Coupling of Magnetohydrodynamic to Electromagnetic Waves at a Plasma Discontinuity. I. Radiation Field

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Abstract
The coupling of magnetohydrodynamic waves to electromagnetic waves at a plasma‐vacuum interface is studied for a general orientation of the external, constant magnetic field and of the direction of the incident wave propagation vector with respect to the normal to the boundary. It is found that there is a cone with its axis along the normal to the boundary within which the incident magnetohydrodynamic wave vector must fall in order that the transmitted electromagnetic wave be radiated, i.e., in order that the Poynting vector be real. The cone has a half‐angle of the order of the ratio of the Alfvén velocity to the light velocity, which is very small in most cases. The ratio of the transmitted to the incident Poynting vectors is usually of the order of the ratio: Alfvén velocity/light velocity, but exhibits an anomaly for the extraordinary magnetohydrodynamic wave propagating in a constant magnetic field inclined to the interface. The variation of the individual electric and magnetic fields across the interface is discussed. In some cases, the fields in the vacuum are greatly increased over their values in the plasma.