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-consistent'' models of polar plasma outflow, apart from the fluid simulation by Swift [1990>. Here we include approximations for this acceleration, which we broadly characterize as centrifugal in nature, in our time-dependent, semikinetic model of polar plasma outflow and describe the effects on the bulk parameter profiles and distribution functions of H+ and O+. For meridional convection across the pole the approximate parallel force along a polar magnetic field line may be written as Fcent,pole=1.5m(Ei/Bi)2(r2/r3i) where m is ion mass, r is geocentric distance; and Ei, Bi and ri refer to the electric and magnetic field magnitudes and geocentric distance at the ionosphere, respectively. For purely longitudinal convection along a constant L shell the parallel force is Fcent,long=Fcent,pole [1-(r/(riL)>3/2/[1-3r/(4riL)>5/2. For high latitudes the difference between these two cases is relatively unimportant below ~5 RE. We find that the steady state O+ bulk velocities and parallel temperatures strongly increase and decrease, respectively, with convection strength. In particular, the bulk velocities increase from near 0 km s-1 at 4000 km altitude to ~10 km s-1 at 5 RE geocentric distance for a 50-mV/m ionospheric convection electric field.

However, the centrifugal effect on the steady O+ density profiles depends on the exobase ion and electron temperatures: for low-base temperatures (Ti=Te=3000 K) theO+ density at high altitudes increases greatly with convection, while for higher base temperatures (Ti=5000 K, Te=9000 K), the high-altitude O+ density decreases somewhat as convection is enhanced. The centrifugal force further has a pronounced effect on the escaping O+ flux, especially for cool exobase conditions; as referenced to the 4000-km altitude, the steady state O+ flux increases from 105 ions cm-2 s-1 when the ionospheric convection field Ei=0 mV/m to ~107 ions cm-2 s-1 when Ei=100 mV/m. The centrifugal effect also decreases the time scale for approach to steady-state. For example, in the plasma expansion for Ti=Te=3000 K, the O+ density at 7 RE reaches only 10-7 of its final value ~1.5 hours after expansion onset for Ei=0. For meridional convection driven by Ei=50 mV/m, the density at the same time after initial injection is 30--50% of its asymptotic level. The centrifugal acceleration described here is a possible explanation for the large (up to ~10 km s-1 or more) O+ outflow velocities observed in the midaltitude polar magnetosphere with the Dynamics Explorer 1 and Akebone spacecraft.