1 / 35

The solar dynamo problem EULAG-MHD The « millenium simulation »

Double cycles and instabilities in EULAG-MHD simulations of solar convection Paul Charbonneau Département de Physique, Université de Montréal. The solar dynamo problem EULAG-MHD The « millenium simulation » Magnetically-mediated cyclic modulation of convective energy transport Conclusions.

svanmeter
Télécharger la présentation

The solar dynamo problem EULAG-MHD The « millenium simulation »

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Double cycles and instabilities in EULAG-MHD simulations of solar convection Paul Charbonneau Département de Physique, Université de Montréal • The solar dynamo problem • EULAG-MHD • The « millenium simulation » • Magnetically-mediated cyclic modulation of convective energy transport • Conclusions Collaborators:Piotr Smolarkiewicz, Mihai Ghizaru, Dario Passos, Antoine Strugarek, Jean-François Cossette, Patrice Beaudoin, Caroline Dubé, Nicolas Lawson, Étienne Racine, Gustavo Guerrero, Aimee Norton, Mark Miesch Solar Metrology 2014

  2. The solar magnetic cycle Solar Metrology 2014

  3. The magnetic self-organization conundrum How can turbulent convection, a flow with a length scale <<R and coherence time of ~month, generate a magnetic component with scale ~R varying on a timescale of ~decade ?? Mechanism/Processes favoring organization on large spatial scales: 1. rotation (cyclonicity); 2. differential rotation (scale ~R); and 3. turbulent inverse cascades. Solar Metrology 2014

  4. The MHD equations Solar Metrology 2014

  5. Selected milestones Gilman 1983:Boussinesq MHD simulation, producing large-scale magneticfields with polarity reversals on yearly timescale; but non-solar large-scaleorganization. Glatzmaier 1984, 1985: Anelastic model including stratification, large-scale fields with polarity reversals within a factor 2 of solar period; tendency for poleward migration of the large-scale magnetic field. Brun et al. 2004:Strongly turbulent MHD simulation, producing copious small-scale magnetic field but no large-scale magnetic component. Browning et al. 2006: Demonstrate the importance of an underlying, convectively stable fluid layer below the convection zone in producing a large-scale magnetic component in the turbulent regime. Brown et al. 2010, 2011: Obtain irregular polarity reversals of thin, intense toroidal field structure in a turbulent simulation rotating at 5X solar. Ghizaru et al.2010: Obtain regular polarityreversals of large-scale magnetic component on decadal timescales, showingmany solar-like characteristics. Nelson et al. 2012, 2013: Autonomous generation ofbuoyantly rising flux-ropesstructures showing sunspot-likeemergencepatterns. Solar Metrology 2014

  6. EULAG-MHD simulations Solar Metrology 2014

  7. Simulation framework Simulate anelastic convection in thick, rotating and unstably stratified fluid shell of electrically conducting fluid, overlaying a stably stratified fluid shell. Recent such simulations manage to reach Re, Rm ~102-103, at best; a long way from the solar/stellar parameter regime. Throughout the bulk of the convecting layers, convection is influenced by rotation, leading to alignment of convective cells parallel to the rotation axis. Stratification leads to downward pumping of the magnetic field throughout the convecting layers. Solar Metrology 2014

  8. Rotation and differential rotation (1) No rotation Rotation at solar rate Vertical (radial) flow velocity, in Mollweide projection [ from Guerrero et al. 2013, Astrophys. J., 779, 176 ] Solar Metrology 2014

  9. EULAG-MHD EULAG: a robust, general solver for geophysical flows; developed by Piotr Smolarkiewicz and collaborators at MMM/NCAR EULAG-MHD: MHD generalization of above; developed mostly at UdeM in close collaboration with Piotr Smolarkiewicz Core advection scheme: MPDATA, a minimally dissipative iterative upwind NFT scheme; equivalent to a dynamical, adaptive subgrid model. Thermal forcing of convection via volumetric Newtonian cooling term in energy equation, pushing reference adiabatic profile towards a very slightly superadiabatic ambiant profile Strongly stable stratification in fluid layers underlying convecting layers. Model can operate as LES or ILES Solar Metrology 2014

  10. MHD simulation of global dynamos [ Ghizaru et al. 2010, ApJL, 715, L133 ] Temperature perturbation Radial flow component Radial magnetic field component http://www.astro.umontreal.ca/~paulchar/grps> Que faisons nous > Simulations MHD Electromagnetic induction by internal flows is the engine powering the solar magnetic cycle. The challenge is to produce a magnetic field well-structured on spatial and temporal scales much larger/longer than those associated with convection itself. This is the magnetic self-organisation problem. Solar Metrology 2014

  11. Simulated magnetic cycles (1) Large-scale organisation of the magnetic field takes place primarily at and immediately below the base of the convecting fluid layers Solar Metrology 2014

  12. Zonally-averaged Bphi at r/R =0.718 Magnetic cycles (1) Zonally-averaged Bphi at -58o latitude Solar Metrology 2014

  13. Successesandproblems KiloGauss-strength large-scale magnetic fields, antisymmetric about equator and undergoing regular polarity reversals on decadal timescales. Cycle period four times too long, and strong fields concentrated at mid-latitudes, rather than low latitudes. Internal magnetic field dominated by toroidal component and strongly concentrated immediately beneath core-envelope interface. Well-defined dipole moment, well-aligned with rotation axis, but oscillating in phase with internal toroidal component. Reasonably solar-like internal differential rotation, and solar-like cyclic torsional oscillations (correct amplitude and phasing). On long timescales, tendency for hemispheric decoupling, and/or transitions to non-axisymmetric oscillatory modes. Cyclic modulation of the convective energy flux, in phase with the magnetic cycle. Solar Metrology 2014

  14. The « millenium simulation »[ Passos & Charbonneau 2014, A&A, 568, A113 ] Define a SSN proxy, measure cycle characteristics (period, amplitude…) and compare to observational record. Solar Metrology 2014

  15. Characteristics of simulated cycles (1)[ Passos & Charbonneau 2014, A&A, 568, A113 ] Define a SSN proxy, measure cycle characteristics (period, amplitude…) and compare to observational record. Solar Metrology 2014

  16. Characteristics of simulated cycles (2)[ Passos & Charbonneau 2014, A&A, 568, A113 ] r = 0.957/0.947 [ 0.763/0.841 ] r = -0.395/-0.147 [ -0.552/-0.320 ] r = 0.688/0.738 [ 0.322/0.451 ] r = -0.465/-0.143 [ 0.185/-0.117 ] Solar Metrology 2014

  17. Magnetically-mediated cyclic modulation of convective energy transport [ with J.-F. Cossette, P. Smolarkiewicz ] Solar Metrology 2014

  18. Observed variations of total solar irradiance PMOD Composite Reconstruction by Crouch et al. (2008) Club Math 2014/UdeM

  19. Two schools of thoughts • All TSI variation on all relevant timescales are due to varying surface coverage of magnetic features (spots, faculae, network, etc.). Strongest evidence: reconstructions based on photospheric data can reproduce 95% of observed variance. • Some TSI variations on timescales decadal and longer originate from deep inside the sun (changes in solar radius, photospheric temperature gradient, magnetic modulation of convective energy flux, etc.). Strongest evidence: cyclic modulation of p-mode frequencies. Solar Metrology 2014

  20. Magnetic modulation of convective energy transport in EULAG-MHD millenium simulation[ Cossette et al. 2013, ApJL, 777, L29 ] The simulation is more « luminous » at magnetic cycle maximum, by a solar-like 0.2% Lsol ! Solar Metrology 2014

  21. How to modulate convective energy transport Vertical flow speed Temperature deviation from horizontal mean • Lorentz force modulates convective velocity ur; • Change in magnitude of temperature perturbations; • Change in degree of correlation between the two; • Change in latitudinal distribution of F . • All of above ? And/or something else … ? Solar Metrology 2014

  22. Spatiotemporal variabilityof the convective flux[ Cossette et al. 2013, ApJL, 777, L29 ] Zonally-averaged toroidal field and convective flux at r/R=0.87 Solar Metrology 2014

  23. Convective entrainment and « hot spots » Solar Metrology 2014

  24. Pinning it down…[ Cossette et al. 2013, ApJL, 777, L29 ] Differences are in the tails of the flux distributions: hot spots are enhanced, turbulent entrainment is suppressed. The strongest (anti)correlations with the magnetic cycle are for the negative convective fluxes. Solar Metrology 2014

  25. Small (multi)periodic signal in temperature[ Beaudoin et al. 2014, in prep. ] 95% confidence Solar Metrology 2014

  26. Convection is not diffusion ! • The Newtonian diffusive heat flux is proportional to the temperature gradient; the heat flux is entirely determined by local conditions. • The convective heat flux is proportional to temperature at point of origin of upflows and downflows; for strongly turbulent convection, these flow structures can cross many scale heights; the heat flux is strongly non-local. Solar Metrology 2014

  27. Convection is not diffusion ! Solar Metrology 2014

  28. Where do we go next ? Understand what sets the cycle period(s) Understand physical underpinnings of the cyclic modulation of the convective energy flux Understand role of tachocline instabilities in long term stability of simulations, and possible role in triggering Maunder-Minimum-like period of strongly reduced activity Comparative benchmark with ASH simulations Get closer to surface !! (Compressible version of EULAG-MHD in the works) Solar Metrology 2014

  29. FIN Collaborators: Piotr Smolarkiewicz (NCAR, ECMWF), Mihai Ghizaru, Étienne Racine (CSA), Jean-François Cossette, Patrice Beaudoin, Nicolas Lawson, Amélie Bouchat, Corinne Simard, Caroline Dubé, Dario Passos, Roxane Barnabé Solar Metrology 2014

  30. Formation of magnetic flux strands (1)[ Nelson et al. 2013, Astrophys. J., 762: 73 ] Recent, very high resolution 3D MHD simulationsof solar convection Have achieved the formation of flux-rope-like super-equipartition-strength « magnetic strands » characterized by a significant density deficit in their core; ripped from the parent large-scale structure by turbulent entrainement, subsequent buoyant rise ensues. Solar Metrology 2014

  31. Formation of magnetic flux strands (2)[ Nelson et al. 2014, Solar Phys., 289, 441 ] The strands « remember » their origin ! The strands develop a pattern of East-West tilt similar to that inferred obervationally for the sun Solar Metrology 2014

  32. Kinetic and magnetic energies (120 s.d.=10 yr) Solar Metrology 2014

  33. Characteristics of simulated cycles (3) Hemispheric cycle amplitude show a hint of bimodality Usoskin et al. 2014, A&A 562, L10; From 3000yr 14C time series Solar Metrology 2014

  34. Hemispheric cycle amplitude show a hint of bimodality Characteristics of simulated cycles (4) Usoskin et al. 2014, A&A 562, L10; From 3000yr 14C time series Solar Metrology 2014

  35. Rotation and differential rotation (2) Helioseismology HD simulation MHD simulation Angular velocity profiles, in meridional quadrant Differential rotation in the Sun and solar-type stars is powered by turbulent Reynolds stresses, arising from rotationally-induced anisotropy in turbulent transport of momentum and heat Solar Metrology 2014

More Related