Hydromagnetic surface waves

Plane and filamentary structures aligned with a magnetic field abound on the sun and in both interplanetary and interstellar space. When the Alfven speed changes across such boundaries, hydromagnetic surface waves can travel along them, carry energy, and provide heating. This paper surveys the nature of such surface waves, with emphasis on the dispersion relations, the spatial extent of the waves, the degree of gas compression, and the possibility of coupling to ordinary hydromagnetic waves. Explicit results are provided for the cases where the gas pressure is either much smaller or much larger than the magnetic pressure on either side of the surface. The waves are shear waves wherever the gas pressure is much smaller than the magnetic pressure. All surface waves involve finite gas compression, but this compression is negligible when the product of wave vector times magnetic-field vector is much smaller than the product of wave number times magnetic-field strength.