Diurnal variation of the neutral thermospheric winds determined from incoherent scatter radar data

Abstract

Incoherent scatter radar measurements of the diurnal variation of the electron and ion temperatures, electron density, and vertical plasma drift are used to determine the diurnal variation of the neutral thermospheric winds. The ionospheric properties needed to determine the neutral winds are (1) the horizontal neutral wind component along the magnetic declination, SD (determined from plasma drift measurements and F region theory) and (2) the longitudinal derivative of the neutral exospheric temperature, ∂T∞/∂γ (derived from ion and electron temperatures and density measurements above 300 km). The atmospheric response to various forcing functions is determined from a three-dimensional dynamic model of the neutral thermosphere, the ion drag being specified by the electron density measurements. Harmonic components of the pressure force in terms of the latitudinal and longitudinal variations of exospheric temperature are used to determine the SD response coefficients from the model. A least squares fit of the measured and calculated diurnal variation of SD and ∂T∞/∂γ gives the Fourier coefficients necessary to construct the appropriate latitudinal and longitudinal pressure gradients. The pressure gradients are then used to calculate the diurnal winds for two geomagnetically quiet days over Millstone Hill during equinox. Amplitudes of 200 m s−1 during the night and 50 m s−1 during the day are obtained. The diurnal winds determined by using three different empirical models of the thermospheric structure are also calculated and compared with the winds determined from the least squares fit.