Travel Time Anisotropy Due to Subsurface Magnetic Fields

While there has been near-continuous observation of the Sun’s surface magnetic field over the past few decades, there remains little insight concerning the configuration of the subsurface field. This work presents a simple helioseismic technique to estimate the orientation of the subsurface horizontal magnetic field as well as a proxy for the horizontal field’s magnitude. The technique is applied to several HMI Dopplergram series of active regions and quiet-sun regions, with notable differences between the two. Some discussion on the challenges and limitations of the technique is provided.