Vol. 6 (2009) > lrsp-2009-2

doi: 10.12942/lrsp-2009-2
Living Rev. Solar Phys. 6 (2009), 2

Solar Surface Convection

1 JILA, University of Colorado, and Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 København Ø, Denmark
2 Physics and Astronomy Department, Michigan State University, East Lansing, MI 48824, U.S.A.
3 Max Planck Institute for Astrophysics, Postfach 1317, D-85741 Garching, Germany

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Article Abstract

We review the properties of solar convection that are directly observable at the solar surface, and discuss the relevant underlying physics, concentrating mostly on a range of depths from the temperature minimum down to about 20 Mm below the visible solar surface. The properties of convection at the main energy carrying (granular) scales are tightly constrained by observations, in particular by the detailed shapes of photospheric spectral lines and the topology (time- and length-scales, flow velocities, etc.) of the up- and downflows. Current supercomputer models match these constraints very closely, which lends credence to the models, and allows robust conclusions to be drawn from analysis of the model properties. At larger scales the properties of the convective velocity field at the solar surface are strongly influenced by constraints from mass conservation, with amplitudes of larger scale horizontal motions decreasing roughly in inverse proportion to the scale of the motion. To a large extent, the apparent presence of distinct (meso- and supergranulation) scales is a result of the folding of this spectrum with the effective "filters" corresponding to various observational techniques. Convective motions on successively larger scales advect patterns created by convection on smaller scales; this includes patterns of magnetic field, which thus have an approximately self-similar structure at scales larger than granulation. Radiative-hydrodynamical simulations of solar surface convection can be used as 2D/3D time-dependent models of the solar atmosphere to predict the emergent spectrum. In general, the resulting detailed spectral line profiles agree spectacularly well with observations without invoking any micro- and macroturbulence parameters due to the presence of convective velocities and atmosphere inhomogeneities. One of the most noteworthy results has been a significant reduction in recent years in the derived solar C, N, and O abundances with far-reaching consequences, not the least for helioseismology. Convection in the solar surface layers is also of great importance for helioseismology in other ways; excitation of the wave spectrum occurs primarily in these layers, and convection influences the size of global wave cavity and, hence, the mode frequencies. On local scales convection modulates wave propagation, and supercomputer convection simulations may thus be used to test and calibrate local helioseismic methods. We also discuss the importance of near solar surface convection for the structure and evolution of magnetic patterns: faculae, pores, and sunspots, and briefly address the question of the importance or not of local dynamo action near the solar surface. Finally, we discuss the importance of near solar surface convection as a driver for chromospheric and coronal heating.

Keywords: Granulation, Convection, Supergranulation, Mesogranulation

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Article Citation

Since a Living Reviews in Solar Physics article may evolve over time, please cite the access <date>, which uniquely identifies the version of the article you are referring to:

Åke Nordlund and Robert F. Stein and Martin Asplund,
"Solar Surface Convection",
Living Rev. Solar Phys. 6,  (2009),  2. URL (cited on <date>):
http://www.livingreviews.org/lrsp-2009-2

Article History

ORIGINAL http://www.livingreviews.org/lrsp-2009-2
Title Solar Surface Convection
Author Åke Nordlund / Robert F. Stein / Martin Asplund
Date accepted 24 February 2009, published 4 April 2009
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