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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