Surface Convection

Stein, R. F.; Georgobiani, D.; Nordlund, Åke; Schaffenberger, W.; Stancliffe, Richard J.; Houdek, G.; Martin, Rebecca G.; Tout, Christopher A.

doi:10.1063/1.2818958

Cited by 17 publications

(6 citation statements)

References 9 publications

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“…They also calculated the wave travel times between different levels in the atmosphere corresponding to commonly observed lines and showed that such observations may be used to map the topography of the magnetic field. Georgobiani et al ( 2007 ) showed that supergranulation scale (48 Mm wide by 20 Mm deep) simulations of quiet Sun surface convection of Stein et al ( 2007a ) possessed a k − ω diagram with f- and p1–p4-modes very similar to those observed by MDI and with travel-time maps that were also nearly the same (Figure 33 ). Zhao et al ( 2007b ) showed that the horizontal velocities inferred from the ray-tracing inversion of f-mode travel times are in good agreement with the flows actually in the simulation down to depths of about 4 Mm (Figure 34 ).…”

Section: Applications To Helioseismologymentioning

confidence: 61%

Solar Surface Convection

Nordlund

Stein

Asplund

2009

Living Rev. Solar Phys.

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430

489

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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 MaterialSupplementary material is available for this article at 10.12942/lrsp-2009-2.

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Section: Applications To Helioseismologymentioning

confidence: 61%

Solar Surface Convection

Nordlund

Stein

Asplund

2009

Living Rev. Solar Phys.

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430

489

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“…So they suggested that these large magnetic features are created in the tachocline, and the smaller features (<10 20 Mx) would continue to be produced in the convection zone throughout solar maximum and solar minimum. This idea has some support from the recent numerical convection simulations of Stein et al (2008) and Nordlund (2008), which show that convection does not occur at two discrete scales (granulation and supergranulation), but rather that it occurs at a continuum of scales whose scale-length increases with depth.…”

Section: Implications For the Generation And Surface Processing Of Mamentioning

confidence: 87%

“…i) A visual inspection of the feature results reveals that they have equivalent areas and fluxes to intranetwork features, which are known to emerge on scales of less than 1 Mm (e.g. Strous and Zwaan, 1999). ii) After running trials using values of 0 between five and nine pixels, we found that 0 = 7 pixels gave the most reasonable pairings according to visual inspection (e.g., a separation of just five pixels appeared to miss a number of obvious emerging pairs, whilst a separation of nine pixels paired too many features).…”

Section: B1 Bipole Identificationmentioning

confidence: 99%

Small-Scale Flux Emergence Observed Using Hinode/SOT

2010

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The aim of this paper is to determine the flux emergence rate due to small-scale magnetic features in the quiet Sun using high-resolution Hinode SOT NFI data. Small-scale magnetic features are identified in the data using two different feature identification methods (clumping and downhill); then three methods are applied to detect flux emergence events. The distribution of the intranetwork peak emerged fluxes is determined. When combined with previous emergence results, from ephemeral regions to sunspots, the distribution of all fluxes are found to follow a power-law distribution which spans nearly seven orders of magnitude in flux (10 16 -10 23 Mx) and 18 orders of magnitude in frequency. The power-law fit to all these data is of the formwhere 0 = 10 16 Mx and is used to predict a global flux emergence rate of ≈ 450 Mx cm −2 day −1 from all features with fluxes of 10 16 Mx or more. Since the slope of all emerged fluxes is less than −2, this implies that most of the new flux that is fed into the solar atmosphere is from small-scale emerging events. This suggests that the rate of flux emergence is independent of the solar cycle and is equivalent to a global rate of flux emergence of more than a few times 10 25 Mx day −1 . The single power-law distribution over all emerged fluxes implies a scale-free dynamo, therefore indicating that a turbulent dynamo may act throughout the convection zone. Moreover, from the slope of the emerging flux distribution the (turbulent?) dynamo producing small-scale features produces considerably more flux than the active-region dynamo at the tachocline.

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“…Stein et al (2009) find f ≈ 1/3, nearly independently of depth, which yields φ kin ≈ √ 2/4 ≈ 0.35; see Table 1, where we list φ kin and −U ↓ /u rms = [(1 − f )/f ] 1/2 for selected values of f . The enthalpy flux is proportional to u z s and, using Equations (9) and (29) together with Equations (25) and (27) …”

Section: Depth Dependence Of the Deardorff Termmentioning

confidence: 89%

Stellar Mixing Length Theory With Entropy Rain

Brandenburg

2017

ApJ

106

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The effects of a non-gradient flux term originating from the motion of convective elements with entropy perturbations of either sign are investigated and incorporated into a modified version of stellar mixing length theory (MLT). Such a term, first studied by Deardorff in the meteorological context, might represent the effects of cold intense downdrafts caused by the rapid cooling in the granulation layer at the top of the convection zone of late-type stars. These intense downdrafts were first seen in the strongly stratified simulations of Stein & Nordlund in the late 1980s. These downdrafts transport heat nonlocally, a phenomenon referred to as entropy rain. Moreover, the Deardorff term can cause upward enthalpy transport even in a weakly Schwarzschild-stably stratified layer. In that case, no giant cell convection would be excited. This is interesting in view of recent observations, which could be explained if the dominant flow structures were of small scale even at larger depths. To study this possibility, three distinct flow structures are examined: one in which convective structures have similar size and mutual separation at all depths, one in which the separation increases with depth, but their size is still unchanged, and one in which both size and separation increase with depth, which is the standard flow structure. It is concluded that the third possibility with fewer and thicker downdrafts in deeper layers remains the most plausible, but it may be unable to explain the suspected absence of large-scale flows with speeds and scales expected from MLT.

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Surface Convection

Cited by 17 publications

References 9 publications

Solar Surface Convection

Solar Surface Convection

Small-Scale Flux Emergence Observed Using Hinode/SOT

Stellar Mixing Length Theory With Entropy Rain

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