Sub-Wavelength Resolution Imaging of the Solar Deep Interior

TL;DR

This paper demonstrates the potential to achieve sub-wavelength resolution in helioseismic imaging by analyzing wave travel time shifts caused by small-scale thermal anomalies and jets within the solar interior.

Contribution

It introduces a method to detect sub-wavelength features in the solar interior using time-distance helioseismology and numerical simulations, extending resolution capabilities beyond traditional limits.

Findings

Oscillatory time shift signals reveal sub-wavelength features.

Simulations of localized jets and sound-speed perturbations support the method.

Current observational noise levels limit detection of certain sharp features.

Abstract

We derive expectations for signatures in the measured travel times of waves that interact with thermal anomalies and jets. A series of numerical experiments that involve the dynamic linear evolution of an acoustic wave field in a solar-like stratified spherical shell in the presence of fully 3D time-stationary perturbations are performed. The imprints of these interactions are observed as shifts in wave travel times, which are extracted from these data through methods of time-distance helioseismology \citep{duvall}. In situations where at least one of the spatial dimensions of the scatterer was smaller than a wavelength, oscillatory time shift signals were recovered from the analyses, pointing directly to a means of resolving sub-wavelength features. As evidence for this claim, we present analyses of simulations with spatially localized jets and sound-speed perturbations. We analyze 1 years' worth solar observations to estimate the noise level associated with the time differences. Based on theoretical estimates, Fresnel zone time shifts associated with the (possible) sharp rotation gradient at the base of the convection zone are of the order 0.01 - 0.1 s, well below the noise level that could be reached with the currently available amount of data ($\sim 0.15-0.2$ s with 10 yrs of data).