Helioseismic Travel-Time Definitions and Sensitivity to Horizontal Flows Obtained From Simulations of Solar Convection

TL;DR

This study compares different travel-time definitions in helioseismology, analyzing their sensitivity to horizontal flows in solar convection simulations, and evaluates the accuracy of the ray approximation at various distances.

Contribution

It provides a detailed comparison of travel-time definitions and assesses the non-linear response of travel times to horizontal flows in solar simulations.

Findings

Travel-time difference and flow amplitude relationship is non-linear, especially at higher flow speeds.

The Gabor wavelet fit shows non-linearity in travel-time differences for flows around 300 m/s.

Ray approximation is accurate for distances >17 Mm but can be off by a factor of three at smaller distances.

Abstract

We study the sensitivity of wave travel times to steady and spatially homogeneous horizontal flows added to a realistic simulation of the solar convection performed by Robert F. Stein, Ake Nordlund, Dali Georgobiani, and David Benson. Three commonly used definitions of travel times are compared. We show that the relationship between travel-time difference and flow amplitude exhibits a non-linearity depending on the travel distance, the travel-time definition considered, and the details of the time-distance analysis (in particular, the impact of the phase-speed filter width). For times measured using a Gabor wavelet fit, the travel-time differences become nonlinear in the flow strength for flows of about 300 m/s, and this non-linearity reaches almost 60% at 1200 m/s (relative difference between actual travel time and expected time for a linear behaviour). We show that for travel distances greater than about 17 Mm, the ray approximation predicts the sensitivity of travel-time shifts to uniform flows. For smaller distances, the ray approximation can be inaccurate by more than a factor of three.