (1) the neutral atmosphere and ionosphere and (2) the magnetospheric energy source. The results of this study indicate that the observed seasonal variation in SAR arc intensities can be explained reasonably well by seasonal variations in the neutral atmosphere and ionosphere. True solstice effects are unlikely to result from differences in a near‐equatorial magnetospheric heat source since the same heat source supplies both the summer and the winter hemispheres at opposite ends of a common flux tube. Observed solar cycle variations in SAR arc intensity for a fixed ring current strength (as represented by the Dst index) are not consistent with variations predicted solely on the basis of a solar cycle changes in the neutral atmosphere and ionosphere. A reduction of the magnetospheric heat flux by a factor of between 5 and 20 from solar maximum to solar minimum conditions is necessary to bring model electron temperatures and 6300Å emission intensity into agreement with observational results for moderately disturbed conditions (Dst ∼ −80γ). The required reduction in the magnetospheric energy source with decreasing solar cycle is attributed to compositional changes in the magnetospheric plasma.

The properties of a low-latitude aurora of September 22, 1982, observed with photometers and a spectrograph from McDonald Observatory, Texas, and an all-sky mapping photometer from a site near Boulder, Colorado, are discussed. The dominant emission was [OI] 630 nm with a 630/558 nm emission ratio of 3.8. Other weaker emissions were present that are characteristic of heavy particle aurorae and can be attributed to energetic (keV) ions and neutral atoms from the ring current. The properties of this and other low-latitude aurorae are discussed in the context of stable auroral red (SAR) arcs and type d aurorae, and of energetic ion and neutral aurorae. The emissions associated with heavy particle precipitation are characteristic of main phase conditions. The observed equatorial movement of main phase low-latitude red aurorae is consistent with the inward convection of ring current particles and the inward movement of the plasmapause. Previous work has hypothesized that coulomb collisions of plasmaspheric electrons with ring current O + cause the energization of the electrons exciting SAR arcs. Calculations of [OI] 630 and 558 nm emissions show that for red aurorae with 630/558 nm ratios ~4, additional energization processes producing electrons in the energy range 10-10,000 eV are required. We propose some clarification of nomenclature for several types of aurorae occurring at low latitudes. O, O +, N, and N + over molecular bands. These spectral features distinguish low-or mid-latitude aurorae from "ordinary" aurorae• excited by keV electrons and found in the high latitude auroral •ones. The distinction between lowand mid-latitude aurorae and so-called stable auroral red(SAR) arcs is much less precise and in this paper we will attempt to provide a clear distinction. However, in order to set the stage for such a classification scheme, background information about other known aurorae relevant to low-latitude aurorae is presented in the next section.In this paper we compare the result of our observations of an aurora that occurred on September 22, 1982, with observations of other red aurorae and S AR arcs. The spectral content, the latitudinal or spatial variation, and the temporal evolution of the emission features are discussed. Optical features presumably associated with the penetration of the storm-time ring current to lower L values are presented, including excitation by precipitation of heavy particles and low-energy electrons. In an effort to clarify the nomenclature for low-latitude aurorae of types other than those caused by keV electrons, a more precise definition of SAR arcs, type d, and heavy particle aurorae is presented. 2. BACKGROUND During magnetic storms, particles which originate from the ring current and/or are energized by the ring current enter the lower thermosphere and cause optical emissions associated with low-latitude aurorae. Three types of precipitating particles have been identified during the main phase of magnetic storms. The precipitation of energetic (~ keV) neutral atoms, originating fr...

Auroral emissions at low latitudes appear to be due to energy loss from trapped ring current ions or electrons of energy of order of 1eV rather than keV electrons accelerated along field lines as in the ordinary polar aurora. Two types of low latitude aurorae have been observed in Texas in our measurements during the last few years. The features observed to predominate in the first type are N2+1N bands showing high vibrational/rotational development, OI 7774 Å emissions, and sometimes Hydrogen Balmer β emissions. These spectral characteristics indicate precipitation of the ring current particles themselves, directly as ions, and/or indirectly as energetic neutrals following charge transfer with exospheric H or O neutrals. The second type consists predominantly of [OI]6300Å and [NI] 5200 Å emission and other features of excitation potential only a few eV, with spectral characteristics indicating excitation by electrons from a population with kT ∼ 1eV. The second type can give the brighter displays more easily visible to the naked eye, although the first appears to be more frequent. The time variations of the emissions have been found to be closely related to fluctuations of periods less than an hour in the magnetograms from nearby observatories.