Investigating toroidal flows in the Sun using normal-mode coupling

TL;DR

This paper uses normal-mode coupling to invert helioseismic data and map the Sun's toroidal convective flows, revealing their depth, scale, and frequency characteristics, and benchmarking results against other methods.

Contribution

It introduces a novel inversion technique for toroidal flows using mode-coupling measurements, providing detailed flow maps and noise evaluation, advancing solar interior dynamics understanding.

Findings

Convective power becomes more isotropic with smaller scales.

No peak in toroidal-flow power at supergranular scales.

Supergranulation is predominantly poloidal.

Abstract

Helioseismic observations have provided valuable datasets with which to pursue the detailed investigation of solar interior dynamics. Among various methods to analyse these data, normal-mode coupling has proven to be a powerful tool, used to study Rossby waves, differential rotation, meridional circulation, and non-axisymmetric multi-scale subsurface flows. Here, we invert mode-coupling measurements from Helioseismic Magnetic Imager (HMI) and Michelson Doppler Imager (MDI) to obtain mass-conserving toroidal convective flow as a function of depth, spatial wavenumber, and temporal frequency. To ensure that the estimates of velocity magnitudes are proper, we also evaluate correlated realization noise, caused by the limited visibility of the Sun. We benchmark the near-surface inversions against results from Local Correlation Tracking (LCT). Convective power likely assumes greater latitudinal isotropy with decrease in spatial scale of the flow. We note an absence of a peak in toroidal-flow power at supergranular scales, in line with observations that show that supergranulation is dominantly poloidal in nature.