High-latitude ground-based observations of the thermospheric ion-drag timeconstant

From previous studies, it has been conclusively demonstrated that F-region thermospheric winds follow, but generally lag behind, the ion drift pattern of magnetospheric convection. Analysis of the ion-neutral momentum exchang e equation shows that ion-drag and thermal pressure are the major contribut ors to neutral momentum forcing at F-region heights with relatively minor e ffects from coriolis, advection and viscous forces. An ion-neutral coupling time constant (e-folding time) has been defined which describes the time t aken for the neutral gas velocity to approach the ion velocity after a step change in convection. In this study, F-region ion drift and neutral winds have been observed by the EISCAT tristatic incoherent scatter facility and a ground-based Fabry-Perot interferometer, respectively, from northern Scan dinavia. The e-folding time varies in the range 0.5 - 6.5 hours, with an av erage of 1.8 and 3.3 hours for a geomagnetically active and quiet period, r espectively, which compares well with previous satellite measurements of 0. 5 - 3.5 hours.