Supergranulation Scale Convection Simulations

Stein, R. F.; Georgobiani, D.; Schafenberger, Werner; Nordlund, Årke; Benson, David B.

doi:10.1063/1.3099227

Cited by 28 publications

(36 citation statements)

References 2 publications

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“…This would imply that the network field is not strictly a passive tracer of the supergranular flow, but rather may play an active dynamic role in supergranular scale selection by inducing radiative perturbations at depth where the density and pressure scale heights are greater. This may provide an explanation for why strictly hydrodynamic simulations have difficulty unambiguously obtaining supergranular scales (Ustyugov 2008;Stein et al 2009), though domain sizes are also currently still quite restrictive.…”

Section: Resultsmentioning

confidence: 99%

The Intensity Profile of the Solar Supergranulation

Goldbaum¹,

Rast²,

Ermolli³

et al. 2009

ApJ

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We have measured the average radial (cell center to network boundary) profile of the continuum intensity contrast associated with supergranular flows using data from the Precision Solar Photometric Telescope at the Mauna Loa Solar Observatory. After removing the contribution of the network flux elements by the application of masks based on Ca ii K intensity and averaging over more than 10 5 supergranular cells, we find a ∼0.1% decrease in red and blue continuum intensity from the supergranular cell centers outward, corresponding to a ∼1.0 K decrease in brightness temperature across the cells. The radial intensity profile may be caused either by the thermal signal associated with the supergranular flows or a variation in the packing density of unresolved magnetic flux elements. These are not unambiguously distinguished by the observations, and we raise the possibility that the network magnetic fields play an active role in supergranular scale selection by enhancing the radiative cooling of the deep photosphere at the cell boundaries.

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Section: Resultsmentioning

confidence: 99%

The Intensity Profile of the Solar Supergranulation

Goldbaum¹,

Rast²,

Ermolli³

et al. 2009

ApJ

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“…For the Sun, we have deep (20 Mm) convection calculations (Stein et al 2009) and the determination of the mass mixing length is straightforward, as shown in Figure 3. From this we see that the ratio of the mass mixing length to the pressure scale height, α P , is nearly constant with depth in the interior of the convection zone until the bottom boundary is approached.…”

Section: Resultsmentioning

confidence: 99%

“…The mass mixing length, the distance over which, because of mass conservation, most of the ascending fluid turns over and starts to descend, is the inverse of the logarithmic derivative of the unidirectional mass flux (Stein et al 2009):…”

Section: Mass Mixing Lengthmentioning

confidence: 99%

The Mass Mixing Length in Convective Stellar Envelopes

Trampedach

Stein

2011

ApJ

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116

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The scale length over which convection mixes mass in a star can be calculated as the inverse of the vertical derivative of the unidirectional (up or down) mass flux. This is related to the mixing length in the mixing length theory of stellar convection. We give the ratio of mass mixing length to pressure scale height for a grid of threedimensional surface convection simulations, covering from 4300 K to 6900 K on the main sequence, and up to giants at log g = 2.2, all for solar composition. These simulations also confirm what is already known from solar simulations that convection does not proceed by discrete convective elements, but rather as a continuous, slow, smooth, warm upflow and turbulent, entropy deficient, fast down drafts. This convective topology also results in mixing on a scale comparable to the classic mixing length formulation, and is simply a consequence of mass conservation on flows in a stratified atmosphere.

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“…Simple models like the mixing-length theory or plume dynamics (Rieutord & Zahn 1997) do not predict any special spectral feature when the scale (horizontal or vertical) grows. Hydrodynamic numerical simulations like the recent ones of Stein et al (2009) seem to go in the same direction and do not exhibit any spectral feature reminiscent of supergranulation (see also Nordlund et al 2009). However, kinetic energy spectra derived from the radial velocity measured by SOHO/MDI (i.e.…”

Section: Introductionmentioning

confidence: 95%

“…Despite several attempts, supergranulation has not been identified in large-eddy or direct numerical simulations (e.g. Rieutord et al 2002;Rincon et al 2005;Stein et al 2009). Its origin remains unknown, although some scenarios have been proposed.…”

Section: Introductionmentioning

confidence: 99%

On the power spectrum of solar surface flows

Rieutord

Roudier

Rincon

et al. 2010

A&A

113

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Context. The surface of the Sun provides us with a unique and very detailed view of turbulent stellar convection. Studying its dynamics can therefore help us make significant progress in stellar convection modelling. Many features of solar surface turbulence like the supergranulation are still poorly understood. Aims. The aim of this work is to give new observational constraints on these flows by determining the horizontal scale dependence of the velocity and intensity fields, as represented by their power spectra, and to offer some theoretical guidelines to interpret these spectra. Methods. We use long time-series of images taken by the Solar Optical Telescope (SOT) on board the Hinode satellite; we reconstruct both horizontal (by granule tracking) and vertical (by Doppler effect) velocity fields in a field-of-view of ∼ 75×75 Mm 2 . The dynamics in the subgranulation range can be investigated with unprecedented precision thanks to the absence of seeing effects and the use of the modulation transfer function of SOT for correcting the spectra. Results. At small subgranulation scales down to 0.4 Mm the spectral density of kinetic energy associated with vertical motions exhibits a k −10/3 -like power law, while the intensity fluctuation spectrum follows either a k −17/3 or a k −3 -like power law at the two continuum levels investigated (525 and 450 nm respectively). We discuss the possible physical origin of these scalings and interpret the combined presence of k −17/3 and k −10/3 power laws for the intensity and vertical velocity as a signature of buoyancy-driven turbulent dynamics in a strongly thermally diffusive regime. In the mesogranulation range and up to a scale of 25 Mm, we find that the amplitude of the vertical velocity field decreases like λ −3/2 with the horizontal scale λ. This behaviour corresponds to a k 2 spectral power law. Still in the 2.5-10 Mm mesoscale range, we find that intensity fluctuations in the blue continuum also follow a k 2 power law. In passing we show that granule tracking cannot sample scales below 2.5 Mm. We finally further confirm the presence of a significant supergranulation energy peak at 30 Mm in the horizontal velocity power spectrum and show that the emergence of a pore erases this spectral peak. We tentatively estimate the scale height of the vertical velocity field in the supergranulation range and find 1 Mm; this value suggests that supergranulation flows are shallow.

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Supergranulation Scale Convection Simulations

Cited by 28 publications

References 2 publications

The Intensity Profile of the Solar Supergranulation

The Intensity Profile of the Solar Supergranulation

The Mass Mixing Length in Convective Stellar Envelopes

On the power spectrum of solar surface flows

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