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Title:
The Local Helioseismology of Inclined Magnetic Fields and the Showerglass Effect
Authors:
Schunker, H.; Braun, D. C.; Cally, P. S.; Lindsey, C.
Affiliation:
AA(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Clayton, VIC 3800, Australia; , .), AB(NorthWest Research Associates, Inc., Colorado Research Associates Division, 3380 Mitchell Lane, Boulder, CO 80301; , .), AC(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Clayton, VIC 3800, Australia; , .), AD(NorthWest Research Associates, Inc., Colorado Research Associates Division, 3380 Mitchell Lane, Boulder, CO 80301; , .)
Publication:
The Astrophysical Journal, Volume 621, Issue 2, pp. L149-L152. (ApJL Homepage)
Publication Date:
03/2005
Origin:
UCP
ApJ Keywords:
Magnetic Fields, Methods: Data Analysis, Sun: Helioseismology, Sun: Sunspots
DOI:
10.1086/429290
Bibliographic Code:
2005ApJ...621L.149S

Abstract

We present evidence for the dependence of helioseismic Doppler signatures in active regions on the line-of-sight angle in inclined magnetic fields. Using data from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory, we performed phase-sensitive holography in the penumbrae of sunspots over the course of several days as the spots traversed the solar disk. Control correlations, which comprise a correlation of the surface wave amplitude with the incoming acoustic wave amplitude from a surrounding region, were mapped. There is a direct dependence of control-correlation phase signatures on the line-of-sight angle in the plane defined by the vertical and magnetic field vectors. The phase shift of waves observed along directions close to the orientation of the magnetic field is smaller than the phase shift observed when the line of sight is at a significant angle with respect to the field orientation. These findings have important implications for local helioseismology. The variation in phase shift (or the equivalent acoustic travel-time perturbations) with line-of-sight direction suggests that a substantial portion of the phase shift occurs in the photospheric magnetic field. Observations of the vector components of the field may be used to develop a proxy to correct these phase perturbations (known as the acoustic showerglass) that introduce uncertainties in the signatures of acoustic perturbations below the surface.
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