Middle‐ and low‐latitude emissions from energetic neutral atom precipitation seen from ATLAS 1 under quiet magnetic conditions

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During the ATLAS 1 mission spectral observations were made at middle and low latitudes of features expected from the precipitation of energetic neutral atoms. The Imaging Spectrometric Observatory was used at night in the UV and visible with maximum gain. The tangent ray heights of the look directions ranged from near 100 km to near 200 km, and the geomagnetic conditions were quiet during the observations, which were made March 28 to April 3, 1992. The N2+1N 391.4‐nm and O I 130.4 and 135.6‐nm emissions were observed at all latitudes, with lower emission rates at lower magnetic dip latitudes, except that enhancements in the O I lines were seen within 30° of the dip equator due to radiative recombination of ionospheric plasma. The latitude profile observed for the N2+1N emission did not show an equatorial or midlatitude peak. This implies that the source of energetic neutrals is more consistent with prompt charge exchange loss of freshly injected trapped ions with relatively low mirror heights (i.e., ions on higher L shells with equatorial pitch angle distributions nearly isotropic to the loss cone) than loss of highly eroded populations of particles with high mirror heights (i.e., ions on lower L shells with pancake equatorial pitch angle distributions). The N2+1N emission rates have been compared with models of atmospheric emission due to fluxes of O/O+ and H/H+ in the thermosphere, as produced by energetic neutral oxygen or hydrogen atom precipitation. Energy deposition rates are inferred that range from about 12 (±3) × 10−4 erg cm−2 s−1 near 45° geomagnetic latitude to about 3 (±1) × 10−4 erg cm−2 s−1 near the geomagnetic equator. These can be compared with energy deposition rates a factor of 10² higher during magnetic storms.