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This is the force exerted on a particle by collision. It is equal to the product of the particle mass,
its velocity relative to colliding particle and the collision frequency. In the ionosphere ions and
electrons can collide with each other or with the neutral particles. If there is a wind in the neutral
atmosphere then this exerts a drag force on the ions and electrons through collisions. This drag force
in combination with the Lorentz force gives rise to a current. Below about 75 km both ion
and electron collision frequencies are higher than their respective gyrofrequencies and both species are
carried along with the wind. No current flows. Above about 120 km both collision frequencies are less
than the respective gyrofrequencies. In this case ions and electrons move perpendicular to the neutral
wind in opposite directions and a weak current flows across the wind. Between 75 km and 120 km the ions
move roughly with the wind whereas the electrons move roughly across it. A more substantial current
flows at a smaller angle to the wind.
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