Detection of Travel Time Anisotropy from Subsurface Horizontal Magnetic Fields

TL;DR

This paper develops a technique to measure and interpret travel time anisotropy caused by subsurface horizontal magnetic fields in the Sun, verified through simulations and applied to sunspot observations.

Contribution

It introduces a novel method to isolate and estimate the orientation of subsurface magnetic fields from helioseismic travel time data.

Findings

Significant travel time anisotropies detected in sunspots.

Subsurface magnetic azimuths align with surface magnetic fields.

Method validated with simulations and applied to real data.

Abstract

A time-distance measurement technique is derived to isolate phase travel time anisotropy caused by subsurface horizontal magnetic fields, and a method which uses the measured anisotropy to estimate the field's orientation is also derived. A simulation of acoustic waves propagating in a uniform, inclined magnetic field with solar background structure is used to verify the derived technique. Then, the procedure is applied to a numerical simulation of a sunspot, for which the subsurface state is known, to provide context for the results obtained from the study of several sunspots observed by the Helioseismic and Magnetic Imager. Significant anisotropies are detected, on the order of one minute, and the subsurface field's azimuth is estimated and compared with the azimuth of the surface magnetic field. In all cases, the subsurface azimuth is found to be well-aligned with that of the surface, and the results from the numerical simulation are used to interpret features in the detected travel time anisotropy.