Convection and the Origin of Evershed Flows in Sunspot Penumbrae

We discuss a numerical 3D radiation-MHD simulation of penumbral fine structure in a small sunspot. This simulation shows the development of short filamentary structures with horizontal flows, similar to observed Evershed flows, and an inward propagation of these structures at a speed compatible with observations. Although the lengths of these filaments are much shorter than observed, we conjecture that this simulation qualitatively reproduces the mechanisms responsible for filament formation and Evershed flows in penumbrae. We conclude that the Evershed flow represents the horizontal flow component of overturning convection in gaps with strongly reduced field strength. The top of the flow is always directed outward—away from the umbra— because of the broken symmetry due to the inclined magnetic field. Upflows occur in the inner parts of the gaps and most of the gas turns over radially (outwards and sideways), and descends back down again. The ascending, cooling and overturning flow tends to bend magnetic field lines down, forcing a weakening of the field that makes it easier for gas located in an adjacent layer—further in—to initiate a similar sequence of motion, aided by lateral heating, thus causing the inward propagation of the filament. Subject headings: sunspots – magnetic field