A study of the variability of dynamics and temperatures near the mesopause from observations of the hydroxyl (OH) Meinel band emission

The hydroxyl (OH) molecule has a significant role in the energy budget of the mesosphere. It can also be an important diagnostic of the physical and dynamical nature in the mesopause region since the OH Meinel emission bands contain signatures of the temperature and wind velocity of the mesopause region, around 85 km altitude. In this study the OH (6,2) emission line (843.0 nm) is investigated using a ground-based Fabry-Perot interferometer. A computer simulation has been carried out to predict the performance of the Fabry-Perot interferometer. The instrument function is obtained using a He-Ne laser at the wavelength of 632.8 nm, and is converted to the OH wavelength of 843.0 nm. The theoretical response of the interferometer to the OH emission is calculated by convolving a modelled OH spectrum and the converted laser calibration profile. This is compared with experimental data by using a non-linear least-squares method to extract Doppler temperature and velocity. Temperature and wind velocity errors are calculated analytically and numerically to increase credibility, and both show a good agreement. Systematic peak drift and broadening of instrument function have been found, which can be compensated for by using properly positioned instrument functions, and monitoring calibration lamp broadenings and subtracting the broadening effect from the instrument function. For tidal analysis, a combined Fourier and least-squares technique has been developed to overcome the problem that observation time is shorter than longer tidal period (diurnal and semidiurnal tide) in summer. In addition, a scheme for detecting and rejecting bad data, due to bad weather and system failure etc., has been also developed and which enables to save a great deal of data for a long term analysis. All these efforts are important for the long term analysis to achieve precise result and higher data density. An annual variation of temperature in the mesopause region has been confirmed. In addition, sudden mesospheric coolings by ∼25 K have been found during the months of January and February. These periods of strong cooling appear to be connected with stratospheric warming events. From the one month averaged hourly mean temperature, tidal variations on OH temperature are found as reducing error bound and smoothing out of uncorrelated fluctuations. The seasonal variations of amplitude and phases are obtained using the combined Fourier and least-squares method. Winter has the maximum amplitudes of ∼15 K, while summer has the minimum amplitudes of ∼2 K. The correlation between OH temperature and intensity has been investigated by introducing [eta] parameter and squared coherency spectrum. Unlike O2 emission, OH temperature and intensity are out-of-phase for the most of time. Mesopause wind velocities are also obtained from the measurement of Doppler shift of OH emission spectra. The meridional wind near the mesopause is usually southward (equatorward) throughout the summer period. This might be the evidence of a systematic inter-hemispheric circulation from the summer to the winter hemisphere. The zonal wind near the mesopause shows a semi-annual variation. This phenomenon may be explained in terms of momentum transfer from gravity waves which are filtered and modulated by the semi-annual zonal wind in the stratosphere. For tidal analysis, one month averaged hourly mean wind velocities are calculated and the combined Fourier and least-squares method is also applied. The data from almost every month show tidal wind variations, however in summer and autumn there are larger amplitudes (∼20 m/sec), while winter shows smaller amplitudes (∼3 m/sec). In phases, it is difficult to find any structure except that the winter phase advances the summer phase by 5 hours in semidiurnal tide.