Solar Dynamo - National Solar Observatory
Transcript of Solar Dynamo - National Solar Observatory
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Solar Dynamo recent progress with global simulations
1. Introduction!!2. The « millenium » simulation!!3. Double-cycle dynamos!!4. Conclusions
With Paul Charbonneau & the GRPS members !Département de Physique, Université de Montréal
Antoine Strugarek
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2Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Source: http://solarscience.msfc.nasa.gov/images/magbfly.jpg
The solar magnetic cycle
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3Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Dynamo problems
The kinematic dynamo problem:
« To find a flow u that can lead to field amplification when!substituted in the MHD equation »
HARDTURBULENCE
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3Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Dynamo problems
The kinematic dynamo problem:
« To find a flow u that can lead to field amplification when!substituted in the MHD equation »
HARDTURBULENCE
« To find a flow u that can lead to field amplification when!substituted in the MHD equation, while being dynamically!consistent with the fluid equations including the Lorentz force »
The self-excited dynamo problem: MUCH HARDERTURBULENCE
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3Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Dynamo problems
The kinematic dynamo problem:
« To find a flow u that can lead to field amplification when!substituted in the MHD equation »
HARDTURBULENCE
« To find a flow u that can lead to field amplification when!substituted in the MHD equation, while being dynamically!consistent with the fluid equations including the Lorentz force »
The self-excited dynamo problem: MUCH HARDERTURBULENCE
« To find a flow u that leads to a magnetic field amplification!and evolution in agreement with observational inferences for!the Sun and stars »
The solar/stellar dynamo problem(s): HARDEST????????
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4Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
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)!! 3. turbulent inverse cascades
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5Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Rotation
Vertical (radial) flow velocity, in Mollweide projection![Guerrero et al. 2013, Astrophys. J., 779, 176]
No rotation Rotation at solar rate
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6Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Differential rotation
Angular velocity profiles, in meridional quadrant
Helioseismology HD simulation MHD simulation
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
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EULAG-MHDEULAG: 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
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8Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
The « millenium » simulation
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The self-organization of the magnetic field is produced at the base of the convective layer, with a field accumulation inside the underlying stable zone
9Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Simulated magnetic cycles[ Ghizaru+ 2010, ApJL]
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The self-organization of the magnetic field is produced at the base of the convective layer, with a field accumulation inside the underlying stable zone
9Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Simulated magnetic cycles[ Ghizaru+ 2010, ApJL]
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The self-organization of the magnetic field is produced at the base of the convective layer, with a field accumulation inside the underlying stable zone
9Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Simulated magnetic cycles[ Ghizaru+ 2010, ApJL]
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HAO 07/2014 15
Magnetic cycles (1)Zonally-averaged Bφ at r/R =0.718
Zonally-averaged Bφat -58o latitude
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11Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Successes and problemskGauss-strength large-scale magnetic fields, antisymmetric about equator and undergoing regular polarity reversals on decadal timescales
Cycle period 4 times too long, strong fields concentrated at mid-latitudes
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11Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Successes and problemskGauss-strength large-scale magnetic fields, antisymmetric about equator and undergoing regular polarity reversals on decadal timescales
Cycle period 4 times too long, strong fields concentrated at mid-latitudes
Internal magnetic field dominated by toroidal component and strongly concentrated immediately beneath core-envelope interface
No clear equatorial migration during the cycle
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11Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Successes and problemskGauss-strength large-scale magnetic fields, antisymmetric about equator and undergoing regular polarity reversals on decadal timescales
Cycle period 4 times too long, strong fields concentrated at mid-latitudes
Well-defined dipole moment, well-aligned with rotation axis,!but oscillating in phase with internal toroidal component
Internal magnetic field dominated by toroidal component and strongly concentrated immediately beneath core-envelope interface
No clear equatorial migration during the cycle
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11Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Successes and problemskGauss-strength large-scale magnetic fields, antisymmetric about equator and undergoing regular polarity reversals on decadal timescales
Cycle period 4 times too long, strong fields concentrated at mid-latitudes
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
Internal magnetic field dominated by toroidal component and strongly concentrated immediately beneath core-envelope interface
No clear equatorial migration during the cycle
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11Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Successes and problemskGauss-strength large-scale magnetic fields, antisymmetric about equator and undergoing regular polarity reversals on decadal timescales
Cycle period 4 times too long, strong fields concentrated at mid-latitudes
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
Internal magnetic field dominated by toroidal component and strongly concentrated immediately beneath core-envelope interface
No clear equatorial migration during the cycle
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11Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Successes and problemskGauss-strength large-scale magnetic fields, antisymmetric about equator and undergoing regular polarity reversals on decadal timescales
Cycle period 4 times too long, strong fields concentrated at mid-latitudes
Well-defined but
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
Internal magnetic field dominated by toroidal component and strongly concentrated immediately beneath core-envelope interface
No clear equatorial migration during the cycle Does the statistics of the modelled dynamo
coincide with the solar statistics?
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The « millenium » simulation [Passos & Charbonneau 2014, A&A]
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13Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Simulated solar cycles: SSN [Passos & Charbonneau 2014, A&A]
Define a SSN proxy, measure cycle characteristics (period, amplitude…) and compare to observational record
SunSimulation
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Simulated solar cycles: Correlations[Passos & Charbonneau 2014, A&A]
0.957/0.947![ 0.763/0.841 ]
-0.465/-0.143![ 0.185/-0.117 ]
0.688/0.738![ 0.322/0.451 ]
-0.395/-0.147![ -0.552/-0.320 ]
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15Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Simulated solar cycles: Bimodality [Passos & Charbonneau 2014, A&A]
Usoskin et al. 2014,!A&A 562, L10;!!From 3000yr 14C!time series
Hemispheric cycle amplitude show a hint of bimodality
North hem. South hem.
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Simulated solar cycles: Hemispheric lag [Passos & Charbonneau 2014, A&A]
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Simulated solar cycles: Hemispheric lag [Passos & Charbonneau 2014, A&A]
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17Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Simulated solar cycles: Gnevyshev-Ohl Pattern [Passos & Charbonneau 2014, A&A]
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Double-cycle dynamo
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19Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Short quasi-periodic variability in the Sun
Evidence for short-term (~0.5 – 2 yr) quasi-periodic variability!is found is a great many indicators of solar activity:
Sunspot number and area!Radio flux!Total and spectral irradiance!p-mode acoustic frequencies!Interplanetary magnetic field!Flaring rate!Solar wind speed
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Short quasiperiodic variability (1)
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HAO 07/2014
GONG!!1.6-2.5 nHz!2.5-3.5 nHz
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Short quasiperiodic variability (2)
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24Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Short quasi-periodic variability in simulations [Beaudoin+ 2014, in prep]
Bφ @ r/R = 0.920
Bφ @ θ = 15o
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25Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Short quasi-periodic variability in simulations [Beaudoin+ 2014, in prep]
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26Space Weather and Early Sun, Tucson, 20/10/2014 A. Strugarek
Double cycle origin
Peaks at high latitude, but significant Amplitude down to equatorial regions
m s-1
Peaks at low latitudes within convection zone; tachocline mostly at high latitudes.
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Double cycle origin
Scenario:!!« Long » dynamo mode powered by turbulent emf!(α2Ω dynamo mode)!!« Short » dynamo mode powered by rotational shear!in equatorial portion of convection zone (αΩ dynamo mode)!!A Parker-Yoshimura sign rule should apply:!dynamo wave propagates away from equatorial plane!along iso-contours of Ω
[Racine+ 2010, ApJ]
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HAO 07/2014 32
Short quasiperiodic variability (4)
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Validation against mean-field model
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Related « short » cycles in other simulations?
Bφ @
r/R
= 0
.95
Bφ @
r/R
= 0
.98
[Augustson+ 2014]!Cycle period ~ 3.1 yr!(+ grand minima)
[Warnecke+ 2014]!Cycle period ~ 2.5 yr!Interpretation as a αΩ dynamo mode
ASH Code
Pencil Code
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Conclusions and Open questions
Can we identify physical parameter(s) that set the cycle(s) period ?!!What is the exact role of the tachocline for the longer cycle?!!Can we get the longer cycle, seated in the upper tachocline, to produce flux emergence throughout the convection zone?!!How to obtain regular cycles in high-Re simulations?!!☛ Direct comparison runs between ASH and EULAG-MHD ongoing…
EULAG-MHD simulations unveils double-cycle dynamos running in solar-like interior!!The two cycles appear to originate from different locations in the interior!!At least one of the two cycles seems akin cycles found with other simulation codes
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Supplemental material
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Kinetic and magnetic energies
(120 s.d.=10 yr)
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Simulation of the solar dynamo[ Ghizaru et al. 2010, ApJL]
A. Strugarek
Temperature perturbations Radial fluid velocity Radial magnetic field
http://www.astro.umontreal.ca/~paulchar/grps
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.
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34Space Weather and Early Sun, Tucson, 20/10/2014
Simulation of the solar dynamo[ Ghizaru et al. 2010, ApJL]
A. Strugarek
Temperature perturbations Radial fluid velocity Radial magnetic field
http://www.astro.umontreal.ca/~paulchar/grps
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.
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34Space Weather and Early Sun, Tucson, 20/10/2014
Simulation of the solar dynamo[ Ghizaru et al. 2010, ApJL]
A. Strugarek
Temperature perturbations Radial fluid velocity Radial magnetic field
http://www.astro.umontreal.ca/~paulchar/grps
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.
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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. !
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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. !
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MagnetoHydroDynamics (MHD) equations
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Correlations for each hemisphere
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Temperature signature of the two cycles
f1 f3
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Solar differential rotation
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Variability in the spectral solar irradiance
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Variability in the spectral solar irradiance
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Variability in the spectral solar irradiance
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