A theoretical study of downward H+ field-aligned velocities at mid-latitudes

This paper examines the magnitude of downward H⁺ field-aligned velocities at mid-latitudes. For the isothermal, collisionless, steady-state case an analytical form is derived for the critical temperature, below which the plasma temperature must lie for there to be a possibility of supersonic H⁺ flow. Some simple equations are presented which yield velocity profiles and an aid to the understanding of energy distribution in the H⁺ gas. Very little gravitational potential energy is converted to kinetic energy. For the general case the situation is far more complicated, but we note in particular the importance of the O⁺ contribution to the electrostatic field. For the simpler case the flow is always subsonic regardless of the plasma temperature and it appears unlikely that supersonic flow will occur in the more general case.