CENTRIFUGAL ACCELERATION OF THE POLAR WIND

The effect of parallel ion acceleration associated with convection was first applied to energization of test particle polar ions by Cladis ( 1986). However, this effect is typically neglected in ''self-consisten t'' models of polar plasma outflow, apart from the fluid simulation by Swift [1990]. Here we include approximations for this acceleration, w hich we broadly characterize as centrifugal in nature, in our time-dep endent, semikinetic model of polar plasma outflow and describe the eff ects on the bulk parameter profiles and distribution functions of H+ a nd O+. For meridional convection across the pole the approximate paral lel force along a polar magnetic field line may be written as F(cent,p ole) = 1.5m(E(i)/B(i))2(r2/r(i)3) where m is ion mass, r is geocentric distance; and E(i), B(i) and r(i) refer to the electric and magnetic field magnitudes and geocentric distance at the ionosphere, respective ly. For purely longitidinal convection along a constant L shell the pa rallel force is F(cent,long) = F(cent,pole) [1-(r/(r(i)L)]3/2/[1-3r/(4 r(i)L)]5/2. For high latitudes the difference between these two cases is relatively unimportant below approximately 5 R(E). We find that the steady state O+ bulk velocities and parallel temperatures strongly in crease and decrease, respectively, with convection strength. In partic ular, the bulk velocities increase from near 0 km s-1 at 4000 km altit ude to approximately 10 km s-1 at 5 R(E) geocentric distance for a 50- mV/m ionospheric convection electric field. However, the centrifugal e ffect on the steady O+ density profiles depends on the exobase ion and electron temperatures: for low-base temperatures (T(i) = T(e) = 3000 K) the O+ density at high altitudes increases greatly with convection, while for higher base temperatures (T(i) = 5000 K, T(e) = 9000 K), th e high-altitude O+ density decreases somewhat as convection is enhance d. The centrifugal force further has a pronounced effect on the escapi ng O+ flux, especially for cool exobase conditions; as referenced to t he 4000-km altitude, the steady state O+ flux increases from 10(5) ion s cm-2 s-1 when the ionospheric convection field E(i) = 0 mV/m to appr oximately 10(7) ions cm-2 s-1 when E(i) = 100 mV/m. The centrifugal ef fect also decreases the time scale for approach to steady-state. For e xample, in the plasma expansion for T(i) = T(e) = 3000 K, the O+ densi ty at 7 R(E) reaches only 10(-7) of its final value approximately 1.5 hours after expansion onset for E(i) = 0. For meridional convection dr iven by E(i) = 50 mV/m, the density at the same time after initial inj ection is 30-50% of its asymptotic level. The centrifugal acceleration described here is a possible explanation for the large (up to approxi mately 10 km s-1 or more) O+ outflow velocities observed in the midalt itude polar magnetosphere with the Dynamics Explorer 1 and Akebono spa cecraft.