Statistical analysis of thermospheric temperatures using four Fabry-Perot interferometers at high and low latitudes
Wednesday, 17 December 2014
Fabry-Perot interferometer (FPI) is an instrument that measures the temperature and wind velocity of the thermosphere by observing from the ground the 630-nm airglow emission. The Solar-Terrestrial Environment Laboratory (STEL), Nagoya University, has five FPI observatories that represent the Optical Mesosphere Thermosphere Imagers. Two of these FPIs using a large aperture etalon (diameter: 116mm) were installed at Shigaraki (FP00), Japan in 2000 and in Tromsø (FP01), Norway, in 2009. The other three smaller FPIs, using 70-mm diameter etalons, were installed in Thailand (FP02), Indonesia (FP03), and Australia (FP04) in 2010-2011. These FPI instruments all use highly-sensitive cooled-CCD cameras (1024x1024 pixels) to observe the interference fringes. However, the error values of temperature estimates were much larger than expected, i.e., 200 K, and the temperature value found in the preliminary analysis was not within the expected range of 600 to 1200 K. In the present study we improved the procedure of temperature determination using these small-etalon FPIs and carried out a statistical analysis of the temperature data obtained by FP01-04 for 3-5 years after 2009. The FPIs scan the sky in north, south, east, west, zenith directions repeatedly by rotating two 45-degree mirrors. We determined the centers of the HeNe 632.8 nm laser and sky fringes for each direction. When we found that these ring centers may vary slightly by a few pixels from one direction to the next, we realized that this difference of fringe centers was a result of the distortion of the optical mounting frame, which is caused by the motion of the heavy scanning mirror on top of the FPI optics. When we determine the fringe center for each direction, then the error values of temperature estimation improved from 200 K to about 30 K, and the results became more in line with the range of expected results. We estimated airglow intensity from the data and performed statistical analysis of the data obtained by FP01-04 for 3-5 years after 2009. Then we found that the low temperatures are obtained when the airglow intensity is very small. In this presentation, we show results derived from the evaluation of the reliability of the obtained temperatures with consideration regarding the accuracy of FPIs and the relation between airglow intensity and obtained temperatures.