Modelling photospheric magnetoconvection

Abstract

The increasing power of computers makes it possible to model the nonlinear interaction between magnetic elds and convection at the surfaces of solar-type stars in ever greater detail. We present the results of idealized numerical experiments on twodimensional magnetoconvection in a fully compressible perfect gas. We rstvary the aspect ratio of the computational box and show that the system runs through a sequence of convective patterns, and that it is only for a su ciently wide box( 6) that the ow becomes insensitive to further increases in . Next, setting =6,we decrease the eld strength from a value strong enough to halt convection and nd transitions to small-scale steady convection, next to spatially modulated oscillations ( rst periodic, then chaotic) and then to a new regime of ux separation, with regions of strong eld (where convection is almost completely suppressed) separated by broad convective plumes. We also explore the e ects of altering the boundary conditions and show that this sequence of transitions is robust. Finally, we relate these model calculations to recent high-resolution observations of solar magnetoconvection, in plage regions as wellasinlight bridges and the umbrae of sunspots.

Fine structure and dynamics in a light bridge inside a solar pore

“…This means that the size of the grains decreases and their lifetime increases the deeper they are located in the umbra. This is in good agreement with the model simulations of Weiss et al (1996) and Blanchflower et al (1998) if it is assumed that the magnetic field strength is higher in the inner end of the light bridge than in its outer end. Measurements of Rüedi et al (1995) show that the field strength in SLBs is about 1500 G lower than in the surrounding umbra.…”

“…8). This result agrees very well with numerical simulations of Weiss et al (1996) and Blanchflower et al (1998), who have shown that the structure of the convective cells is strongly dependent on the magnetic field strength.…”

“…Numerical simulations of magnetoconvection (Weiss et al 1996;Blanchflower et al 1998) show that when increasing the intrinsic magnetic field strength up to the conditions present in a sunspot umbra, the flow structure becomes completely controlled by the magnetic field. Thus the convective cells are prevented from expanding.…”

Swift-Hohenberg model for magnetoconvection

“…Numerical simulations of magnetoconvection in regions of large horizontal extent exhibit a phenomenon known as flux separation, where the convection cells and magnetic field rearrange spontaneously into areas where there is vigorous convection and weak magnetic field and other areas where the magnetic field is strong and the convection is reduced [5,9,10]. In its most extreme form, this process leads to the formation of "convectons" [11,12], which are isolated, stationary convection cells surrounded by practically stationary regions of strong magnetic field.…”

Space Weather and the Changing Sun

“…We wonder if the individual magnetic fibril is the basic mag netic entity rather than the mean field conventionally used in dynamo theory. The effect sometimes called "turbulent diamagnetism" (Blanchflower, Rucklidge, and Weiss, 1998; concentrates the magnetic field into regions of weak tur bulence and may play a role in forming the fibrils. The continual stretching of a fibril caught up in the convective flow and nonuniform rotation would continually intensify the field of the fibril.…”