dept. of applied mathematics & research computing
TRANSCRIPT
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Stellar Dynamos in 90 minutes or less!
( with contributions and inspiration from Mark Miesch )
Nick FeatherstoneDept. of Applied Mathematics &
Research Computing
University of Colorado
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Why are we here?
• Stellar dynamos: Outstanding questions• A closer look at Solar Magnetism• Dynamo theory in the solar context• The big question: convective flow speeds• Today: emphasize induction• Tomorrow: more on convection
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What even is a dynamo?
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Dynamo
The process by which a magnetic field is sustained against decay through the motion of a conducting fluid.
Stellar Dynamo
The process by which a star maintains its magnetic field via the convective motion of its interior plasma.
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106
104
102
1
10-2
10-4
Lum
inos
ity (s
olar
units
)
30,000 K 10,000 K 6,000 K 3,000 K
107 yrs
108 yrs
1010 yrs
The Stars
Bennett, et al. 2003
The Sun
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106
104
102
1
10-2
10-4
Lum
inos
ity (s
olar
units
)
30,000 K 10,000 K 6,000 K 3,000 K
107 yrs
108 yrs
1010 yrs
Low-Mass Stars
High-Mass Stars
One Broad Distinction
Bennett, et al. 2003
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As far as dynamo theory is concerned, what do you think is
the main difference between massive and low-mass stars?
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Massive vs. Low-Mass Stars
The key difference is……mass, …luminosity, …size … geometry!
massive stars low-mass stars
radiative envelope
convective envelope
radiative interior
convective core
Different magnetic field configurations arise in different geometries.
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Massive vs. Low-Mass Stars
Fundamental questions similar, but some differences.
massive stars low-mass stars
radiative envelope
convective envelope
radiative interior
convective core
Different magnetic field configurations arise in different geometries.
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• Magnetic activity increases with integrated sunspot area• Mean magnetic polarity reverses every 11 years
Stellar Dynamo Question: The Sun’s Magnetic Cycle
Equatorward Migration
Disk-Integrated Sunspot Area
How do stars generate a magnetic field?What causes the cyclic behavior?
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Big Question:Magnetism and Stellar Age?
Brown et al. 2010
“Young Sun” simulation ( 3:sun)
• Stars rotate more slowly as they age.
• Rapid rotation favors large-scale magnetic fields.
• Is a tachocline necessary for organized field growth?
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Big Question:Is the Sun “normal”?
Metcalf et al. 2016Bohm-Vitense (2007)
Survey of solar analogues
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Big Question:Massive Star Dynamos?
Featherstone et al. 2010 (A stars)see also Augustson et al. 2016 (B stars)
•How is the magnetic field structured?
•Are core-generated fields visible at the surface?
•Can core-generated fields becomes buoyant?
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• Field strengths and line widths vary periodically: Oblique Rotator Model
• Rotation periods from days to decades (magnetic braking?)
Peculiar Question: Ap and Bp Stars• Abnormally strong abundances of Si
and various rare earth metals (Hg, etc.)
• Magnetic: typical field strengths of a few hundred Gauss.
Rotation Axis
Dipole Axis
Source of Magnetic Field?
• Core-dynamo? (but diffusion time through radiative zone very long)
• Primordial magnetic field?
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B
Magnetic Field
Abundance Maps
Kochukhov et al. 2004
B
Magnetic Doppler Imaging
Ap Star HR3831
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Big Question:Exoplanets and Host-Star Magnetism
Credit: NASA/JPL-Caltech
•Exoplanets most easily found around low-mass M dwarfs•Vigorous dynamo action + fierce flaring•How does magnetism impact habitable zone?
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Big Question:Relationship between Magnetism and
Stellar Rotation Rate?
Rotation-Activity Correlation
•Solar-like stars shown•General trend holds for all low-mass stars•What causes saturation?
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At the end of the day…
• The Sun is a star• Stellar magnetism is the superset of solar
magnetism• We study stellar dynamos indirectly via the
Sun as much as we do directly via observations
• Let’s turn to the Sun, which we can RESOLVE!
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The Sun ( 9/7/2016)
visible light617 nm
HMI/SDO
sunspots
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Rice Univ.
Detailed sunspot records from 1600s onward(Galileo Galilei, 1612)
Naked-eye observations from China since 23 BCE
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Detailed sunspot records from 1600s onward(Galileo Galilei, 1612)
Naked-eye observations from China since 23 BCE
Rice Univ.
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Cosmic Perspective, 3rd Ed.
photosphericconvection
“granulation”
Swedish Solar Telescope (visible; 430 nm)
Sunspots: A Closer Look
~1 kG
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• Magnetic activity increases with integrated sunspot area• Mean magnetic polarity reverses every 11 years
The Sun’s Magnetic Cycle
Equatorward Migration
Disk-Integrated Sunspot Area
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• Magnetic activity increases with integrated sunspot area• Mean magnetic polarity reverses every 11 years
The Sun’s Magnetic Cycle
Equatorward Migration
Disk-Integrated Sunspot Area
Where does all this magnetism come from?
What makes it so ordered in space and time?
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Solar Dynamo: The Big Questions
Where does solar magnetism originate? Why is there a solar cycle?
Line-of-sight magnetic Field (HMI)
Sunspot clusters ( active regions )
A deep origin for solar magnetism?
large-scale organization
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What Lies Beneath: Solar Helioseismology
Dopplergram(radial velocity)
ResonantAcoustic Modes
Spherical Harmonic Degree l
MDI Medium-l Power Spectrum
Freq
uenc
y (m
Hz)
MDI
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Helioseismology Key Result: Differential Rotation
:
360 nHz
500 nHz
Howe et al. 2000; Schou et al. 2002
Equator (370 m/s)
North Pole (-240 m/s)• Sun rotates differentially
• 24-day period equator• 30-day period poles
• Latitudinal Shear• Radial Shear
NASA/ESA
near-surface shear layer
tachocline
414
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Zhao et al. 2012 Greer et al. 2013
Ring DiagramsTime Distance
Shallow or Deep reversal? Latitudinal variation?
Helioseismology Key Result: Meridional Circulation
20 m s-1
poleward
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radius
Helioseismology Key Result: Solar Structure
Core
Radiative Zone
Convection Zone
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Fusion Core
Central Temperature: 15.7 million KCentral Density: 154 g cm-3
Expanse: 0 – 0.25 Rsun
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Radiative Zone
Temperature: 2.3 million KDensity: 0.2 g cm-3
Radius: 0.7 Rsun
Temperature: 8 million KDensity: 20 g cm-3
Radius: 0.25 Rsun
• Convectively Stable• Energy Transport: photon scattering• Random walk time: 100,000 years
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Convection Zone
Temperature: 2.3 million KDensity: 0.2 g cm-3
Radius: 0.7 Rsun
Temperature: 5800KDensity: 2x10-7 g cm-3
Radius: 1 Rsun
• Convectively unstable• Convective timescale: months, years?
region of extreme contrasts…… ionized plasma!
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Temperature: 14,400KDensity: 2x10-6 g cm-3
Radius: 0.9985 Rsun
Temperature: 5,800KDensity: 2x10-7 g cm-3
Radius: 1 Rsun
The convection that we can see occurs in a region thinner than the width of this line (about 1,000 km in depth)
Strong variation with depth
Note:
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Convection Zone Bulk
Temperature: 2.3 million KDensity: 0.2 g cm-3
Temperature: 14,400KDensity: 2x10-6 g cm-3
• 11 density scaleheights• 17 pressure scaleheights• Reynolds Number | 1012 – 1014
• Rayleigh Number | 1022 – 1024
• Magnetic Prandtl Number | 0.01• Prandtl Number | 10-7
• Ekman Number | 10-15
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Where does the energy in the solar magnetic field come from?
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The Solar Dynamo generates magnetic fields from flows
convectiondifferential rotation
meridional circulation
magnetic energy ultimately comes from the Sun’s own mass
Fusion
mass ⇒ radiation & thermal energy
Convection
thermal energy ⇒ kinetic energyDynamo
kinetic energy ⇒ magnetic energy
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More on convection tomorrow.
Today: Induction!
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MHD Magnetic Induction equation
Comes from Maxwell’s equations (Faraday’s Law and Ampere’s Law)
Magnetic diffusivityAnd Ohm’s Law
electrical conductivity
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Creation and destruction of magnetic fields
Source of Magnetic Energy
Sink of Magnetic Energy
How would you demonstrate this?
(Hint: have a sheet handy with lots of vector identities!)
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Creation and destruction of magnetic fields
Source of Magnetic Energy
Sink of Magnetic Energy
Poynting Flux Ohmic Heating
Dot with B…
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Creation and destruction of magnetic fields
Source of Magnetic Energy
~ U B / D
Sink of Magnetic Energy
~ 𝜼 B / D2
If Rm >> 1 the source term is much bigger than the sink term
….Or is it???ratio of source and sink terms
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Creation and destruction of magnetic fields
Source of Magnetic Energy
~ U B / D
Sink of Magnetic Energy
~ 𝜼 B / 𝛅2
𝛅 can get so small that the two terms are comparable
It’s not obvious which term will “win” - it depends on the subtleties of the flow, including geometry & boundary conditions
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Creation and destruction of magnetic fields
Source of Magnetic Energy
~ U B / D
Sink of Magnetic Energy
~ 𝜼 B / 𝛅2
What is a Dynamo? (A corollary)
A dynamo must sustain the magnetic energy (through the conversion of kinetic energy) against Ohmic dissipation
How exactly does this work? It depends on the nature of the flow…
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Large Scale Dynamos: The Mean Induction Equation
Go back to our basic induction equation
Now just average over longitude and rearrange a bit (other averages are possible but we’ll stick to this for simplicity)
𝛀-effect(large-scale
shear)
Meridional circulation(transport)
Diffusion(molecular)
Fluctuating emf
(small-scale shear)
Note: The B field in the Sun is
clearly not axisymmetric. Still, the solar cycle does have
an axisymmetric component so that’s a
good place to start
The equation for the mean field comes out to be
No assumptions made up to this point beyond the basic
MHD induction equation
Straightforward to show that if E=0, the dynamo dies (Cowling’s theorem)
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The MHD Induction Equation: Alternate View
𝜕𝑩𝜕𝑡
= −𝑩𝛻 ∙ 𝒗 + 𝑩 ∙ 𝛻𝒗 − 𝒗 ∙ 𝛻𝑩 − 𝛻 × 𝜂𝛁 × 𝑩
compression
shear production
advection diffusion
Rotation
Non-rotating Rapidly-rotating
red upflow blue downflow
Large-Scale Shear (differential rotation)
Small-Scale Shear (helical rolls)
Convection Simulations
Rotating convection naturally generates both small-scale and large-scale shear!
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The 𝛀-effect(large-scale
shear)
J. Werne
Converts poloidal to toroidal field and amplifies it
...by tapping the kinetic energy of the differential rotation
𝛻𝑣𝜙
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:
360 nHz
500 nHz
Howe et al. 2000; Schou et al. 2002Equator
North Pole
• 24-day period equator• 30-day period poles
“Omega-effect”
Bennett, et al. 2003
Mean shear:• Latitudinal (Omega effect)• Radial (Tachocline; Interface Dynamo)
Converts poloidal to toroidal field and amplifies it
...by tapping the kinetic energy of the differential rotation
The 𝛀-effect (large-scale shear via helical convection)
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Moffat (1978)
Helical motions (lift, twist) can induce an emf that is parallel to the
mean field
Mean Toroidal Field(east-west)
Mean Poloidal Field(r, θ plane)
(north/south, up/down)
which closes the Dynamo Loop
Linked to kinetic, magnetic helicity
Linked to large-scale dynamo action
Illustrates the 3D nature of dynamos
This creates mean poloidal (r, θ) field from toroidal (𝝓) field
The turbulent α-effect(small-scale shear via helical convection)
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Rotation Yields Helical ConvectionNon-rotating
or Fast Convection
Rapidly-rotatingor
Slow Convection
toroidal field poloidal field poloidal field toroidal field
“alpha-effect”
Olson et al., 1999, JGR
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Helical Convection(Deep Shells)
Northern HemisphereTilted orbits
Streamlines
:
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Fast Convection / slow rotation
Helical Convection:Solar-like Simulations
Radial VelocityUpper Convection Zone
red upflows
blue downflows
Slow convection / rapid rotationFeatherstone & Hindman 2016
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Another Way: Starts with Magnetic Buoyancy
magnetic flux tube
If
Fan(2008)
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The Babcock-Leighton Mechanism
Trailing member of the spot pair is displaced poleward relative to leading edge by the Coriolis force (Joy’s law: the higher the latitude, the more the tilt)
Polarity of trailing spot is opposite to pre-existing polar field
Dispersal of many spots by convection and meridional flow acts to reverse the pre-existing poloidal field
Dikpati & Gilman 2006
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Babcock-Leighton Dynamo Models
Dikpati & Gilman 2006
Poloidal field is generated by the Babcock-Leighton
Mechanism
Cycle period is regulated by the equatorward advection of
toroidal field by the meridional circulation at the
base of the CZ
2-3 m/s gives you about 11 years
For this reason, they are also called
Flux-Transport Dynamo Models
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Final Thoughts on the Dynamo Process
• v influences B …•… AND vice versa
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This suggests two classes of dynamos:
Essentially Kinematic:Small seed field that is initially kinematic (too weak to induce a significant Lorentz force) grows exponentially until it becomes big enough to modify the velocity field
This brings up the crucial issue of: Dynamo Saturation
Essentially Nonlinear:The velocity field that gives rise to the dynamo mechanism depends on the
existence of the fieldThe focus then shifts toward: Dynamo Excitation + Dynamics
Final Thoughts on the Dynamo Process
• v influences B …•… AND vice versa
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Ultimately, convective flows drive a stellar dynamo.
How fast are those flows?
What is their structure?
These are NOT easy questions to answer
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Granulation in the Quiet Sun Lites et al (2008)
L ~ 1-2 MmU ~ 1 km s-1
𝝉c ~ 10-15 minDominant size scale of solar convection
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The MagneticNetwork
CaIIKnarrow-band core filter
PSPT/MLSO
SupergranulationL ~ 30-35 MmU ~ 500 m s-1
𝝉c ~ 20 hr
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Most prominent in horizontal velocities
near the limb
Supergranulationin Filtered Dopplergrams
D. Hathaway(NASA MSFC)
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A hierarchy of convective scales
Spruit, Nordlund & Title (1990)
Supergranulation and mesogranulation are part of a
continuous (self-similar?) spectrum of convective motions
Most of the mass flowing upward does not make it to the surface
Nordlund, Stein & Asplund (2009)
Density increases dramatically with depth below the solar surface
Downward plumes merge into superplumes that penetrate deeper
Fast, narrow down flows (plumes)Slow, broad upflows
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But still stops at 0.97R!what lies deeper still?
simulation by Stein et al (2006), visualization by Henze (2008)
Size, time scales of convection cells increases with depth
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radial velocity, r = 0.98R
Giant Cells Eventually the heirarchy must culminate in motions large enough to sense the spherical geometry and rotation
(Loosely, anything bigger than
supergranulation)
L ~ 100 MmU ~ 100 m s-1
𝝉c ~ days - months
Miesch et al (2008)
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N. Featherstone
Giant cells are notoriously difficult to detect (possibly masked by more vigorous surface
convection)How do we know they
are there?
Slow Convection
Rapid Convection
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We don’t.This is a major puzzle for solar
(and also stellar) dynamo theory and brings us to one of
the major outstanding questions.
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What is the solar Rossby Number?
:
L
Ro2v
Convective Timescale
Rotational Timescale
typical velocity
rotation rate typical length scale
What is Ro in the Sun?
What is v?
Ro influences the structure of the convection and so too MANY aspects of the dynamo!
Aside1. Consider system-scale Ro2. v = rms convective flow speed3. L = shell depth
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Ro determines differential rotation
Aside: differential rotation may be indirect evidence of giant cells, but see tomorrow’s discussion!
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Low Ro Promotes Magnetic Cycles (models)
Ghizaru, Charbonneau & Schmolarkiewicz2010
Ro | 0.01
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… and Equatorward Propagation of Magnetic Features (models)
Ro | 0.02
Käpylä et al. 2013
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… and Large-Scale Magnetic Structure (models)
Brown et al. 2010 Ro | 0.03“wreathy” dynamos
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Ring-Diagram Analysis (Greer et al. 2015)
Time-Distance(Hanasoge et al. 2012)
Convection Models
Disagreeing Observations( 0.96 Rsun )
What do measurements tell us?
Open Question:How fast is deep solar convection?
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Helioseismology(Greer et al. 2016)
What we know: at Depth … Ro is LOW
Possibly even lower: See non-detection of Hanasoge et al. 2012, PNAS, 109, 11928
But how low?Open Question!
Upper 15% of solar CZ
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Helioseismology(Greer et al. 2016)
What we know: at Depth … Ro is LOW
Possibly even lower: See non-detection of Hanasoge et al. 2012, PNAS, 109, 11928
But how low?Open Question!
Upper 15% of solar CZ
Challenge:
We can directly image convective flows in only the upper 15% of the convection zone.
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What do we expect at the solar surface?
1. Small-scale motions: • Near-surface features• high-Ro• Spatial-scale depends
on Ro-transition depth
2. Large-scale motions:• deep-seated, global• low-Ro
High Ro
Ro
Dept
h Ro-Transition Region
Low Ro(solar-like differential rotation)
photosphere
Deep CZ
We might expect to see two types of convection…
Lack of #2 may be telling us something…
Observed: 30 Mm sizeRo = 3
Unobserved…
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Deep Motions & Their Spectra
Cellular Structure Photospheric Power
Deep Motions(marginal Ro)
Near-Surface Motions(high-Ro)
Total Power
Deep Motions(Low- Ro)
Classic Cartoon(not observed)
AlternativePossibility
Featherstone & Hindman 2016
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Granulation (1 Mm; photospheric convection)
Giant Cells (200 Mm, Not here)
Hathaway et al. 2015
Photospheric Doppler Velocity Spectra
Supergranulation (30 Mm)
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Supergranulation
AIA 1700
L | 30 Mm U | 400 m s-1
Ro | 2.7
The largest, distinctly visible mode of solar convection is not rotationally constrained at the surface.
?
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Where is the Sun?
Until we know this, it is very difficult to make meaningful statements about how the solar dynamo works.