Large-scale Flows and Dynamo In Solar-Like Stars

Transcription

1 Large-scale Flows and Dynamo In Solar-Like Stars Gustavo Guerrero Physics Department Universidade Federal de Minas Gerais Brazil P. Smolarkiewicz (ECMWF) A. Kosovichev (NJIT), Elisabete M. de G. Dal Pino (IAG-USP) N. Mansour (NASA Ames) A. Stejko (NJIT)

2 Magnetic fields in late-type stars Magnetic field in late type stars, Ca II H (396.8nm) and K (393.4nm) (Baliunas et al. 1995)

3 u rms Ro= 2 Ω0 L 0.47 B R ' HK ( ) Beq From Brandenburg, Saar & Turpin (1998) Stellar magnetic activity is directly related to the stellar rotation rate

4 Petit+ (2008)

5 Large-scale magnetic fields in the Sun Poloidal (r,θ) Toroidal (φ) Poloidal (r,θ) Toroidal (φ) How does it work? Turbulent dynamo (mean-field theory), Flux-transport dynamo, Babcock-Leighton α

6 Solar Mean flows: differential rotation

7 Solar Mean flows: meridional circulation Surface Sub-surface old beliefs New findings Zhao+ (2103) Ulrich (2010) See also: Schad+ (2013)

8 Global models

9 Anelastic simulations with the MHD-EULAG code Anelastic approximation (Lipps & Helmer 1982, Lipps 1990) (Guizaru+ 2010, Racine+ 2011, Smolarkiewicz & Charbonneau 2013, Cossette+ 2013, Guerrero+ 2013, Guerrero+ 2015)

10 HD

11 Differential rotation in solar-like stars a) Ω0 = 0 b) Ω0 = Ω /2 c) Ω0 =Ω d) Ω0=2Ω Guerrero+ (2013)

12 a) T = 112d (Ω0=Ω /4 ) c) T = 28d (Ω0=Ω ) b) T = 56d (Ω0=Ω /2) d) T = 14d (Ω0=2Ω ) SOLAR DYNAMO FRONTIERS WORKSHOP HELIOSEISMOLOGY 3D MODELING AND DATAGuerrero+ ASSIMILATION (2013)

13 Differential rotation profile is defined by the balance between buoyancy and Coriolis forces defined by the Rossby number (Ro). Solar-like (fast equator) and anti-solar modes (faster poles) are obtained. (see Käpylä+ 2011, Guerrero+ 2013, Featherstone & Miesch, 2015) Ω eq Ω 60 Ω0 o Gastine+ (2014) Guerrero+ (2013) SOLAR DYNAMO FRONTIERS WORKSHOP HELIOSEISMOLOGY 3D MODELING AND DATA ASSIMILATION

14 MHD

15 Stellar Mean-flows and Dynamo a) T = 7d b) T = 14d d) T = 56d e) T = 112d c) T = 28d f) T = 224d

16 The differential rotation profile depends now on the balance between buoyancy, Coriolis and Lorentz forces Ω eq Ω 60 Ω0 o Rengarajan (1984) SOLAR DYNAMO FRONTIERS HELIOSEISMOLOGY 3D MODELING AND DATA ASSIMILATION Looking forward forworkshop Tim Reinhold's talk

17 Stellar Dynamos T = 7d T = 56d T = 14d T = 112d T = 28d T = 224d

18 B (surface) vs Ro

19 The role of tachoclines in stellar dynamo simulations (Guerrero+ 15)

20 Models without tachocline Models with tachocline

21 Models without tachocline (Butterfly diagrams at r=0.95r ) Models with tachocline

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23 Turbulent coefficients FOSA (Northern hemisphere) T=14d T=28d T=56d

24 Shear source terms C θ Ω C r Ω T=14d T=28d T=56d

25 Model CZ02 Tc=2.2 yr Time [yr] θ Ω>0, r Ω> 0, α> 0 α> 0 Radial DW upwards Lat. DW poleward

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27 Model RC02 Tc=34.5 yr Time [yr] Conv. Zone Tach. (Eq) Tach. (pole) NSSL θ Ω>0, r Ω> 0, r Ω< 0, r Ω< 0, α> 0 α> 0 α> 0 α< 0 upwards poleward equatorward poleward

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29 Who process governs the cycle period? According with MLT estimations, ηt 10 cm s =10 m s (mean field models result in periods ~2-8 yr), η-quenching does not help much with the cycle period (Rudiger+ 94, Tobias 96, Gilman & Rempel 05, Guerrero+ 09, Muñoz+11). MLT Christensen-Dalsgard 96 η-quenching effect on mean-field dynamo models + Beq = 104 Beq = 103 Muñoz+ (2011) Guerrero+ (2009)

30 2 τ ηt = urms ( FOSA) 3 t t RC d CZ d = 2 RC 2 CZ RC t CZ t L /η L /η 19 Despite the field production is distributed over the simulation domain, the dynamo period is regulated by the diffusion time of the magnetic field at the deeper, more stable layers

31 CONCLUSIONS LES & ILES are promising tools to explore turbulent problems. MHD-EULAG code is a powerful code to study solar/stellar mean-flows and dynamo Tachoclines play an important role un solar and stellar dynamos Due to the highly sub-adiabatic stable region, all the simulations exhibit the formation of a tachocline. Besides, by choosing the appropriate stratification, a near-surface shear layer is obtained. For all the simulations at the solar rotation rate the meridional flow is multicellular. The simulations that reproduce the NSSL develop a poleward meridional flow at the surface and higher latitudes. Dynamo models without tachocline result in oscillatory solutions with short cycle period (~2yr). Dynamo models including a tachocline result in oscillatory solutions with long cycle period (>30yr). The strongest field developed in this radial shear layers dominates the evolution of the global field.

32 Thanks

33 The Near Surface Shear Layer

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