Anomalously Weak Solar Convection

TL;DR

This study uses helioseismology to measure solar interior flows, revealing convective velocities much weaker than models predict and suggesting the Sun's convection is dominated by Coriolis forces, indicating a different turbulence paradigm.

Contribution

It provides the first large-scale observational constraints on solar convective velocities across a broad range of scales, challenging existing theoretical models.

Findings

Convective velocities are 20-100 times weaker than theoretical estimates.

Coriolis forces dominate advection for large scales, implying rapid solar rotation.

Large-scale convection may be quasi-geostrophic, with non-aligned iso-rotation contours.

Abstract

Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical-harmonic degree $\ell$. Within the wavenumber band $\ell<60$, Convective velocities are 20-100 times weaker than current theoretical estimates. This suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers $\ell<60$, with Rossby numbers smaller than $\sim10^{-2}$ at $r/R_\odot=0.96$, suggesting that the Sun may be a much faster rotator than previously thought, and that large-scale convection may be quasi-geostrophic. The fact that iso-rotation contours in the Sun are not co-aligned with the axis of rotation suggests the presence of a latitudinal entropy gradient.