Acoustic Signatures of Subphotospheric Structure Underlying Sunspots

Abstract

Helioseismic holography of active regions at frequencies in the range of 5-6 mHz renders acoustic signatures that we think signify acoustic perturbations several megameters beneath the photosphere. The application of holographic diagnostics at 5-6 mHz gives us images with considerably finer horizontal spatial resolution, and likewise much finer depth resolution with respect to focus, than the 3 mHz diagnostics we have recently published. This Letter reports comparative results of standard focus-defocus diagnostics of a single sunspot at 3 and 6 mHz. Images of the ``acoustic egression power'' at 6 mHz show a remarkable, compact acoustic deficit that persists in acoustic focal planes submerged up to 11.2 Mm beneath the solar surface. We propose that this and other similar features associated with other active regions are the result of refraction or scattering by submerged thermal or Doppler perturbations of an acoustic deficit that is caused by strong wave absorption in the overlying photosphere of the active region.