Spectrometric and photometric observations of low‐latitude aurorae

Abstract

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...

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