Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...479.1012z&link_type=abstract
Astrophysical Journal v.479, p.1012
Astronomy and Astrophysics
Astronomy
4
Sun: Magnetic Fields, Sun: Oscillations, Sun: Sunspots
Scientific paper
We decomposed the oscillations inside the annular region surrounding a sunspot (excluding the spot itself) into incoming waves (waves that propagate toward the spot) and outgoing waves (away from the spot) under a cylindrical coordinate system. We made the same decomposition for the oscillations in the annular region surrounding a spotless area. Incoming waves in the region surrounding the spot can be considered as waves that propagate inside the medium magnetized by the spot flux tube (the magnetic fields in the medium are basically the horizontal), incoming waves in the spotless region are considered as waves that propagate inside the nonmagnetic medium. By comparing the distributions of power with the frequency for the two kinds of waves in fixed wavenumbers, it can be found that the frequencies at which the ridges of the mode are placed have been reduced systematically for different radial order n of the mode in the magnetic field and that the power of the waves in the magnetic field has been reduced as well. The reduced power of the waves is up to 30% of that in the nonmagnetic medium. Also, comparing the variations of power versus wavenumber in fixed frequency for the two waves, we found that the wavenumbers in which the ridges of the mode in the magnetic field are placed have been shifted in the direction of higher wavenumbers with respect to the fundamental and p-modes of n = 1, 2, 3 and that the wavenumber shifts increase as the frequency increases. However, the power depression is modulated by frequency; i.e., the acoustic waves suffer absorption at some limited zones of frequency. These observations indicate that the mode coupling (or mixing) of p-modes with the corresponding magnetoacoustic-gravity (MAG) waves is generated in the magnetic field. The generation of mode mixing supports the explanation that mode conversion is probably the prime mechanism for absorption of p-modes by sunspots. In addition, the converted MAG waves are absorbed further when they propagate into the spot flux tube (in which the magnetic fields are basically vertical). The power absorbed is up to 20% of the (residual) power of the waves. No further conversion inside the tube is observed.
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