MHD Simulations of Penumbra Fine Structure

Abstract

We present the results of numerical 3D magnetohydrodynamic ( MHD) simulations with radiative energy transfer of fine structure in a small sunspot of about 4 Mm width. The simulations show the development of filamentary structures and flow patterns that are, except for the lengths of the filaments, very similar to those observed. The filamentary structures consist of gaps with reduced field strength relative to their surroundings. Calculated synthetic images show dark cores like those seen in the observations; the dark cores are the result of a locally elevated ¼ 1 surface. The magnetic field in these cores is weaker and more horizontal than for adjacent brighter structures, and the cores support a systematic outflow. Accompanying animations show the migration of the dark-cored structures toward the umbra, and fragments of magnetic flux that are carried away from the spot by a large-scale ''moat flow.'' We conclude that the simulations are in qualitative agreement with observed penumbra filamentary structures, Evershed flows, and moving magnetic features. Subject headingg s: magnetic fields -sunspots Online material: mpeg animations

On the emergence of toroidal flux tubes: general dynamics and comparisons with the cylinder model

“…Petrie & Neukirch 1999) to studies of magnetoconvection (e.g. Heinemann et al 2007). A useful collection of these methods, and general sunspot physics, is presented in Thomas & Weiss (1992, 2008.…”

Recent Evidence for Convection in Sunspot Penumbrae

“…3D MHD simulations of such structures (Nordlund 2006;Heinemann 2006) reproduce observed dark lanes running along the center of such structures (Lites et al 2004) and demonstrate that the origin of this dark structure is the same as proposed for the convective gap model. Of considerable importance is therefore that dark-cored light bridge structures occasionally show smooth transitions to dark-cored penumbral filaments (Langhans 2006;Scharmer et al 2007), strongly suggesting similar origin.…”

“…Evidence for this conclusion comes from recent SOT/Hinode and SST observations Zakharov et al 2008), showing vertical flows of the right magnitude to explain the penumbral radiative heat flux. Recent numerical 3D MHD simulations (Heinemann et al 2007;Rempel et al 2008) reproduce fundamental properties of observed penumbrae and confirm the convective origin of penumbral filaments. The simulations show that the nature of this convection takes place in gaps with up to 2 Mm depth and that any roll-like convection (Danielson 1961;Rempel et al 2008), if present, is of small importance (Rempel et al 2008).…”

“…Added to this flow pattern is a radially outward (Evershed) flow, explained by Scharmer et al (2008b) on the basis of 3D MHD simulations (Heinemann et al 2007) as being identical to the horizontal component of this convection. In our model, the dark cores of the penumbral filaments correspond to locations of convective upflows, in contradiction with what we expect from field-free convection.…”

Solar surface magnetoconvection simulations: A brief review of solar dermatology

“…Similarly pores, which are the smallest features clearly associated with the solar cycle have been studied and compared to observations [18] . More recently sunspots have been tackled, including sunspot umbrae and umbral dots [19] , light bridges [20] , and penumbral filaments [21,22] . Figure 5 is an image from the simulation by Rempel et al [22] , showing a sunspot umbra including umbral dots, a rudimentary penumbra as well as the quietSun.…”