Simulation of Solar Radiative Magneto-Convection

Abstract

The term ‘magneto-convection’ summarizes the variety of processes arisingfrom the dynamic interaction between convective motions and magnetic fieldsin an electrically conducting medium. Magneto-convective processes play animportant role in many astrophysicalsystems; their effects can be best studiedin the case of the Sun, where the relevant spatial and temporal scales ofthe phenomena can (in principle, at least) be observed. The generation ofmagnetic flux in the Sun by a self-excited dynamo process and the variousspectacular phenomena of solar activity, like sunspots, coronal loops, flares,and mass ejections all are, directly or indirectly, driven by magneto-convectiveinteractions.The large length scales of the typical convective flow structures on theSun lead to high (hydrodynamic and magnetic) Reynolds numbers, so thatthe magneto-convective processes typically involve nonlinear interactions andformation of structures and patterns. Fig.1 illustrates typical regimes ofmagneto-convection near the visible solar surface, differing mainly in theamount of magnetic flux per unit area (i.e., average magnetic field strength)and the orientation of the field. In the ‘quiet’ Sun, some magnetic flux is con-centrated in bright magnetic elements, isolated patches with field strengthsexceeding 1000G (corresponding to 0.1Tesla). In magnetically active regions,such magnetic elements densely populate the dark convective downflow net-work and decrease the size of convective upflows (‘granules’ in solar physicslingo). In the dark core of a sunspot (the so-called umbra), the strong verti-cal magnetic field is space-filling and largely suppresses the convective energytransport. The less dark, striated periphery of a sunspot (the penumbra) har-