Vortices in simulations of solar surface convection

Moll, R.; Cameron, R. H.; Schüßler, M.

doi:10.1051/0004-6361/201117441

Cited by 83 publications

(120 citation statements)

References 26 publications

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“…Either it diffuses into the magnetic features across field lines, which runs counter to the estimates of Hasan & Schüssler (1985), or the lifetimes of BPs, i.e., kilogauss features are rather short, or the plasma with the strong field is continually mixing with relatively field-free plasma in the immediate surroundings of the magnetic elements. This last process may be related to the vortices found in the simulations around magnetic elements by, e.g., Moll et al (2011), and observationally by Bonet et al (2010) and Wedemeyer-Böhm et al (2012).…”

Section: Discussionmentioning

confidence: 58%

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

Riethmüller

Solanki

Berdyugina

et al. 2014

A&A

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Bright points (BPs) in the solar photosphere are thought to be the radiative signatures (small-scale brightness enhancements) of magnetic elements described by slender flux tubes or sheets located in the darker intergranular lanes in the solar photosphere. They contribute to the ultraviolet (UV) flux variations over the solar cycle and hence may play a role in influencing the Earth's climate. Here we aim to obtain a better insight into their properties by combining high-resolution UV and spectro-polarimetric observations of BPs by the Sunrise Observatory with 3D compressible radiation magnetohydrodynamical (MHD) simulations. To this end, full spectral line syntheses are performed with the MHD data and a careful degradation is applied to take into account all relevant instrumental effects of the observations. In a first step it is demonstrated that the selected MHD simulations reproduce the measured distributions of intensity at multiple wavelengths, line-of-sight velocity, spectral line width, and polarization degree rather well. The simulated line width also displays the correct mean, but a scatter that is too small. In the second step, the properties of observed BPs are compared with synthetic ones. Again, these are found to match relatively well, except that the observations display a tail of large BPs with strong polarization signals (most likely network elements) not found in the simulations, possibly due to the small size of the simulation box. The higher spatial resolution of the simulations has a significant effect, leading to smaller and more numerous BPs. The observation that most BPs are weakly polarized is explained mainly by the spatial degradation, the stray light contamination, and the temperature sensitivity of the Fe i line at 5250.2 Å. Finally, given that the MHD simulations are highly consistent with the observations, we used the simulations to explore the properties of BPs further. The Stokes V asymmetries increase with the distance to the center of the mean BP in both observations and simulations, consistent with the classical picture of a production of the asymmetry in the canopy. This is the first time that this has been found also in the internetwork. More or less vertical kilogauss magnetic fields are found for 98% of the synthetic BPs underlining that basically every BP is associated with kilogauss fields. At the continuum formation height, the simulated BPs are on average 190 K hotter than the mean quiet Sun, the mean BP field strength is found to be 1750 G, and the mean inclination is 17 • , supporting the physical flux-tube paradigm to describe BPs. On average, the synthetic BPs harbor downflows increasing with depth. The origin of these downflows is not yet understood very well and needs further investigation.

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Section: Discussionmentioning

confidence: 58%

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

Riethmüller

Solanki

Berdyugina

et al. 2014

A&A

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“…The comparison with the magnetic-field distribution in the third row indicates that the horizontal vortices near z = 0 are a hydrodynamical phenomenon of the overturning motions at the borders of granules, which is essentially unrelated to the intergranular magnetic flux concentrations (cf. Steiner et al 2010;Moll et al 2011). In contrast to this, the vertical vortices are almost exclusively found in magnetic flux concentrations.…”

Section: Vortex Identificationmentioning

confidence: 87%

“…For instance, a purely bidirectional shear flow without any rotational component nevertheless has a non-vanishing vorticity. For the detection of vortices, i.e., fluid elements rotating about a local, possibly moving axis, we followed the procedure employed in Moll et al (2011): vortices (swirling flows) are defined as regions where the velocity gradient tensor has a pair of complex conjugate eigenvalues (Zhou et al 1999). The strength of the vortical motion is determined by the swirling strength, λ ci , which is the magnitude of the unsigned imaginary part of the complex eigenvalues.…”

Section: Vortex Identificationmentioning

confidence: 99%

Vortices, shocks, and heating in the solar photosphere: effect of a magnetic field

Moll

Cameron

Schüßler

2012

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Aims. We study the differences between non-magnetic and magnetic regions in the flow and thermal structure of the upper solar photosphere. Methods. Radiative MHD simulations representing a quiet region and a plage region, respectively, which extend into the layers around the temperature minimum, are analyzed. Results. The flow structure in the upper photospheric layers of the two simulations is considerably different: the non-magnetic simulation is dominated by a pattern of moving shock fronts while the magnetic simulation shows vertically extended vortices associated with magnetic flux concentrations. Both kinds of structures induce substantial local heating. The resulting average temperature profiles are characterized by a steep rise above the temperature minimum due to shock heating in the non-magnetic case and by a flat photospheric temperature gradient mainly caused by Ohmic dissipation in the magnetic run. Conclusions. Shocks in the quiet Sun and vortices in the strongly magnetized regions represent the dominant flow structures in the layers around the temperature minimum. They are closely connected with dissipation processes providing localized heating.

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“…Bonet et al 2008Bonet et al , 2010Steiner et al 2010;Vargas Domínguez et al 2011;Wedemeyer-Böhm et al 2012) and also in solar simulations (e.g. Stein & Nordlund 1998;Moll et al 2011). Ludwig et al (2006 reported swirling downflows also in simulations of cool main-sequence stars of spectral type M.…”

Section: Vortex Motionsmentioning

confidence: 95%

“…Analogous to small-scale bright magnetic features (magnetic bright points), for which the depressions of the optical surface are caused by the magnetic pressure, the vortex flows sometimes can appear as features of enhanced intensity due to side-wall heating of the interior of the depression. The vortices in solar MURaM simulations are described in detail by Moll et al (2011Moll et al ( , 2012.…”

Section: Vortex Motionsmentioning

confidence: 99%

Three-dimensional simulations of near-surface convection in main-sequence stars

Beeck

Cameron

Reiners

et al. 2013

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Context. The atmospheres of cool main-sequence stars are structured by convective flows from the convective envelope that penetrate the optically thin layers and lead to structuring of the stellar atmospheres analogous to solar granulation. The flows have considerable influence on the 3D structure of temperature and pressure and affect the profiles of spectral lines formed in the photosphere. Aims. For the set of six 3D radiative (M)HD simulations of cool main-sequence stars described in the first paper of this series, we analyse the near-surface layers. We aim at describing the properties of granulation of different stars and at quantifying the effects on spectral lines of the thermodynamic structure and flows of 3D convective atmospheres. Methods. We detected and tracked granules in brightness images from the simulations to analyse their statistical properties, as well as their evolution and lifetime. We calculated spatially resolved spectral line profiles using the line synthesis code SPINOR. To enable a comparison to stellar observations, we implemented a numerical disc-integration, which includes (differential) rotation. Results. Although the stellar parameters change considerably along the model sequence, the properties of the granules are very similar. The impact of the 3D structure of the atmospheres on line profiles is measurable in disc-integrated spectra. Line asymmetries caused by convection are modulated by stellar rotation. Conclusions. The 3D structure of cool stellar atmospheres as shaped by convective flows has to be taken into account when using photospheric lines to determine stellar parameters.

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Vortices in simulations of solar surface convection

Cited by 83 publications

References 26 publications

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

Vortices, shocks, and heating in the solar photosphere: effect of a magnetic field

Three-dimensional simulations of near-surface convection in main-sequence stars

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