Retrieval of Rotational Temperatures From the Arecibo Observatory Ebert-Fastie Spectrometer and Their Inter-Comparison With Co-Located K-Lidar and SABER Measurements

Rotational temperatures in the Mesosphere-Lower Thermosphere region are estimated by utilizing the OH(6,2) Meinel band nightglow data obtained with an Ebert-Fastie spectrometer (EFS) operated at Arecibo Observatory (AO), Puerto Rico (18.35 degrees N, 66.75 degrees W) during February-April 2005. To validate the estimated rotational temperatures, a comparison with temperatures obtained from a co-located Potassium Temperature Lidar (K-Lidar) and overhead passes of the Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) instrument onboard NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite is performed. Two types of weighting functions are applied to the K-Lidar temperature profiles to compare them with EFS temperatures. The first type has a fixed peak altitude and a fixed full width at half maximum (FWHM) for the whole night. In the second type, the peak altitude and FWHM vary with the local time. SABER measurements are utilized to estimate the OH(6,2) band peak altitudes and FWHMs as a function of local time and considerable temporal variations are observed in both the parameters. The average temperature differences between the EFS and K-Lidar obtained with both types of weighting functions are comparable with previously published results from different latitude-longitude sectors. We found that the temperature comparison improves when the time-varying weighting functions are considered. Comparison between EFS, K-Lidar, and SABER temperatures reveal that on average, SABER temperatures are lower than the other two instruments and K-Lidar temperatures compare better with SABER in comparison to EFS. Such a detailed study using the AO EFS data has not been carried out previously. In the Earth's atmosphere, a strong band of near-infrared (NIR) emissions exists at similar to 90 km altitude due to the vibrational-rotational excitation of the Hydroxyl (OH) molecules. The intensity of these OH emissions depends on the temperatures of their rotational states. Under the assumption of thermodynamic equilibrium, the rotational temperatures should represent the background atmospheric temperatures. A scanning spectrometer of the Ebert-Fastie configuration (EFS) was operated on the nights of February-April 2005 at Arecibo Observatory (AO), Puerto Rico (18.35 degrees N, 66.75 degrees W) to record the intensity of the OH(6,2) band which is used to estimate rotational temperatures in this work. During the same period, a co-located Potassium temperature Lidar (K-Lidar) utilized resonance scattering from the atmospheric K-atoms to measure temperatures in the 80-100 km altitude range. By nature, the EFS-derived temperatures are an altitude-integrated quantity while the K-Lidar provides altitude-resolved temperatures. First, we followed a commonly used methodology to compare the EFS and K-Lidar temperatures by using the SABER data onboard NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. We found good agreement between our results and previously published studies. Then, we propose a modified methodology with the help of SABER data to show that it provides a better comparison between the EFS and K-Lidar temperatures. Good inter-comparison is obtained between Arecibo Observatory spectrometer and Lidar temperatures, and results are similar to previous studies The temperature comparison improves when time-varying weighting functions are used instead of a single weighting function Rotational temperature estimation from the spectrometer data depends strongly on the choice of Einstein coefficients for the P1 line transitions