Published online by Cambridge University Press: 13 March 2009
The characteristics of Alfvén surface waves along a cylindrical annular streaming column with a solid core and surrounded by a tenuous medium, and pervaded by a longitudinal magnetic field are studied analytically and numerically. Both axisymmetric (m = 0) and non-axisymmetric (m ≠ 0) modes are found to be dispersive. The Alfvén-wave velocity decreases with increasing magnetic field. Under an infinitesimal perturbation, the motion of the system is laminar and stable for all values of m for all short and long wavelengths if the unperturbed state is non-streaming. If the system is streaming, there will be unstable domains not only for m = 0 but also for m ≠ = 0 owing to the strong destabilizing influence of the streaming. However, this instability can be suppressed completely if the magnetic field is sufficiently high that the Alfvén-wave velocity is greater than the streaming velocity. The thicker the solid core the greater is its stabilizing influence for m = ±1, and vice versa for m = 0. A longitudinal field imparts a degree of rigidity to the conducting fluid, so it always has a strong stabilizing influence.