Frictional forces and collision frequencies between moving ion and neutral gases

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Two different terms are used in literature for the frictional force between moving ion and neutral gases. It is shown how the corresponding collision frequencies have to be defined in order to keep both terms valid. An expression is derived for the momentum transfer collision frequency ν in between ions and neutral particles as a function of temperature and the relative flow velocity ¦ν i − ν n¦ . The result indicates that for most practical applications v in may be considered as independent of ¦ν i − ν n¦ . However, for very high velocities, e.g. a convection flow from the magnetosphere into the ionosphere, this dependence may be significant. Numerical values for ν in are presented. In the case of collisions between ions and their parent neutral gases, laboratory values are used for the resonant charge exchange cross section obtained for high energies, but a correction is made for the much smaller thermal energies in the temperature range typical for ionospheric conditions in order to take into account the effect of curved particle trajectories due to attractive electrostatic forces.