Magnetoseismic signatures and flow diagnostics beneath magnetic regions

Abstract

AbstractOne of the major, important developments in local helioseismology was the discovery by Duvall et al. (1996) that the travel times of seismic waves into sunspots from the surrounding quiet Sun significantly exceed the same in the reverse direction, a behavior they suggested was the result of rapid downflows directly beneath the sunspot photosphere. This led to the need for rapid near‐surface horizontal inflows to replace the mass evacuated from the sunspot subphotosphere by such downflows. The lack of independent evidence for such inflows led to the suggestion that the travel‐time asymmetry could be explained by a relative phase delay in the response of the sunspot photosphere to incoming waves with respect to that of the quiet Sun. In the succeeding ten years major progress has been made in our understanding of how magnetic photospheres respond to incoming waves, at the instigation of theoretical work by Spruit, Cally and Bogdan. This has led to the recognition of inclined penumbral magnetic fields as a major avenue for control work on the subject of the traveltime asymmetry and its relation to the absorption of p‐modes by magnetic regions. A major recent development has been the discovery by Schunker et al. (2005) that the phase of this response in Doppler observations of penumbral photospheres depends strongly on the vantage of the Doppler measurements projected into the vertical plane of the magnetic field. This discovery heavily reinforces the proposition that the travel‐time asymmetry is largely the signature of the same irreversible damping processes that are responsible for the strong absorption of p‐modes in magnetic regions. We will elaborate on the implications of the foregoing developments respecting the diagnostics of subphotospheric flows based on seismic observations in which magnetic regions cannot be avoided. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)