Magnetic and Thermal Phase Shifts in the Local Helioseismology of Sunspots

Abstract

Phase perturbations due to inclined surface magnetic field of active region strength are calculated numerically in quiet Sun and simple sunspot models in order to estimate and compare the direct and indirect (thermal) effects of the fields on helioseismic waves. It is found that the largest direct effects occur in highly inclined field characteristic of penumbrae, and scale roughly linearly with magnetic field strength. The combined effects of sunspot magnetic and thermal anomalies typically yield negative travel-time perturbations in penumbrae. Travel-time shifts in umbrae depend on details of how the thermal and density structure differs from the quiet Sun. The combined shifts are generally not well approximated by the sum of the thermal and magnetic effects applied separately, except at low field strengths of around 1 kG or less, or if the thermal shift is small. A useful rule-of-thumb appears to be that travel-time perturbations in umbrae are predominantly thermal, whereas in penumbrae they are mostly magnetic.

11 pages, 11 figures (3 colour), 1 table. Accepted by MNRAS

References in corpus (5)

• Three Dimensional MHD Wave Propagation and Conversion to Alfven Waves near the Solar Surface. I. Direct Numerical Solution
• Helioseismology of Sunspots: Confronting Observations with Three-Dimensional MHD Simulations of Wave Propagation
• Surface-focused Seismic Holography of Sunspots: I. Observations
• Numerical Models of Travel-Time Inhomogeneities in Sunspots
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Cited by in corpus (4)

• Waves in the lower solar atmosphere: the dawn of next-generation solar telescopes
• Helioseismic Holography of Simulated Sunspots: dependence of the travel time on magnetic field strength and Wilson depression
• Directional Time-Distance Probing of Model Sunspot Atmospheres
• Helioseismic holography of simulated sunspots: magnetic and thermal contributions to travel times