Time-Dependence of Subsurface Solar Convection Using the Time-Distance Deep-Focus Method

TL;DR

This study evaluates the accuracy of time-distance helioseismology in measuring deep solar convection, finds limitations at smaller scales, and observes cycle-dependent variations in convective power, highlighting discrepancies with theoretical models.

Contribution

It validates the deep-focus helioseismic method using simulations and applies it to solar data, revealing scale-dependent accuracy and solar cycle variations in convection power.

Findings

Helioseismic measurements are accurate for large-scale convection ($\

Power spectrum at larger scales ($\

Cycle-dependent variations in convective power observed

Abstract

We re-examine the deep-focus methodology of time-distance helioseismology previously used to estimate the power spectrum of the solar convection at a depth of about 30 Mm, which was found to be significantly weaker than predicted by theory and simulations. The Global Acoustic, Linearized Euler (GALE) and Eulerian Lagrangian (EULAG) codes are used to generate ground-truth simulations to evaluate the accuracy of the inferred convective power spectrum. This validation process shows that the power spectrum derived using the time-distance methodology diverges significantly from ground truth beyond spatial scales corresponding to the spherical harmonic degree $\ell=15$--$30$ because of the limited resolution of helioseismic measurements at that depth. However, the power estimated at larger spatial scales ($\ell<15$) is sufficiently accurate. We then apply the methodology to solar data selected from throughout Solar Cycle 24 and find some evidence that the magnitude of the convective power changes throughout the Cycle. An average of the convective power across the Solar Cycle reveals a spectrum that is qualitatively similar to previous estimates, though about half an order of magnitude greater. The disagreement between observations of solar convection and the magnitudes predicted by simulations persists.