GEOPHYSICAL RESEARCH LE•'•ERS, VOL. 11, NO. 6, PAGES 572-575, JUNE 1984 SPECTRAL CHARACTERISTICS OF TWO TYPES OF LOW LATITUDE AURORAE B.A. Tinsley l, R.P. Rohrbaugh l, H. Rassoul l, E.S. Barker 2,A.L. Cochran 2, W.D. Cochran 2, B.J. Wills 2, D.W. Wills 2, D. Slater 3 1University of Texas at Dallas, Richardson, Texas 75080 2University of Texas at Austin, Texas 3Battelle Pacific Northwest Laboratory, Richland, Washington Abstract. Auroral emissions at low latitudes appear to be due to energy loss from trapped ring current ions or electrons of energy of order of leV rather than keV electrons acceleratedalong field lines as in the ordinary polar aurora. Two typesof low latitude auroraehavebeen observedin Texas in our measurements during the last few years. The features observedto predominate in the first type areN2*IN bands showing high vibrational/rotational development, OI 7774,•emissions, and sometimes Hy- drogen Balmer/3 emissions. These spectralcharacteristics indicate precipitation of the ring current particlesthem- selves, directly as ions, and/or indirectly as energetic neutrals following charge transfer with exospheric H or O neutrals. Thesecond toype consists predominantly of [OI] 6300,• and [NI] 5200 Aemission and other features of excitation potential only a few eV, with spectral charac- teristics indicating excitation by electrons from a popu- lation with kT--- leV. The second type cangivethe brighter displaysmore easilyvisibleto the naked eye, although the first appearsto be more frequent. The time variations of the emissions have been found to be closely related to fluctuations of periodsless than an hour in the magneto- grams from nearby observatories. Introduction Low latitude aurorae bright enough to be visible have been recorded for many centuries from Japan, India, Singapore, Cuba, and Hawaii, for example, and reviews of such observationshave been given by Loomis (i86i), and Chamberlain (1961). For a number of years we have been observing with sensitive spectrographs and evenmore sensitive photometers the spectral characteristics of aurorae detectable at McDonald Observatory, Southwest Texas (lat. 30.7øN, long. 101.0øW, dip lat. 40øN). We have found that spectral characteristics of low latitude aurorae are different fromthose of ordinary polar aurorae. For pur- poses of comparison,we define the ordinary polar aurora as producedby keV electrons accelerated along field lines.' These polar aurorae occur at high latitudes for low mag- netic activity and expand to mid latitudes during large magnetic storms. We see two types of aurorae at low latitudes, both of which differ from the ordinary polar aurorae, but which bear more resemblance to lesscommon forms of the polar aurorae. Neutral Atom/Ion Aurorae N2 +IN emissions showing high vibrational/rotational development have been observed at middle and low lati- Copyright 1984 by the American Geophysical Union. Paper number 4L6128. 0094-8276/84/004L-6128503.00 tudes, and sometimes at high latitudes. Such excitation is not present in ordinary polar aurora in the absence of sunlight.Lord Rayleigh observed vibrational development (in a spectrum from Terling, England, Rayleigh, 1922), as also, for example, did Wallace (1959). The initial inter- pretation of such spectra was that the vibrational/rotational excitation resulted directly from collisions of heavy par- ticles, such as protons, as had been demonstrated in the laboratory (e.g., Fan, 1956)but it hasbeen difficult to show that the alternativeinterpretationof resonance fluorescence (Bates, 1949)was not a sufficientexplanation. Until recent- ly long photographic exposures were necessary to obtain spectra of the relatively faint middle and low latitude aurorae, and these were rarely limited to periods where the shadow height on the line of sight was high enough (say above 600 km) to rule out resonance fluorescence. An additional problem is that resonance fluorescence increases the brightness as well as the vibrational developmentof the N2*IN emission, so one must also be able to rule out contributions from bright regions, perhaps on a distant horizon, which are not on the line of sight but from which radiation may be tropospherically scattered into the field of view (Broadfoot, 1968). Observationswith tilting filter photometersscanning over a sufficientlywide wavelength rangeto showthe high rotational and vibrational development were made in Hawaii, Brasil and Texas for the magnetic stormsof 1981 March 5, April 12, April 13, and 1982July 14. The shadow heightswere sufficiently high for most of the data taken on these occasions to rule out contributions from resonance fluorescence(Rohrbaugh et al, 1983). Observations with a ,broad spectral coverageand with high shadow height were made on 1983June 13 with a spectrograph attached to the 2.7 meter telescopeat McDonald Observatory. Emission bright enough to be visible was seen from McDonald Observatory to the north, but only for a short while at the end of astronomical twilight. Three indepen- dent observers recalled that it was of white color, but only about as bright as the Milky Way. J.C. Liddel (personal communication) reported that the display extended to about 15 ø abovethe northern horizon and that the bright- ness was approximately uniform up to that elevation. The display faded after 15-20 minutes, i.e., by about 04 UT, and this behavior is consistent with the presence of N2* 1N emission enhancedat that time by resonantscattering of sunlight. A set of six spectra was recorded between0556 and 0632 UT with zenith angle of observations (Z.A.) 56ø, azimuth (Az.) 356 ø. These spectrawere each actually the sum of two obtained simultaneouslyfrom areas about 4 arc sec square, a few minutes of arc apart on the sky. A three stage image tube with image dissector was used as detector, and the spectral slitwidth was 11 •. The shadow heighton the line of sightfor these exposures ranged from 572