In the last subsection it was shown that an ambipolar electric field is set up in steady-state ionospheres so as to limit charge-separation electric fields caused by the much greater electron mobility (due to their much smaller masses). These electric fields pull ions out of the ionosphere. Schunk [1983] considered the effects of the electric field set up by the major ion species on a second, much lighter ion species (such as protons). He found that combining the number conservation equation and the momentum equation for the minor species (including the polarization field) led to an equation strongly reminiscent of Parker's gasdynamic equation for subsonic to supersonic flow. He found that this equation admitted subsonic to supersonic solutions. This model explains supersonic flows of light ions from Earth's polar ionosphere, called the polar wind.
Finally, it might be wondered why the ionospheric temperature increases with height in the thermosphere and ionosphere. One qualitative explanation is simply that only hotter particles can overcome the gravitational potential and reach those heights, so that the temperature should increase with height. This explanation is not, in fact, quantitatively viable for Maxwellian distributions of particles. However, for non-Maxwellian distributions this idea is quantitatively viable [e.g., Scudder, 1992].