Seismic holography of solar activity

Helioseismic images of sunspots show a remarkable acoustic anomaly surrounding the sunspot. We applied the computational formalism of "helioseismic holography" to SOHO-MDl observations to render acoustic images of NOAA AR 7973, an active region containing a moderately large sunspot. The results of this study are based on simple "acoustic power holography," to image the absorption of p-mode waves by the sunspot. These images clearly show a strong, compact acoustic deficit representing the sunspot, as well as plages in the neighborhood of the sunspot, consistent with earlier results of "Hankel analysis." However, they also show surrounding the sunspot a conspicuous acoustic halo extending out to a radius of approximately 35,000 km. We propose that this "acoustic moat" is the helioseismic manifestation of a single convection eddy that is driven by the thermal disturbance resulting from the local blockage of convective transport in the sunspot subphotosphere. Depth diagnostics based on acoustic focus show a rapidly defocusing sunspot image as the focal plane is submerged. Acoustic noise models in which absorption by the sunspot is entirely superficial yield images that defocus significantly more slowly with increasing focal-plane depth than the SOHO-MDI images of NOAA AR 7973. Extending the absorption significantly beneath the model photosphere enhances the discrepancy. More recent tests tentatively suggest that this "focus anomaly" is the result of neglect of image smearing introduced into the MDI instrument to suppress aliasing, and that a proper account of the instrumental MTF will render defocus profiles roughly consistent with superficial absorption. Our holographic images roughly indicate that the sunspot in NOAA AR 7973 absorbs low-l waves with approximately the same efficiency as it does high-l waves. Contrary to widely held opinion, this result is entirely consistent with that of the Hankel analysis, given that the absorption of waves by magnetic regions is indeed superficial. We expect that the efficient absorption of low-l waves will make it possible to image large active regions on the far side of the Sun by the acoustic-absorption signatures they render at their antipodes.