Hydroxyl airglow temperatures above Davis Station, Antarctica
Creators: name: French, W. John R.
Publisher: name: University Of Tasmania
Year: 2023

description: The hydroxyl airglow (6-2) band has been monitored above Davis station, Antarctica (68.6¬¨‚àûS, 78.0¬¨‚àûE) by means of a Czerny-Turner spectrometer since 1990. This thesis is an investigation of the long-term trends and variability in OH(6-2) rotational-temperatures over an 11 year time span. Tropospheric warming, due to increased greenhouse gases concentrations over the last 150 years, is expected to be associated with enhanced cooling in the stratosphere and mesosphere. Modelling studies indicate a maximum cooling response in the high latitude mesosphere. Some reported observations suggest that pronounced cooling, (up to 7 K/decade) in excess of model predictions, has already taken place. Hydroxyl airglow emissions originate near 87 km. The layer is ideally located to monitor mesopause region temperatures by ground based spectroscopic means. The Davis OH(6-2) database contains 8 years of observations that span 1990 to 2000; over 126,000 spectra, which yield 1310 nightly averages. Observations are limited by day length at Davis. Night-time observations are possible between day-of-year 45 (14-Feb) and 300 (27-Oct). Analysis techniques are developed to optimise the rotational temperature determinations for the P 1 (2), P 1(4) and P 1(5) ratio's in this band for climate change studies. Precise calibration of the instrument spectral response is important for long-term trend assessment. Inter-year spectral response variation of less than 0.3% has been achieved. Temperature errors associated with the response calibration amount to 1.5 K for earlier years and less than 0.3 K since 1996. A detailed investigation of auroral and OH satellite line emissions, solar Fraunhofer and water vapour absorption across the OH(6-2) region (˜í¬™837.5-851.5 nm) is undertaken. These features can modify the apparent intensity of the P-branch lines. P1(3) is rejected from further analysis due to un-thermalised contributions from the OH(5-1)P1(12) lines. A correction is applied to account for the Q1(5) contribution under P1(2). N2 1PG and N¬¿‚àÇvü2 Meinel auroral emissions and solar Fraunhofer absorption are accounted for by appropriate selection of the backgrounds for each line. Water vapour absorption is not found to be significant. Correction factors are also applied to account for the difference in A-doubling between the P-branch lines. These are derived from frequency-stabilised laser determinations of the instrument bandwidth (which is 0.15nm). Errors associated with each correction are assessed. Serendipitously, auroral emissions due to atmospheric Argon are identified for the first time in this investigation. Two argon lines (at ˜í¬™840.82 nm and ˜í¬™842.46 nm, between P1 (2) and P1 (3)) are apparent during intense auroral activity but do not influence the rotational temperatures at the resolution of the Davis instrument. As a scanning instrument is used, a 'sampling' error is also associated with the time taken to acquire each spectrum due to possible intensity variations. A mean trend of ‚ÄövÑvÆ1.1 K due to an average intensity decrease across the night, with a 7 K standard deviation is found from coincident photometer observations. As a result of the investigation of background features, acquisition times were reduced from the order of 1 hour (in 1990) to 7.3 minutes (from the end of 1996) by scanning only selected P-branch lines and background regions, which reduces the standard deviation. Furthermore, an analysis technique for time interpolation of sampled branch lines and backgrounds between spectra is developed, which removes the trend component. Three sets of published transitions probabilities yield a 12 K range in the absolute temperature derived. An experimental investigation of OH(6-2) Qi/Pi and R1/P1 emission intensity ratios, for rotational states up to j'=4.5, is undertaken to determine which set is most suitable for application to the OH(6-2) band. Selection criteria are established to reject spectra that do not yield consistent temperatures for each of the three possible ratios, suffer low signal-to-noise, or are contaminated by strong aurora, scattered moonlight or changing cloud conditions. Nightly averages are determined from spectra that pass all criteria. Annual variations are characterised by an extended warm (206 ¬¨¬± 4 K) winter period, with a gradual decline (-0.04 K/day) over the interval DOY 106-258 and including episodic 10-20 day (planetary scale) variations of amplitude up to 30 K. Equinoctial transitions from cold summer temperatures show a sharp rise in autumn (1.2 K/day; DOY 49-80) and a more gradual spring decline (-0.65 K/day; DOY 275-296). The autumn transition occurs earlier, and the spring transition later than either CIRA86 or MSISE-90 model predictions. The midwinter local minimum in MSISE-90 is also not supported. Mean winter temperatures calculated from the daily averages for each year are consistent with a positive solar cycle association of 0.066 K/solar-flux-unit, considerably lower than most values reported in the literature. Multivariate analysis supports a long term cooling trend of the order of 0.5 K/year in the winter average temperatures over Davis.