Assessing the Observability of Deep Meridional Flow Cells in the Solar Interior

TL;DR

This paper investigates the challenges in detecting multiple meridional flow cells within the Sun's interior using helioseismic methods, highlighting the influence of averaging kernel artifacts and flow strength on observability.

Contribution

It demonstrates how averaging kernel side lobes can obscure deep flow structures and assesses the conditions under which multiple cells can be reliably detected.

Findings

Weak deep flows are obscured by surface signals at mid/high latitudes.

Multiple deep cells produce detectable signals at low latitudes.

Systematic biases from averaging kernels are now comparable to convective noise.

Abstract

Meridional circulation regulates the Sun's interior dynamics and magnetism. While it is well accepted that meridional flows are poleward at the Sun's surface, helioseismic observations have yet to provide a definitive answer for the depth at which those flows return to the equator, or the number of circulation cells in depth. Here, we explore the observability of multiple circulation cells stacked in radius. Specifically, we examine the seismic signature of several meridional flow profiles by convolving time-distance averaging kernels with mean flows obtained from a suite of 3D hydrodynamic simulations. At mid and high latitudes, we find that weak flow structures in the deep convection zone can be obscured by signals from the much stronger surface flows. This contamination of 1--2 m s$^{-1}$ is caused by extended side lobes in the averaging kernels, which produce a spurious equatorward signal with flow speeds that are one order of magnitude stronger than the original flow speeds in the simulations. At low latitudes, the flows in the deep layers of the simulations are stronger ($> 2$ m s$^{-1}$) and multiple cells across the convection zone can produce a sufficiently strong signal to survive the convolution process. Now that meridional flows can be measured over two decades of data, the uncertainties arising from convective noise have fallen to a level where they are comparable in magnitude to the systematic biases caused by non-local features in the averaging kernels. Hence, these systematic errors are beginning to influence current helioseismic deductions and need broader consideration.