Solar Magneto-Atmospheric Waves and Penumbral Waves.
Final rept. Sep 74-30 Sep 76,
ROCHESTER UNIV N Y DEPT OF MECHANICAL AND AEROSPACE SCIENCES
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The linearized theory of magneto-atmospheric waves involving the combined restoring forces due to buoyancy, compressibility, and magnetic field is developed for the case of a horizontal magnetic field which may vary with height. The convective stability of the basic atmosphere is considered, and it is shown that a nonuniform horizontal magnetic field may be destabilizing as well as stabilizing. It is found that penumbral waves may be identified with magneto-atmospheric waves of the plus type that are vertically trapped at photospheric levels. Although most of the wave energy is contained in the penumbral photosphere and subphotosphere, the maximum vertical velocity occurs in the chromosphere where the waves are evanescent and where they are observed in H alpha. An exact analytical solution for magneto-atmospheric wave modes is found in the case of an isothermal atmosphere permeated by a uniform horizontal magnetic field, without making the usual short-wavelength approximation. The exact solution mentioned above is also used in conjunction with a simple two-layer model of a sunspot penumbra to further study the mode of running penumbral waves. The lowest plus eigenmode of the model is in good agreement with observations of penumbral waves.