Structures in compressible magnetoconvection and the nature of umbral dots

Details Structures in compressible magnetoconvection and the nature of umbral dots Structures in compressible magnetoconvection and the nature of umbral dots

2011-05-27

arxiv.org/abs/1105.5590v1

Astronomy and Astrophysics

Astrophysics

Solar and Stellar Astrophysics

12 pages, 10 figures, submitted

Scientific paper

Structures seen in idealized numerical experiments on compressible magnetoconvection in an imposed strong vertical magnetical field show important differences with respect to those detected in observations or realistic numerical simulations of sunspot umbrae.In order to elucidate the origin of these discrepancies, here we present a series of idealized 3D compressible magnetoconvection experiments that differ from previous such experiments in several details, bringing them closer to realistic solar conditions. An initially vertical magnetic field $B_0$ is imposed on a time snapshot of fully developed solar-like turbulent convection in a layer bounded by a stable layer from above. Upon relaxation to a statistically steady state, the structure of the flow field and magnetic field is examined. Instead of the vigorous granular convection (GRC) well known to take place in nonmagnetized or weakly magnetized convection, for high values of $B_0$ heat is transported by small-scale convection (SSC) in the form of narrow, persistent convective columns consisting of slender upflows accompanied by adjacent downflow patches, reminiscent of the "convectons" identified in earlier semianalytic models. For moderate field strengths flux separation (FXS) is observed: isolated field-free inclusions of GRC are embedded in a strongly magnetized plasma with SSC. Between the SSC and FXS regimes, a transitional regime (F/S) is identified where convectons dynamically evolve into multiply segmented granular inclusions and back. Our results are in somewhat closer agreement with observed umbral structures than earlier idealized models, as they do reproduce the strong localized, patchy downflows immediately adjacent to the narrow convective columns. Based on recent observations of umbral dots we suggest that the conditions in sunspot umbr{\ae} correspond to the newly identified F/S transitional regime.

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