A TIME-DEPENDENT GYRO-KINETIC MODEL OF THERMAL ION UPFLOWS IN THE HIGH-LATITUDE F-REGION

Ample evidence supports the significance of the high-latitude ionosphe ric contribution to magnetospheric plasma. Assuming flux conservation along a flux tube, the upward field-aligned ion flows observed in the magnetosphere require high-latitude ionospheric field-aligned ion upfl ows of the order of 10(8) to 10(9) cm(-2) s(-1). Since radar and satel lite observations of high-latitude F region flows at times exceed this flux requirement by an order of magnitude, the thermal ionospheric up flows are not simply the ionospheric response to a magnetospheric flux requirement. Several ionospheric ion upflow mechanisms have been prop osed, but simulations based on fluid theory do not reproduce all the o bserved features of ionospheric ion upflows. Certain asymmetries in th e statistical morphology of high-latitude F region ion upflows suggest that the ion upflows may be generated by ion-neutral frictional heati ng. We developed a single-component (O+), time-dependent gyro-kinetic model of the high-latitude F region response to frictional heating in which the neutral exobase is a discontinuous boundary between fully co llisional and collisionless plasmas. The concept of a discontinuous ne utral exobase and the assumption of a constant and uniform polarizatio n electric field reduce the ion guiding center motion in the frame of a convecting flux tube to simple one-dimensional ballistic trajectorie s. We thus are able to analytically calculate a time and height-depend ent ion velocity distribution function, from which we can compute the ion density, parallel velocity, parallel and perpendicular temperature , and parallel flux. Using our model, we simulated the response of a c onvecting flux tube between 500 km and 2500 km to various frictional h eating inputs; the results were both qualitatively and quantitatively different from fluid model results, which may indicate an inadequacy o f the fluid theory approach. The gyro-kinetic frictional heating model responses to the various simulations were qualitatively similar: (1) initial perturbations of all the modeled parameters propagated rapidly up the flux tube, (2) transient values of the ion parallel velocity, temperature, and flux exceeded 3 km s(-1), 2 x 10(4) K, and 10(9) cm(- 2) s(-1), respectively, (3) a second transient regime developed wherei n the parallel temperature drops to very low values (a few hundred Kel vins), and (4) well after heating ceased, large parallel temperatures and large downward parallel velocities and fluxes developed as the flu x tube slowly returned to diffusive equilibrium. The ion velocity dist ributions during the simulation are often non-Maxwellian and are somet imes composed of two distinct ion populations.