Giant Convection Cells Found on the Sun

Hathaway, David H.; Upton, Lisa; Colegrove, Owen

doi:10.1126/science.1244682

Cited by 100 publications

(128 citation statements)

References 17 publications

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“…Although detection of giant cells in the Sun has been reported (e.g. Hathaway et al 2013), local time-distance helioseismology appears to indicate a gaping discrepancy between the Sun and current global simulations in that the latter produce significantly too much power at large scales (Hanasoge et al 2012). Local ring-diagram helioseismology, on the other hand, gives much larger convective velocities (Greer et al 2015).…”

Section: Discussionmentioning

confidence: 91%

Magnetic flux concentrations from turbulent stratified convection

Käpylä

Brandenburg

Kleeorin

et al. 2016

A&A

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Context. The formation of magnetic flux concentrations within the solar convection zone leading to sunspot formation is unexplained. Aims. We study the self-organization of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods. We perform simulations of magnetoconvection in Cartesian domains representing the uppermost 8.5−24 Mm of the solar convection zone with the horizontal size of the domain varying between 34 and 96 Mm. The density contrast in the 24 Mm deep models is more than 3 × 10 3 or eight density scale heights, corresponding to a little over 12 pressure scale heights. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow in set-ups where no small-scale dynamos are present. In the most highly stratified cases we employ the reduced sound speed method to relax the time step constraint arising from the high sound speed in the deep layers. We model radiation via the diffusion approximation and neglect detailed radiative transfer in order to concentrate on purely magnetohydrodynamic effects. Results. We find that super-equipartition magnetic flux concentrations are formed near the surface in cases with moderate and high density stratification, corresponding to domain depths of 12.5 and 24 Mm. The size of the concentrations increases as the box size increases and the largest structures (20 Mm horizontally near the surface) are obtained in the models that are 24 Mm deep. The field strength in the concentrations is in the range of 3-5 kG, almost independent of the magnitude of the imposed field. The amplitude of the concentrations grows approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its derivative with respect to the mean magnetic field, however, is positive in most of the domain, which is unfavourable for the operation of the negative effective magnetic pressure instability (NEMPI). Simulations in which a passive vector field is evolved do not show a noticeable difference from magnetohydrodynamic runs in terms of the growth of the structures. Furthermore, we find that magnetic flux is concentrated in regions of converging flow corresponding to large-scale supergranulation convection pattern. Conclusions. The linear growth of large-scale flux concentrations implies that their dominant formation process is a tangling of the large-scale field rather than an instability. One plausible mechanism that can explain both the linear growth and the concentration of the flux in the regions of converging flow pattern is flux expulsion. A possible reason for the absence of NEMPI is that the derivative of the effective magnetic pressure with respect to the mean magnetic field has an unfavourable sign. Furthermore, there may not be sufficient scale separation, which is required for NEMPI to work.

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Section: Discussionmentioning

confidence: 91%

Magnetic flux concentrations from turbulent stratified convection

Käpylä

Brandenburg

Kleeorin

et al. 2016

A&A

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“…It is thus highly relevant if one can show that the generation of the differential rotation of the solar convection zone is based on the existence of nondiffusive Λ terms in the Reynolds stress in a similar sense to the solar dynamo being based on the existence of nondiffusive α terms in the induction equation. We argue that the Λ effect in rotating anisotropic turbulence is indeed needed to explain the current observation of positive (negative) horizontal cross correlation Q θφ in the northern (southern) hemisphere at the solar surface, which Hathaway et al (2013) indicated as a result of the proper motions of giant cells. All analytical and numerical studies lead to positive functions H, which in the bulk of the convection zone are able to overcompensate for the diffusive term, which alone would lead to the signs that are not observed.…”

Section: Discussionmentioning

confidence: 99%

“…According to (1), an accelerated equator provides a negative (positive) horizontal Reynolds stress in the northern (southern) hemisphere. From the data of the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), Hathaway et al (2013) have recently isolated a giant cell pattern at the solar surface where the proper motions form a horizontal cross-correlation Q θφ , which is -as a fulfilled necessary condition -antisymmetric with respect to the equator. The correlation is positive (negative) at northern (southern) midlatitudes, and its amplitude of 20 m 2 /s 2 suggests rms velocities of the cells of (say) 10 m/s (Fig.…”

Section: Introductionmentioning

confidence: 99%

The existence of the Λ effect in the solar convection zone as indicated by SDO/HMI data

Rüdiger

Küker

Tereshin

2014

A&A

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The empirical finding with data from the Helioseismic and Magnetic Imager instrument onboard the Solar Dynamics Observatory of positive (negative) horizontal Reynolds stress at the northern (southern) hemisphere for solar giant cells is discussed for its consequences on the theory of solar/stellar differential rotation. By solving the nonlinear Reynolds equation for the angular velocity Ω when neglecting the meridional circulation, we show that the horizontal Reynolds stress of the northern hemisphere is always negative at the surface but positive in the bulk of the solar convection zone by the action of the Λ effect. The Λ effect, which describes the angular momentum transport of rigidly rotating anisotropic turbulence and which avoids a rigid-body rotation of the convection zones, is in the horizontal direction of cubic power in Ω and proves to be always positive (negative) in the northern (southern) hemisphere in both theory and simulations. In contrast, theories without the Λ effect, which may provide the observed solar rotation law only by the action of a meridional circulation, will lead to a horizontal Reynolds stress with signs that are not observed.

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“…This argument may be too naive and would obviously be in conflict with the results of Greer et al (2015), which show an increase with depth at low spherical harmonic degrees. On the other hand, Bogart et al (2015) have argued that the flows reported by Hathaway et al (2013) may be non-convective in nature and, in fact, magnetically driven and perhaps related to the torsional oscillations.…”

Section: Introductionmentioning

confidence: 99%

“…It should be mentioned that small flow speeds of giant cell convection have been found by correlation tracking of super-granule proper motions (Hathaway et al 2013). The typical velocities are of the order of 20 m s −1 at spherical degrees of around 10.…”

Section: Introductionmentioning

confidence: 99%

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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Giant Convection Cells Found on the Sun

Cited by 100 publications

References 17 publications

Magnetic flux concentrations from turbulent stratified convection

Magnetic flux concentrations from turbulent stratified convection

The existence of the Λ effect in the solar convection zone as indicated by SDO/HMI data

Stellar Mixing Length Theory With Entropy Rain

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