  The charged component of the insterstellar medium consists of atomic and
molecular ions, electrons, and charged dust grains, coupled to the local
Galactic magnetic field. Collisions between neutral particles (mostly atomic or
molecular hydrogen) and charged species, and between the charged species
themselves, affect the magnetohydrodynamical behaviour of the medium and the
dissipation of electric currents. The friction force due to elastic collisions
between particles of different species in the multi-component interstellar
plasma is a nonlinear function of the temperature of each species and the Mach
number of the relative drift velocity. The aim of this paper is to provide an
accurate and, as far as possible, complete set of momentum transfer rate
coefficients for magnetohydrodynamical studies of the interstellar medium.
Momentum transfer rates are derived from available experimental data and
theoretical calculations of cross sections within the classic approach
developed by Boltzmann and Langevin for a wide range of values of the
temperature and the drift velocity. Accurate numerical values for momentum
transfer rates are obtained and fitted to simple analytical formulae expressing
the dependence of the results on the gas temperature and the relative drift
velocity. The often used polarization approximation is in satisfactory
agreement with our results only for collisions between H2and molecular ions
(HCO+, H3+). For other kinds of collisions, the polarization approximation
fails by large factors, and must be replaced by more accurate expressions.
