Magnetosonic Waves

The compressional fast and slow MHD waves in which magnetic and thermal pressure jointly provide restoring force.

Two Compressional Branches

Besides the shear Alfven wave, ideal MHD supports two compressional waves called magnetosonic. Both compress the plasma and the magnetic field, and both are driven by a combination of thermal pressure and magnetic pressure. They are distinguished by whether the two pressures reinforce (fast wave) or oppose (slow wave) one another during the compression.

Fast Magnetosonic Wave

In the fast wave the magnetic and thermal pressures oscillate in phase, adding their restoring forces, so it has the highest phase speed of the three MHD modes and propagates in all directions, including across the field. Perpendicular to B its speed approaches sqrt(v_A^2 + c_s^2). The fast wave is the workhorse of ion-cyclotron-range heating, since it can carry energy across field lines to a central resonance.

Slow Magnetosonic Wave

In the slow wave the magnetic and thermal pressures are out of phase and partly cancel, giving the lowest phase speed. It propagates nearly along the field, becoming the ordinary sound wave when magnetic effects are weak. The slow wave is strongly damped in hot plasmas and plays a smaller role in heating but matters for parallel dynamics and shocks.

Relevance

Fast-wave propagation and accessibility govern ion-cyclotron heating scheme design, including mode conversion at hybrid layers. For the Hyperion breeder concept, magnetosonic accessibility through the varying field profile is part of the design-stage heating analysis; the high on-axis field near 8 T sets the fast-wave speeds and resonance structure in modeling of a machine not yet built.