A computer model of global thermospheric winds and temperatures

Abstract

A computer model of the global, time-dependent, thermospheric horizontal vector neutral wind and neutral temperature fields has been constructed based on output from the NCAR thermospheric general circulation model (NCAR-TGCM). The wind field is represented by a vector spherical harmonic (VSH) expansion in the horizontal, a fourier expansion in Universal Time, and a polynomial expansion in altitude. The global temperature field representation differs in that a scalar spherical harmonic expansion is used in the horizontal and a Bates model temperature profile is used in altitude. A set of suitably-truncated spectral coefficients contains the wind and temperature description for a diurnally-reproducible run of the NCAR-TGCM. The VSH model is coded in a FORTRAN subroutine that returns vector wind and temperature values for a given UT, geographic location, and altitude. The model has applicability for studies of thermospheric and/or ionospheric physics were reasonable time-dependent neutral wind and temperature values are of interest. The routine is novel since portable computer models of thermospheric wind fields have not previously been available to researchers. The current version of the model is valid for solar maximum, December solstice only, although the model can be extended to any season and specific set of geophysical conditions for which TGCM results are available. Results from the VSH computer model are presented to compare with global-scale wind measurements from the Dynamics Explorer (DE-2) satellite. The agreement between the computer model results and data from individual orbits of DE-2 is good, indicating that the model provides reasonable wind values, having the appropriate characteristic latitudinal, diurnal, and Universal-Time-dependent signatures observed from the satellite at upper thermospheric altitudes. The VSH thermospheric temperature values are in general agreement with MSIS-83 temperatures but illustrate smaller-scale horizontal temperature structures than are resolved by MSIS-83, owing to the larger number of spectral harmonics retained.