The Local Helioseismology of Inclined Magnetic Fields and the Showerglass Effect

Abstract

We present evidence for the dependence of helioseismic Doppler signatures in active regions on the line-ofsight 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-ofsight 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.

Local Helioseismology of Sunspots: Current Status and Perspectives

“…In particular, one concern was about the influence of the "inclined-field or magnetic shower-glass effect" (Schunker et al, 2005;Schunker, Braun, and Cally, 2007), which shows that helioseismic travel times measured from Dopplergrams may depend on the line-of-sight angle in inclined magnetic fields of active regions. However, the Hinode results were obtained from the Ca II H intensity data, and this effect does not exist in the travel times measured from intensity oscillations (Zhao and Kosovichev, 2006).…”

“…It has been noticed by Schunker et al (2005) that when a sunspot is located near the limb the phase shifts of acoustic waves (corresponding to travel times) vary in a sunspot penumbra relative to the direction to the disk center. They attributed this to a phase change of acoustic waves traveling through the inclined magnetic field of the penumbra, calling these phase perturbations "the acoustic showerglass", and suggesting that this effect might substantially affect the inversion results.…”

Physical Properties of Wave Motion in Inclined Magnetic Fields within Sunspot Penumbrae

“…The dependence of the phase shift of the observed ingression correlations with azimuthal angle around sunspot penumbrae, as viewed from different observational vantage points, shows that the incoming phase shifts must be (at least partly) photospheric in origin and are influenced by the presence of inclined magnetic fields (Schunker et al, 2005). Analysis of the variation of both the amplitude and phase of the surface velocities provides an opportunity to characterize the magnetically influenced acoustic signature as an ellipse with properties determined by the magnetic fields.…”

“…In Schunker et al (2005) and Schunker, Braun, and Cally (2007) the phase shift, caused by the surface perturbations to the incident wave, is calculated at various frequencies. Here we extend that research and monitor the variation of the correlation amplitude within the sunspot penumbral region, which is then combined with the phase information to form estimates of the surface velocity ellipse.…”

“…It is difficult to assess the validity of this, and so it is used as a reasonable working assumption subject to some caution. We use the same angle θ p as defined in Schunker et al (2005). The line-of-sight vector is projected onto the plane containing the local magnetic field vector and the radial vector.…”