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2009 ; 6
(ä): 2
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Solar Surface Convection
#MMPMID27194960
Nordlund Å
; Stein RF
; Asplund M
Living Rev Sol Phys
2009[]; 6
(ä): 2
PMID27194960
show ga
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. ELECTRONIC SUPPLEMENTARY MATERIAL:
Supplementary material is available for this article at 10.12942/lrsp-2009-2.
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