GEOPHYSICAL RESEARCH LETTER, VOL. 20, NO. 4,PAG ES 305-308, FEBRUARY 19, 1993 OH(vdV) COLUM N DENSITIES FROM I-HGH-RESOLUTIO N EARTHLIMB SPECTRA J.A.Dodd,' W.A.M. Blumberg, S.J. Lipson, J.R. Lowell, P.S.Armstrong, DR. Smith, R.M. Nadile, N.B. Wheeler, and E.R. Huppi Phillips Laboratory, Geophysics Directorate, Optical Environment Division Abstract IndividualOH(v,N) rotational state population column densities havebeen derived from spectral analysis of CIRRIS 1A nighttime earthlimbairglow data. Both pure rotation and vibration-rotation fundamental spectra havebeen examined, providing unique informationon highly excited rotational states of OH(v=0-6). The relativepopulations of the four spin sublevels have also been determined. These findings provideimportant insights into OH dynamics at the mesopause. Introduction Recently we reported the first observation of OH(N-•N- 1) purerotation transitions in the earth's infraredairglow [Smith et at, 1992]. Limbspectra withtangent heights between 80 and 110 km, obtained by the CIRRIS 1A interferometer aboard the Space Shuttle, were analyzed. No OH(v,N) emission features areobserved for tangent heights greater than110km. Upper state rotation quantum numbers ashigh as N=33 are inferred, much higher than the highestvalue previously reported, i.e. OH(N'=12) in theMeinel (7-4) band [Pendleton et aL, 1989]. The pure rotationemission probabilities for the high-Nlevels (hereafter denoted N*) are an order of magnitude larger than those for the Meinel vibration-rotation bands, facilitatingdetection in the longer wavelength region. Syntheticspectral fits have now been performedusing spectrally calibratednighttime data in several passbands, including the OH(v-•v-1) Meinel fundamental bands andthe pure rotation bands. Absolute OH(v,N) Column densities have been obtained for a large number of levels, including 1ow-N states for v=l-9, as well as highly excitedrotational states for v=0-6. The relative populations of the 1-I3a(A'), l'I3a(A •), II,a(A'), and II•a(A •) spin sublevel states have also been determinedfrom the pure rotation spectrum. This effort hasprovideda unique, detailed database of hydroxyl rotational and vibrational state distributions at the mesopause. Experimental The CIRRIS 1A instrumentatiov is described in our first report [Smith et al., 1992] and more fully elsewhere [Ahmadjian et al., 1990]. A cryogenic Michelson 'Stewart Radiance Laboratory Copyright 1993 by the American Geophysical Union. Paper Number93GL00091 0094- 8534/93/93 GL-00091 $03.00 interferometer operated with Si:As detectors was used to collect data, with response between 400 and 4000 cm". Optical passbandfilters were used to enhance detector sensitivity by rejectingintenseemission from CO2(v2) (600-750 cm") and O•(v•) (990-1070 cm"). Hydroxyl emission was examined in threewavenumber passbands: a "fundamental" passband between 2000 and 4000 cm", and two "pure rotation" passbands in the 400-600 cm" and 750-1000 cm" regions. Spectra in the fundamental band exhibit OH(v) vibrational fundamental emission fromN=1-4. Spectrain the pure rotationbands exhibit OH(v) pure rotation emission fromapproximately N=I 4-18 and N=25-33, respectively, depending on the v level. The verticalfield-of- view of the detector usedhere is approximately 15 km; detectors with smaller fields-of-view were present but produced data with poorerS/N. Tangent heights for the center of the detector field-of-view, originally determined by an IR horizon sensor,have been corrected since our first report through the use of celestial background images. Details of the correction will be described in a forthcoming publication from this group. We believethe current values to be accurate to ñ 3 km. Spectra with absolute radiance units (W/cm2/sr/cm 4) were used for this study. Filter-dependent calibration spectra were generated by subjecting calibration interferograms to numerical processing identical to that of data interferograms [Doddet aL, 1991 ]. Uncalibrated spectra werethendivided by an appropriate calibration spectrum to generate corrected spectra with absolute radiance units. In theabsence of large neighboring features, unapodized interferograms were used to generate spectra with the maximumpossible resolution (0.63cm"FWHM). Spectral Fitting Spectral fittingwas performed on a scan-by-scan basis in order to determine OH(vdV)populations radiating in the field-of-view. Basisfunctions were synthesized for each filter region by convolving a setof OH(v,N) lineswith an appropriate lineshape function. Vibrational population multipliers were taken to be independent, adjustable parameters in the spectral fits, while rotational populations were parameterized usinga Boltzmanndistribution, with the effective temperature T as an adjustable parameter. Literature line positions[Coxon and Foster, 1982] and intensifies [Nelson et aL, 1990] were used for fits to the fundamental band. Fits to this region determined T = 200 ñ 20 K for N=l-4, consistent with the kinetic temperature of the mesopause. Within the limits of the S/N (about10 for the strongest lines), no evidence of departure from a Boltzmann distribution was observed in the fundamental band. For OH pure rotation emission, basis functions were 305