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 spectrum. We are particularly interested in the deficit brought about by strong absorption by the sunspot itself in the 3-4 mHz range and in the helioseismic character of the acoustic recently discovered by Braun et al. The holographic images computed here clearly show that over a broad range in frequency the sunspot replaces the radiation impinging into it from the ambient solar interior with an outgoing flux that is only a fraction of that which it receives. This 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 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 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 moat has its own absorption mechanism, they show little independent evidence to indicate that the moat otherwise behaves very differently from the quiet Sun where absorption and reemission of flux are concerned. Helioseismic images of conspicuous halos that appear in 6 mHz power maps show no significant enhancement of 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 power. This seems to explain peculiarities in the spectrum of 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 power halos seen in 6 mHz power maps.