How close are ground-based Fabry-Perot thermospheric wind and temperature measurements to exospheric values? A simulation study

Abstract

Ground-based Fabry-Perot interferometers (FPIs) have been used extensively to determine thermodynamical and hydrodynamical properties of the upper atmosphere by measuring emission line profiles from the O( 1D) nightglow at high spectral resolution. The resulting thermospheric winds and temperatures are normally referred to the altitude of the peak of the O( 1D) emission layer, near 250 km, and the effects of wind shear and temperature gradations along the line of sight are neglected. In order to quantify the significance of these effects, and to obtain a better understanding of the “effective” (volume-emission-rate-weighted) altitude of the FPI measurement, a computer simulation of the measurement technique has been performed, incorporating realistic profiles of winds, temperatures and O( 1D) volume emission rates along the instrumental line-of-sight. The atmospheric profiles used for the simulations are derived from a recently-developed thermospheric computer subroutine based on the predictions of the NCAR thermospheric general circulation model. The simulation code is used to calculate synthetic FPI spectrograms for different viewing geometries and FPI station locations, and the spectrograms are analyzed using standard data reduction techniques to derive fitted winds and temperatures. These are then compared with the atmospheric profiles used as input to the simulation code to determine the effective altitude of each simulated measurement and to study the differences between recovered winds and temperatures and the corresponding exospheric values. A first investigation using the simulation code for geophysical conditions corresponding to December solstice at solar maximum has indicated that FPI-derived Doppler temperatures may be lower than exospheric temperatures by ∼10% in the winter hemisphere and ∼15% in the summer hemisphere. Furthermore, FPI measurements of neutral winds, particularly at high latitudes, can differ appreciably from exospheric values due to the weighting of the FPI measurement to altitudes near ∼250 km.