Centrifugal acceleration of the polar wind

Abstract

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/ri³) 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 longitidinal 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) the O+ 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 Akebono spacecraft.