Linear Sensitivity of Helioseismic Travel Times to Local Flows

TL;DR

This paper develops a Born approximation method to compute how subsurface flows affect helioseismic travel times, improving flow inference accuracy in solar studies.

Contribution

It introduces a new Born approximation-based approach for calculating travel time sensitivities to subsurface flows, accounting for depth dependence and vector components.

Findings

Sensitivity functions for each flow component are derived.

Depth sensitivity relates to the kinetic energy density of oscillation modes.

Born approximation results differ significantly from ray approximation for strong depth-dependent flows.

Abstract

Time-distance helioseismology is a technique for measuring the time for waves to travel from one point on the solar surface to another. These wave travel times are affected by advection by subsurface flows. Inferences of plasma flows based on observed travel times depend critically on the ability to accurately model the effects of subsurface flows on time-distance measurements. We present a Born approximation based computation of the sensitivity of time distance travel times to weak, steady, inhomogeneous subsurface flows. Three sensitivity functions are obtained, one for each component of the 3D vector flow. We show that the depth sensitivity of travel times to horizontally uniform flows is given approximately by the kinetic energy density of the oscillation modes which contribute to the travel times. For flows with strong depth dependence, the Born approximation can give substantially different results than the ray approximation.