Authors: John Stefan (NJIT), Alexander Kosovichev (NJIT)

A long-standing challenge for helioseismology has been to extend subsurface measurements of quantities such as flow speed and sound-speed perturbations to the magnetic field. Here, we present a time-distance technique which isolates the perturbation caused by subsurface horizontal magnetic fields to helioseismic travel times. The technique enables the direct measurement of the horizontal field’s orientation as well as a proxy for the field’s magnitude. After validating the technique using several independent MHD simulations, it is applied to several sunspots observed by the Helioseismic and Magnetic Imager (HMI). We examine how properties of the horizontal magnetic field change with depth. Additionally, we explore how the SNR from HMI-derived measurements is impacted by duration of observation, input and output resolution, and existing surface magnetic fields. We provide some insight as to how this methodology can be extended to the radial magnetic field using frequency-dependent helioseismic measurements.