Seismic probes of solar interior magnetic structure

TL;DR

This paper advances solar interior imaging by employing PDE-constrained optimization to accurately characterize magnetic structures using multiple seismic wave speeds, accounting for anisotropies often neglected in prior methods.

Contribution

It introduces a new parameterization of magnetic interior properties using seven wave speeds, improving the interpretation of helioseismic data with anisotropic effects.

Findings

Significant errors occur if anisotropies are ignored in inversions.

Magnetic media are characterized by multiple wave modes with distinct speeds.

Translation invariance is lost when anisotropic effects are considered.

Abstract

Sunspots are prominent manifestations of solar magnetoconvection and imaging their subsurface structure is an outstanding problem of wide physical importance. Travel times of seismic waves that propagate through these structures are typically used as inputs to inversions. Despite the presence of strongly anisotropic magnetic waveguides, these measurements have always been interpreted in terms of changes to isotropic wavespeeds and flow-advection related Doppler shifts. Here, we employ PDE-constrained optimization to determine the appropriate parameterization of the structural properties of the magnetic interior. Seven different wavespeeds fully characterize helioseismic wave propagation: the isotropic sound speed, a Doppler-shifting flow-advection velocity and an anisotropic magnetic velocity. The structure of magnetic media is sensed by magnetoacoustic slow and fast modes and Alfv\'{e}n waves, each of which propagates at a different wavespeed. We show that even in the case of weak magnetic fields, significant errors may be incurred if these anisotropies are not accounted for in inversions. Translation invariance is demonstrably lost. These developments render plausible the accurate seismic imaging of magnetoconvection in the Sun.