Effect of collisions on hydromagnetic waves

The effect of collisions on the theory of low-frequency wave propagation in a low-τ plasma is taken into account by using the moment equations derived by Kaufman. The general dispersion relation is obtained and examined in the various limiting cases. We find that the inclusion of Hall current and finite ion Larmor radius makes the medium dispersive. When propagation is along the field lines, the damping of the acoustic mode is due to viscosity, electrical conductivity, thermal conductivity and thermoelectric coefficients, while the damping of the hydromagnetic mode is due to viscosity and electrical conductivity only; this mode gets modified due to the Hall term and the finite ion Larmor radius. In the case of propagation transverse to the magnetic field, all the transport coefficients contribute to the damping of waves