EFFECT OF CENTRIFUGAL ACCELERATION ON THE POLAR WIND

The ionospheric convection electric fields that occur at high latitude s cause plasma to drift across the cusp region and the polar cap. Sinc e the magnetic field at high latitudes is close to vertical, pointing downward (upward) in the northern (southern) hemisphere, the convectin g plasma experiences a centrifugal acceleration as it crosses the pola r region because of the diverging magnetic field geometry. The centrif ugal force is directly proportional to the mass of the plasma particle s, and it is reasonable to ask whether this force has an effect on pol ar plasma outflow, particularly for the more massive ion O+. To date, a number of studies have addressed this question, but the theoretical models used in these studies were either overly simplified (i.e., negl ected processes known to be important in the polar ionosphere) or else did not use appropriate boundary conditions or take account of the ti me variability of the problem, The results of these prior investigatio ns were often contradictory. In order to overcome the limitations of t hese earlier studies, we have used a macroscopic particle-in-cell (PIC ) code, which is sophisticated in the sense that a broad range of phys ical processes are incorporated in its description, in conjunction wit h time-varying boundary conditions obtained from a time-dependent, thr ee-dimensional, hydrodynamic model of the polar ionosphere. This enabl es us to properly account for the variation of boundary conditions alo ng a flux tube trajectory. Initially, our macroscopic PIC model was so lved for steady state conditions. This allowed us to compare results f rom our code with those of a prior study of centrifugal acceleration t hat uses a PIC formulation. Also, by obtaining steady state solutions for both low and high electron temperatures, we have been able to dire ctly compare the effects of electron temperature and centrifugal force on the polar plasma outflow, a comparison that a time-dependent simul ation might obscure. Then time-dependent PIC solutions were obtained f or the plasma in a convecting flux tube, using solutions to a time-dep endent, three-dimensional, hydrodynamic model to provide realistic bou ndary values for the electron and ion temperatures and the H+ and O+ d ensities and drift velocities along a flux tube trajectory. Both stead y state and time-dependent solutions indicate that centrifugal acceler ation does not significantly contribute to the loss of plasma from the polar ionosphere.