NEOCON · New Paradigm of Stellar Convection

Tension between solar observations and theoretical and numericalmodels is shifting the paradigm of stellar convection: simulationsproduce dominant large-scale flows that are absent in the Sun and thesupergranulation scale which according to helioseismology is dominantin the Sun is not captured by simulations. This discrepancy, named theconvective conundrum, is the most likely reason why solar differentialrotation and global dynamo cannot be reproduced by currentsimulations. Canonical theory of convection, adopted in many models,assumes an unstable temperature gradient and local driving ofconvection leading to large-scale flows. More recent models relaxingthis assumption suggest that surface cooling is the main driver ofconvection leading to non-local flows in the form of cool entropyrain. In this scenario the bulk of the convection zone is weaklystably stratified in stark contrast to traditional mixing lengthpicture. Recent solar observations also suggest fast small-scaleplumes and weakly stable stratification.Simulations of stellar convection are limited by computationalresources such that fully realistic models of stars are unreachable inthe foreseeable future. This is most likely why surface driving ofconvection is too weak in current simulations. In the NeoCon projectthis is addressed via a solar observation-inspired subgrid-scale modelincorporating entropy rain to reach the highly non-local regime ofconvection conjectured to exist in stars. This has paradigm-shiftingconsequences for theories of stellar convection and dynamos withpotential ramifications for stellar structure and evolution. Inaddition to capturing convection accurately, another crucialrequirement is ensuring the correct force balance in stellar interiorswith strong multiscale magnetic fields. Strong magnetism is anotherpossible solution to the convective conundrum which is addressed inthe NeoCon project by comprehensive study of magnetic back-reaction onconvection.