Using Observations of High-Latitude Flows to Ascertain the Sun's Convective Regime

Abstract:

Recently performed numerical simulations of the convection in solar-like stars suggest that different regimes of convective morphology are possible depending on the star's rotation rate and the typical speed of the convective flows (i.e., on the Rossby number of the convection). At convective onset when the convective speeds and Rossby number are tiny, convection is isolated to a band surrounding the equator. As the supercriticality of the convection increases, and the typical convective flow speeds and Rossby number rise, the poles destabilize as well. The polar convection initially appears in a cellular form, but as the Rossby number continues to increase, eventually a plumy form emerges. All of these states with low to moderate Rossby number possess a solar-like differential rotation where the equator rotates more rapidly than the poles. If the convective speeds and Rossby number rise past a critical threshold, the sense of the differential rotation reverses. Thus, the most supercritical models possess antisolar differential rotation with rapidly rotating poles and a slow equator. Currently, it is unclear in which convective regime the Sun may actually reside. These different regimes or states are likely to possess very different dynamo properties. Hence, a prerequisite to building successful dynamo models of the Sun's magnetic activity cycle may be observations that ascertain the Sun's convective regime. Here we will emphasize how the convection and differential rotation differ across regimes and how high-latitude flow observations could be used to distinguish between convective states. We will explore observational signatures and discuss key features and structures of the convection for which missions like Solar Orbiter and particularly Solaris should be searching.