On the occurrence of critical levels in solar magnetohydrodynamics

It is shown that the singular behaviour exhibited by a solution of the magnetoatmospheric wave equations for motion in the presence of a horizontal magnetic field is a special case of the 'valve' type critical level discussed by Acheson (1973), with the difference that the 'valve effect' does not strictly occur; waves are captured as they approach the singular level either side and are neither reflected or transmitted, but constrained to propagate along the field line. This effect is also likely to occur for purely vertical fields. The possible importance of such critical levels to solar physics is discussed. (Auth.)