THE CUTOFF FREQUENCY FOR FAST-MODE MAGNETOHYDRODYNAMIC WAVES IN AN ISOTHERMAL ATMOSPHERE WITH A UNIFORM HORIZONTAL MAGNETIC-FIELD

This study analytically examines conditions for reflection of magnetoh ydrodynamic (MHD) fast-mode waves propagating upward in an isothermal atmosphere. A new method of transforming the linearized wave equation into Klien-Gorden form is utilized to calculate a local cutoff (critic al) frequency for these waves. This critical frequency, OMEGA(crit)2 = omega(s)2 + 3/4(V(A)/V(f))2omega(a)2, determines the height in the at mosphere at which reflection dominates and above which wave propagatio n is effectively cut off. Comparison of our results to those previousl y obtained shows that earlier calculations of the critical frequency f or MHD fast mode waves were done incorrectly. The results may be helpf ul in explaining the short-period end of the spectrum of the solar glo bal p-mode oscillations. They may also be important in studies of wave propagation and wave trapping in highly magnetized stellar atmosphere s. Subject headings: MHD - Sun: oscillations