Mesogranulation and small-scale dynamo action in the quiet Sun

Abstract

Regions of quiet Sun generally exhibit a complex distribution of small-scale magnetic field structures, which interact with the near-surface turbulent convective motions. Furthermore, it is probable that some of these magnetic fields are generated locally by a convective dynamo mechanism. In addition to the well-known granular and supergranular convective scales, various observations have indicated that there is an intermediate scale of convection, known as mesogranulation, with vertical magnetic flux concentrations accumulating preferentially at mesogranular boundaries. Our aim is to investigate the small-scale dynamo properties of a convective flow that exhibits both granulation and mesogranulation, comparing our findings with solar observations. Adopting an idealised model for a localised region of quiet Sun, we use numerical simulations of compressible magnetohydrodynamics, in a 3D Cartesian domain, to investigate the parametric dependence of this system (focusing particularly upon the effects of varying the aspect ratio and the Reynolds number). In purely hydrodynamic convection, we find that mesogranulation is a robust feature of this system provided that the domain is wide enough to accommodate these large-scale motions. The mesogranular peak in the kinetic energy spectrum is more pronounced in the higher Reynolds number simulations. We investigate the dynamo properties of this system in both the kinematic and the nonlinear regimes and we find that the dynamo is always more efficient in larger domains, when mesogranulation is present. Furthermore, we use a filtering technique in Fourier space to demonstrate that it is indeed the larger scales of motion that are primarily responsible for driving the dynamo. In the nonlinear regime, the magnetic field distribution compares very favourably to observations, both in terms of the spatial distribution and the measured field strengths.

12 pages, 11 figures, accepted for publication in Astronomy & Astrophysics

References in corpus (14)

• A Substantial Amount of Hidden Magnetic Energy in the Quiet Sun
• Simulations of nonhelical hydromagnetic turbulence
• A solar surface dynamo
• Quiet Sun internetwork magnetic fields from the inversion of Hinode measurements
• On magnetic field generation in Kolmogorov turbulence
• Large-scale dynamos in turbulent convection with shear
• Strong horizontal photospheric magnetic field in a surface dynamo simulation
• Hinode Observations of Magnetic Elements in Internetwork Areas
• DOT tomography of the solar atmosphere. IV. Magnetic patches in internetwork areas
• Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
• Universality of the Small-Scale Dynamo Mechanism
• Mesoscale flows in large aspect ratio simulations of turbulent compressible convection
• Convective intensification of magnetic fields in the quiet Sun
• The Effect of Velocity Shear on Dynamo Action Due to Rotating Convection

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• On the Magnetic Nature of an Exploding Granule as Revealed by Sunrise/IMaX
• Estimating the rate of field line braiding in the solar corona by photospheric flows
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