Imaging the Solar Tachocline by Time-Distance Helioseismology

TL;DR

This paper introduces a new time-distance helioseismology technique for two-dimensional imaging of the solar tachocline, revealing latitudinal variations in sound speed and demonstrating its effectiveness with both simulations and real data.

Contribution

The paper develops and validates a novel 2D helioseismology method for imaging the solar tachocline, enabling detailed analysis of its structure and variations.

Findings

Successfully recovers features of simplified tachocline models

Provides the first 2D sound-speed perturbation image of the tachocline

Finds latitudinal variation in sound-speed perturbation amplitude

Abstract

The solar tachocline at the bottom of the convection zone is an important region for the dynamics of the Sun and the solar dynamo. In this region, the sound speed inferred by global helioseismology exhibits a bump of approximately 0.4% relative to the standard solar model. Global helioseismology does not provide any information on possible latitudinal variations or asymmetries between the Northern and Southern hemisphere. Here, we develop a time-distance helioseismology technique, including surface- and deep-focusing measurement schemes and a combination of both, for two-dimensional tomographic imaging of the solar tachocline that infers radial and latitudinal variations in the sound speed. We test the technique using artificial solar oscillation data obtained from numerical simulations. The technique successfully recovers major features of the simplified tachocline models. The technique is then applied to SOHO/MDI medium-l data and provides for the first time a full two-dimensional sound-speed perturbation image of the solar tachocline. The one-dimensional radial profile obtained by latitudinal averaging of the image is in good agreement with the previous global helioseismology result. It is found that the amplitude of the sound-speed perturbation at the tachocline varies with latitude, but it is not clear whether this is in part or fully an effect of instrumental distortion. Our initial results demonstrate that time-distance helioseismology can be used to probe the deep interior structure of the Sun, including the solar tachocline.