Testing volume integrals of travel-time sensitivity kernels for flows

TL;DR

This study tests the accuracy of volume integrals of helioseismic travel-time sensitivity kernels by comparing measurements with model predictions, finding good agreement for ridge-filtered data but discrepancies for phase-speed filtered data.

Contribution

It introduces a method to validate sensitivity kernels by manipulating tracking rates and measuring travel times, highlighting the reliability of models for certain filtering techniques.

Findings

Good agreement between measurements and models for ridge-filtered kernels.

Discrepancies observed for phase-speed filtered kernels, especially at small phase speeds.

Measured kernel integrals are larger than model predictions at higher phase speeds.

Abstract

Context: Helioseismic inversions rely largely on sensitivity kernels, 3-D spatial functions describing how do the changes in the solar interior translate into the changes of helioseismic observables. These sensitivity kernels come in most cases from the forward modelling utilising state-of-the-art solar models. Aims: We aim to test the sensitivity kernels by comparing their volume integrals with measured values from helioseismic travel times. Methods: By manipulating the tracking rate, we mimic the additional zonal velocity present in the Dopplergram datacubes. These datacubes are then processed by a standard travel-time measurements pipeline. We investigate the dependence of the east--west travel time averaged over a box around the disc centre on the implanted tracking velocity. The slope of this dependence is directly proportional to the total volume integral of the sensitivity kernel corresponding to the used travel-time geometry. Results: We find a very good to acceptable agreement between measurements and models for travel times with ridge filtering applied. The sought dependence indeed resembles a linear function and the slope of it agrees with the expected volume integral from the forward-modelled sensitivity kernel. The agreement is less optimistic for the phase-speed filtered datacubes. A disagreement is particularly large for smallest phase speeds (filters td1--td4), for the larger phase speeds, our result indicate that the measured kernel integrals are systematically larger than expected from the forward modelling. We admit that for large phase-speeds and higher radial modes our testing procedure does not have to be appropriate.