Seismology Reveals Low-Rossby-Number Convection in the Interior of the Sun

Abstract:

Convection in the solar interior is thought to comprise structures on a spectrum of scales. Here, we analyze observations of the wavefield in the solar surface using techniques of time-distance helioseismology to image convective flows in the solar interior. We downsample and synthesize 900 billion wavefield observations taken by the Helioseismic and Magnetic Imager 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. At degrees less than 60, convective velocities are 20–100 times weaker than current theoretical estimates. Advection is dominated by Coriolis forces in this regime, with Rossby numbers smaller than 0.01 in the interior. This constraint suggests the prevalence of a different paradigm of turbulence from that predicted by existing models. Convective Reynolds stresses, thought to power observed large-scale rotational shear and meridional circulation in the Sun, would be too weak by three orders in magnitude. In this talk I will discuss the seismic analyses that went into generating this result and the current understanding of this puzzle.