Umbral oscillations as resonant modes of magneto-atmospheric waves

Details Umbral oscillations as resonant modes of magneto-atmospheric waves Umbral oscillations as resonant modes of magneto-atmospheric waves

May 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981soph...71...21s&link_type=abstract

Solar Physics, vol. 71, May 1981, p. 21-38.

Physics

78

Magnetohydrodynamic Waves, Plasma Oscillations, Plasma Resonance, Solar Atmosphere, Sunspots, Umbras, Atmospheric Models, Chromosphere, Convection, Eigenvalues, Linear Equations, Photosphere, Resonant Frequencies, Solar Magnetic Field, Wave Equations

Scientific paper

Umbral oscillations in sunspots are identified as a resonant response of the umbral atmosphere to forcing by oscillatory convection in the subphotosphere. The full, linearized equations for magnetoatmospheric waves are solved numerically for a detailed model of the umbral atmosphere, for both forced and free oscillations. Resonant 'fast' modes are found, the lowest mode having a period of 153 s, typical of umbral oscillations. A comparison is made with a similar analysis by Uchida and Sakurai (1975), who calculated resonant modes using an approximate ('quasi-Alfven') form of the wave equations. Whereas both analyses give an appropriate value for the period of oscillation, several new features of the motion follow from the full equations. The resonant modes are due to upward reflection in the subphotosphere (due to increasing sound speed) and downward reflection in the photosphere and low chromosphere (due to increasing Alfven speed); downward reflection at the chromosphere-corona transition is unimportant for these modes.

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