Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...451..859b&link_type=abstract
Astrophysical Journal v.451, p.859
Astronomy and Astrophysics
Astronomy
123
Scattering, Sun: Oscillations, Sun: Sunspots
Scientific paper
The acoustic scattering properties of two large sunspots and a plage are determined from a Fourier-Hankel decomposition of p-mode amplitudes as measured from 68 hr subsets of a larger set of observations made at the South Pole in 1988. We present measurements of p-mode absorption and scattering phase shifts as functions of the incident mode properties (degree, radial order, and azimuthal order).
In the two sunspots we observe a distinct modulation of the absorption with temporal frequency in a fashion that is very nearly independent of the degree of the mode. In particular, the absorption exhibits a broad peak at 3 mHz, an absence of absorption at 5 mHz, and a rise in absorption at higher temporal frequencies. This variation is in good qualitative agreement with a prediction of a model of p-mode absorption by slow mode conversion described recently by Spruit & Bogdan (see also the recent work of Cally & Bogdan). The spotless plage also shows significant p-mode absorption, at a level of 20% of the value seen in the spots. Control tests made by repeating the analysis in a region of quiet Sun have confirmed the recent observation by Bogdan et al. that the quiet Sun shows an apparent acoustic emission. In particular, we find a significant anticorrelation between the absorption coefficient in the quiet Sun and the values measured in a nearby sunspot.
The scattering phase shifts are shown to increase with the degree of the modes with an increase that is faster than a linear relation. At constant degree the phase shifts increase with temporal frequency (radial order) before leveling off at a roughly constant value. We suggest that this behavior is consistent with a relatively shallow sound-speed perturbation produced by the spots. The variation of the phase shift and the absorption with azimuthal order suggest that the phase shifts are largely produced within the area of the sunspot, while significant absorption is occurring in a more extended region. Related to this observation is the finding that the plage, while absorbing p-modes, produces no measurable phase shifts. Upper limits place any phase shifts caused by the plage to be less than about 5 % of that observed in the spots.
Finally, we find strong evidence of mode mixing in the two sunspots, as demonstrated by a significant correlation of the phases of incident p-modes of a given radial order with the phases of outgoing modes of adjacent orders at the same temporal frequency.
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