Measurement process and inversions using helioseismic normal-mode coupling

TL;DR

This paper explores a helioseismic technique based on normal-mode coupling to image the solar interior, emphasizing measurement processes, sensitivity analysis, and noise modeling, while addressing observational limitations.

Contribution

It develops a realistic sensitivity framework for normal-mode coupling measurements and proposes methods to mitigate observational leakage effects.

Findings

Normal-mode coupling provides a straightforward helioseismic imaging method.

Leakage effects can be mitigated to improve resolution.

Full-surface observations are crucial for optimal interpretation.

Abstract

Normal modes are coupled by the presence of perturbations in the Sun, providing a novel and under-appreciated helioseismic technique with which to image the solar interior. The process of measuring coupling between normal modes is straightforward, much more so when compared with other prevalent helioseismic techniques. The theoretical framework to interpret these measurements is well developed with the caveat that it applies only in the case where the entire surface of the Sun is observed. In practice however, the limited visibility of the Sun and line-of-sight related effects diminish the resolution of the technique. Here, we compute realistic sensitivities of normal-mode coupling measurements to flows in the solar interior and describe how to mitigate the sometimes-overwhelming effect of leakage. The importance of being able to isolate individual spherical harmonics and observe the full Sun, to which future solar observatories may aspire, is thus highlighted in our results. In the latter part of the article, we describe the noise model for the variance of coupling coefficients, a critical component to the process of inference.