Chromatic Holography of the Sunspot Acoustic Environment
Abstract
We use helioseismic holography to obtain seismic images of a sunspot and its local environment over the 3-8 mHz acoustic spectrum. We are particularly interested in the acoustic deficit brought about by strong acoustic absorption by the sunspot itself in the 3-4 mHz range and in the helioseismic character of the ``acoustic moat'' recently discovered by Braun et al. The holographic images computed here clearly show that over a broad range in frequency the sunspot replaces the acoustic radiation impinging into it from the ambient solar interior with an outgoing acoustic flux that is only a fraction of that which it receives. This acoustic deficit persists uniformly over the 3-7 mHz spectrum, even as the reflectivity of the quiet-Sun photosphere goes from being an almost perfect, specular reflector at 4 mHz to an almost perfect absorber at 5 mHz. As far as we can judge, the acoustic moat surrounding the sunspot need not require a helioseismic absorption mechanism of its own. Its signature in 3-4 mHz images could arise from simple scattering of an acoustic deficit that originates in the nearby sunspot. Such scattering may be the result of a thermal perturbation resulting from the blockage of convective heat transport through the sunspot photosphere. Alternatively, it could be the signature of a Doppler perturbation attendant to the rapid convective outflow that might be driven by such a thermal accumulation. While the results presented here do not rule out the possibility that the acoustic moat has its own absorption mechanism, they show little independent evidence to indicate that the acoustic moat otherwise behaves very differently from the quiet Sun where absorption and reemission of acoustic flux are concerned.
Helioseismic images of conspicuous halos that appear in 6 mHz acoustic power maps show no significant enhancement of acoustic emission from these regions. A fairly broad region surrounding the sunspot appears to render a weak enhancement, ~2.5%, in the local generation of 5 mHz acoustic power. This seems to explain peculiarities in the spectrum of acoustic flux balance measurements based on Hankel analysis. The distribution of the 5 mHz ``acoustic egression'' excess is fairly diffuse and does not seem to be spatially correlated with the strong acoustic power halos seen in 6 mHz acoustic power maps.- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1999
- DOI:
- 10.1086/306560
- Bibcode:
- 1999ApJ...510..494L
- Keywords:
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- SUN: OSCILLATIONS;
- SUN: SUNSPOTS;
- WAVES;
- Sun: Oscillations;
- Sun: Sunspots;
- Waves