On global solar dynamo simulations

Käpylä, Petri J.

doi:10.1002/asna.201012345

Cited by 20 publications

(23 citation statements)

References 94 publications

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“…Due to numerical reasons, the regime of Reynolds, Re and Rm, and magnetic Prandtl, P r m , numbers reached in simulations of magneto-convection and dynamo is different from that of the solar case (see, e.g., Käpylä, 2011). This is so both for the simulations reported here and for those reported previously in the literature (Vögler & Schüssler, 2007;Pietarila Graham et al, 2010;Rempel, 2014;Kitiashvili et al, 2015).…”

Section: Reynolds and Magnetic Prandtl Numbers Of The Simulationscontrasting

confidence: 53%

Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery

Khomenko

Vitas

Collados

et al. 2017

A&A

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The magnetic fields of the quiet Sun cover at any time more than 90% of its surface and their magnetic energy budget is crucial to explain the thermal structure of the solar atmosphere. One of the possible origins of these fields is due to the action of local dynamo in the upper convection zone of the Sun. Existing simulations of the local solar dynamo require an initial seed field, and sufficiently high spatial resolution, in order to achieve the amplification of the seed field to the observed values in the quiet Sun. Here we report an alternative model of seeding based on the action of the Bierman battery effect. This effect generates a magnetic field due to the local imbalances in electron pressure in the partially ionized solar plasma. We show that the battery effect self-consistently creates from zero an initial seed field of a strength of the order of micro G, and together with dynamo amplification, allows the generation of quiet Sun magnetic fields of a similar strength to those from solar observations.

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Section: Reynolds and Magnetic Prandtl Numbers Of The Simulationscontrasting

confidence: 53%

Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery

Khomenko

Vitas

Collados

et al. 2017

A&A

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“…Iskakov et al (2007) found supercritical solutions near PrM ≈ 0.1 when using hyperviscosity in their simulations, which does however affect the strength of the spectral bottleneck, as will be discussed in a moment. Even the re-cent convection simulations of Käpylä et al (2018) at a resolution of 1024 3 meshpoints found only decaying magnetic fields at PrM = 0.1.…”

Section: Introductionmentioning

confidence: 94%

Varying the forcing scale in low Prandtl number dynamos

Brandenburg

Haugen

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Small-scale dynamos are expected to operate in all astrophysical fluids that are turbulent and electrically conducting, for example the interstellar medium, stellar interiors, and accretion disks, where they may also be affected by or competing with large-scale dynamos. However, the possibility of small-scale dynamos being excited at small and intermediate ratios of viscosity to magnetic diffusivity (the magnetic Prandtl number) has been debated, and the possibility of them depending on the large-scale forcing wavenumber has been raised. Here we show, using four values of the forcing wavenumber, that the small-scale dynamo does not depend on the scale-separation between the size of the simulation domain and the integral scale of the turbulence, i.e., the forcing scale. Moreover, the spectral bottleneck in turbulence, which has been implied as being responsible for raising the excitation conditions of small-scale dynamos, is found to be invariant under changing the forcing wavenumber. However, when forcing at the lowest few wavenumbers, the effective forcing wavenumber that enters in the definition of the magnetic Reynolds number is found to be about twice the minimum wavenumber of the domain. Our work is relevant to future studies of small-scale dynamos, of which several applications are being discussed.

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“…It is large in the bulk of the solar convection zone, especially if τ is estimated from mixing length theory, which predicts values of Co ranging from 10 −3 near the surface to more than 10 in the deep layers (e.g. Ossendrijver 2003;Brandenburg & Subramanian 2005;Käpylä 2011). This is not captured by present simulations, which might well be due to insufficient density stratification in most numerical simulations so far.…”

Section: Introductionmentioning

confidence: 99%

Confirmation of bistable stellar differential rotation profiles

Käpylä

Brandenburg

2014

A&A

141

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Context. Solar-like differential rotation is characterized by a rapidly rotating equator and slower poles. However, theoretical models and numerical simulations can also result in a slower equator and faster poles when the overall rotation is slow. Aims. We study the critical rotational influence under which differential rotation flips from solar-like (fast equator, slow poles) to an anti-solar one (slow equator, fast poles). We also estimate the non-diffusive (Λ effect) and diffusive (turbulent viscosity) contributions to the Reynolds stress. Methods. We present the results of three-dimensional numerical simulations of mildly turbulent convection in spherical wedge geometry. Here we apply a fully compressible setup which would suffer from a prohibitive time step constraint if the real solar luminosity was used. To avoid this problem while still representing the same rotational influence on the flow as in the Sun, we increase the luminosity by a factor of roughly 10 6 and the rotation rate by a factor of 10 2 . We regulate the convective velocities by varying the amount of heat transported by thermal conduction, turbulent diffusion, and resolved convection. Results. Increasing the efficiency of resolved convection leads to a reduction of the rotational influence on the flow and a sharp transition from solar-like to anti-solar differential rotation for Coriolis numbers around 1.3. We confirm the recent finding of a large-scale flow bistability: contrasted with running the models from an initial condition with unprescribed differential rotation, the initialization of the model with certain kind of rotation profile sustains the solution over a wider parameter range. The anti-solar profiles are found to be more stable against perturbations in the level of convective turbulent velocity than the solar-type solutions. Conclusions. Our results may have implications for real stars that start their lives as rapid rotators implying solar-like rotation in the early main-sequence evolution. As they slow down, they might be able to retain solar-like rotation for lower Coriolis numbers, and thus longer in time, before switching to anti-solar rotation. This could partially explain the puzzling findings of anti-solar rotation profiles for models in the solar parameter regime.

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On global solar dynamo simulations

Cited by 20 publications

References 94 publications

Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery

Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery

Varying the forcing scale in low Prandtl number dynamos

Confirmation of bistable stellar differential rotation profiles

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