Effects of magnetospheric electrons on polar plasma outflow: A semikinetic model

Abstract

A semikinetic model for the study of the effect of hot magnetospheric electrons on polar plasma outflow is developed. The model is based on a hybrid particle‐in‐cell approach which treats the ions (H+ and O+) as adiabatic, parallel‐drifting gyro‐centers injected as the upgoing portions of drifting bi‐Maxwellian distributions at 1.6 RE, while the electrons are treated as a massless neutralizing fluid. As a first approach to understanding the effects of hot magnetospheric electrons on the outflow, we consider electron temperature profiles which increase from low temperatures at the ionospheric levels to high temperatures at high altitudes. The electric field is determined by both the electron temperature and its gradient. The electric field produced by the electron temperature alone generally accelerates ions outward while that associated with the electron temperature gradient increases the potential barrier and inhibits the outflow. For typical polar wind stream conditions, electron temperature gradients exceeding 3×104 K/RE cause reflection of much of the ion stream back downward toward the ionosphere. Under these circumstances the H+ outflow forms two counterstreaming beams at altitudes below the reflecting potential barrier and a cooler and faster transmitted beam at high altitudes. Above the potential barrier, the O+ density decreases by 7 orders of magnitude for a very large electron temperature gradient. For the case of an electron temperature profile established by thermal conduction the results show inhibition of polar plasma outflow very near the lower boundary, but continuous acceleration of the escaping ions along most of the flux tube. H+ shows a continuous decrease in net outward flux from 3×107 ions cm−2 s−1 when the electron temperature is isothermal at 4400 K to 1.5×107 ions cm−2 s−1 when the upper boundary temperature is increased to 1×106 K. On the other hand, the flux of O+ exhibits a rise and fall with upper boundary electron temperature with a peak of 1.1×107 ions cm−2 s−1 when the upper boundary electron temperature is approximately equal to 2×105 K.