Issues with time--distance inversions for supergranular flows

TL;DR

This study investigates the limitations of time--distance helioseismic inversions for supergranular flows, revealing inconsistencies and potential inaccuracies in inferred flow structures at various depths due to forward problem errors.

Contribution

The paper demonstrates that different filtering schemes produce inconsistent flow inferences and highlights potential issues in the inversion process affecting supergranulation studies.

Findings

Flow inferences vary significantly with filtering scheme.

Inverted flow signs change with depth, indicating possible inaccuracies.

Inversion results may be affected by forward problem errors.

Abstract

Aims. Recent studies showed that time--distance inversions for flows start to be dominated by a random noise at a depth of only a few Mm. It was proposed that the ensemble averaging might be a solution to learn about the structure of the convective flows, e.g., about the depth structure of supergranulation. Methods. Time--distance inversion is applied to the statistical sample of ~$10^4$ supergranules, which allows to regularise weakly about the random-noise term of the inversion cost function and hence to have a much better localisation in space. We compare these inversions at four depths (1.9, 2.9, 4.3, and 6.2 Mm) when using different spatio-temporal filtering schemes in order to gain confidence about these inferences. Results. The flows inferred by using different spatio-temporal filtering schemes are different (even by the sign) even-though the formal averaging kernels and the random-noise levels are very similar. The inverted flows alterates its sign several times with depth. It is suggested that this is due to the inaccuracies in the forward problem that are possibly amplified by the inversion. It is possible that also other time--distance inversions are affected by this issue.