The Acoustic Showerglass. I. Seismic Diagnostics of Photospheric Magnetic Fields

Abstract

A problem of major interest in the helioseismology of active regions is the acoustics of magnetic photospheres and shallow subphotospheres. Magnetic fields suppress the photospheric signatures of acoustic waves impinging onto them from the underlying solar interior and shift their phases. The phase shifts function as a sort of acoustic showerglass that impairs the coherence of seismic waves arriving from below, degrading images of subsurface anomalies derived by mechanical reconstruction of phase-coherent waves. The purpose of this study is to characterize the ``acoustic showerglass'' in general optical terms and make a rough practical assessment of its impact on local seismic diagnostics of the shallow subphotospheres of active regions. We compile statistics comparing the acoustic field in magnetic photospheres with holographic projections of waves arriving from distant surrounding pupils. These ``local control correlations'' are consistent with an acoustic anomaly in the shallow subphotosphere of the active region that is strong but predominantly superficial; we call this the ``acoustic Wilson depression.'' The local control correlations also exhibit a phenomenon we call the ``penumbral acoustic anomaly,'' characterized by a conspicuous phase shift in regions of inclined magnetic field. This appears to be consistent with a fairly straightforward hydromechanical interpretation of the interaction of acoustic waves with photospheric magnetic forces. Detailed numerical simulations of the interaction of acoustic waves with magnetic forces can greatly facilitate our understanding of the acoustic showerglass and the thermal structure of the top few hundred kilometers of active region subphotospheres.