Investigating Toroidal Flows in the Sun Using Normal-mode Coupling

Abstract

Helioseismic observations have provided valuable data sets with which to pursue the detailed investigation of solar interior dynamics. Among various methods to analyze these data, normal-mode coupling has proven to be a powerful tool, used to study Rossby waves, differential rotation, meridional circulation, and nonaxisymmetric multiscale subsurface flows. Here, we invert mode-coupling measurements from the Helioseismic Magnetic Imager and the Michelson Doppler Imager 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. The convective power likely assumes greater latitudinal isotropy with a decrease in spatial scale of the flow. We note the absence of a peak in toroidal-flow power at supergranular scales, in line with observations that show that supergranulation is dominantly poloidal in nature.