Transcript Space Weather Magnetic Field Origins

Space Weather
Magnetic Field Origins
Helioseismology
Dynamo Theory
1
Motivation
• Solar magnetic fields are the driver of
space weather; without the magnetic field
there is no space weather. Can someone
tell me why that is?
• Magnetic fields originate under the solar
surface.
• There are short and long term changes in
the fields.
• How can we predict these changes?
2
Outline
• Helioseismology
– What is it
– What are the measurements
– What is the mathematics
– What do the results look like
• Solar Dynamo
– Basic concept What is a dynamo?
– Current understanding (pun not intended)
3
Solar Structure
4
Pressure and Gravity Mode Rays
Courtesy Juri Toomre and Douglas Gough
5
Helioseismology
•Phenomenon
•5 minute oscillations of the visible surface
•Sound waves
•resonantly trapped below the surface
•excited by convection just below the surface
•Methodology
•107 normal modes of varying radial/latitudinal extent
•Comparison of frequencies with model calculations
•Inverse methods
•Local helioseismology and running waves
•Results
•Physics: GR, neutrinos, EoS, opacities, diffusion,
•Stellar Structure: temperature, density, abundances, rotation, flows,
"turbulence", ... Implies stellar evolution.
•Variability: modes, 3-D structures
•Frontier
•Asteroseismology
•Gravity Modes
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The Solar Surface
Courtesy of Luc Rouppe van der Voort, Oslo, from the Swedish Solar Telescope, La Palma
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“Wiggle Line” Spectrum
100
50
0
5853.7
5856.1
5859.6
Wavelength [ Ångstroms ]
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Telescope
• Ordinary white light telescope, need not be very
large, good enough for 1” resolution
• Blocking filter to isolate one desirable spectral
line
• Tunable interference filter to make
measurements in several locations through line
to make Doppler line profiles.
• CCD detector.
• Output is a line profile at each 1” element on the
Sun
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Data-Velocity Image
10
Disentangling the Modes
Courtesy Frank Hill

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
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
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Mathematical Form
From Annual Reviews of Astronomy and Astrophysics 1984, 22, 593, Deubner
and Gough
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Equation of Motion
13
Surface velocity changes
Steve Musman and Dave Rust, Solar Physics
14
Temporal Power Spectrum
Courtesy Dick White and Milton Cha
15
A Testable Prediction
p-modes
g-modes
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The Outcome
Courtesy Franz-Ludwig Deubner
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And Rapid Improvement
Courtesy Franz-Ludwig Deubner, Roger Ulrich, and Ed Rhodes
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Caricature of the “Scientific Method”
• Serendipitous Discovery
• Many Different Descriptions
• Testable Prediction of Unobserved Phenomenon
• Confirmation
• Use Remaining Small Deviations as Tool
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A Typical ℓ - Diagram
5000
4000

(Frequency)
3000
2000
500
ℓ (Spherical Harmonic Degree)
1000
20
0
The Global Rotation Picture
Surface Shear
Tachocline
21
Courtesy of Rachel Howe
Sub-surface Flows
Jesus Patron et al.
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Structure of Sunspot
Courtesy Tom Ducall
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High Resolution Near Surface Flows
Courtesy of Junwei Zhao
24
The Farside
Courtesy Doug Braun
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What we have learned, are
learning, and hope to learn
• Successes
– The phenomenon itself, Neutrinos, J2, Internal Rotation,
Helium abundance, Opacities, Depth of Convection Zone,
Structures, etc, etc, etc…
• Challenges
– Origins of solar magnetism
– Differential rotation, torsional oscillations, and meridional
circulation
– Sub-surface inhomogeneities
– How all of this should manifest itself in other stars
– How these tools can contribute to a predictive understanding
of space weather.
• Frontiers
– Asteroseismology & G-modes
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•Dynamo
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Coexistence of magnetic fields consisting of
widely different length scales, magnitudes and
temporal scales
Hierarchy in length scale:
 Network fields ~ 100 km size
 Sunspots, ephemeral regions, plage ~ 10000-30000 km diameter
 Unipolar regions ~ 100,000 km extent
Hierarchy in flux and/or field strength:
 Spots, active regions, network fields ~ a few thousand Gauss
 Plage ~ a few hundred Gauss
 Large-scale diffuse fields ~ a few tens of Gauss
Hierarchy in temporal variations:
 Persistent, cyclic features: butterfly diagrams, polar reversal,
active longitudes
 Random features: small-scale mixed-polarity turbulent fields
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What is a
dynamo?
A dynamo is a process by which the magnetic field
in an electrically conducting fluid is maintained
against Ohmic dissipation
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Let’s Build A Homopolar-disc
Dynamo
A copper disc that can rotate about its axis
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Let’s Build A Homopolar-disc
Dynamo
Supply kinetic energy to rotate the disc
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Let’s Build A Homopolar-disc
Dynamo
Introduce magnetic fields; an electromotive force between the
axis and the rim will be generated
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A Homopolar-disc Dynamo
(Complete)
Connect a wire twisted in the same sense as the sense of rotation;
magnetic fields will grow
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Observational signature for systematic, cyclic
evolution of solar magnetic fields
Courtesy:
D.H. Hathaway
Many evidences for coexistence of small-scale and large-scale dynamos
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Large-scale dynamo: historical background
(i) Generation of toroidal field
by shearing a pre-existing
poloidal field by
differential rotation
(Ω-effect )
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Large-scale dynamo: historical background
(contd.)
(ii) Re-generation of poloidal field
by lifting and twisting a
toroidal flux tube by helical
turbulence
(α-effect)
Proposed by Parker (1955)
Mathematically formulated by Steenbeck, Krause & Radler (1969)
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Large-scale dynamo: historical background
(contd.)
• In 1960’s and 70’s, equatorward propagating dynamo wave was obtained by
assuming a radial differential rotation increasing inward throughout the
convection zone.
• Sunspots were identified as that formed from strong toroidal flux tubes which
rise to the surface due to their magnetic buoyancy
• Equatorward migration of sunspot-belt was explained by an equatorward
propagating dynamo wave for the subsurface toroidal fields
Equatorward propagation of dynamo wave was obtained by satisfying
Parker-Yoshimura Sign Rule; α dΩ/dr < 0, In North-hemisphere
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Historical Background (contd.)
• But, In 1980’s, helioseismic analysis
inferred that there is no radial shear in
the convection zone, and the strong
radial shear at or below the base of
the convection zone is decreasing
inward at sunspot latitudes.
(Courtesy: Thierry Corbard)
Therefore, Convection Zone Dynamos Do Not Work
With Solar-like Ω
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FLUX-TRANSPORT DYNAMO
+
Meridional
circulation
Wang & Sheeley, 1991
Choudhuri, Schüssler, & Dikpati, 1995
Durney, 1995
Dikpati & Charbonneau, 1999
Küker, Rüdiger & Schültz, 2001
And certainly many others
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Mathematical Formulation
Under MHD approximation (i.e. electromagnetic variations are nonrelativistic),
Maxwell’s equations + generalized Ohm’s law lead to induction equation :
B
   U  B  η  B  .
t
(1)
Applying mean-field theory to (1), we obtain the dynamo equation as,
B
   U  B  αB  η  B  ,
t
(2)
Diffusion
(turbulent + molecular)
Differential rotation
and meridional circulation
Displacing and
twisting effect
by kinetic helicity
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Evolution of Magnetic Fields
In a Babcock-Leighton Flux-Transport Dynamo
Dynamo cycle period ( T ) primarily governed by meridional flow speed
Dikpati & Charbonneau 1999, ApJ, 518, 508
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Validity test of calibration
Contours: toroidal fields at CZ base
Gray-shades: surface radial fields
Observed NSO map of longitude-averaged photospheric fields
(Dikpati, de Toma, Gilman, Arge & White, 2004, ApJ, 601, 1136)
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Predicting the onset of cycle 24
Simulated solar cycles
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22
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Next cycle will
start late in 2007
or early in 2008
(Dikpati et al., 2004, AAS/SPD)
(Delayed onset of cycle 24 has also been
predicted by Sello 2003 using a different
method)
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Can we go beyond decadal time-scale?
Can we predict Maunder minima or
Medieval maxima?
Maunder minimum is the absence of sunspots,
but not the absence of cycle
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The origin of magnetic fields in the solar interior
 Could solar cycle dynamo be a source
for deep interior magnetic fields?
 Noticed flux-transport dynamo model
diffusing toroidal field into low-diffusivity
domain below tachocline.
Artifact, or reality?
 Long-term transient or permanent?
 Nonreversing fields, but structure
dependent on initial phase of cycle
when diffusion starts?
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Dikpati, Gilman & MacGregor (2005, in preparation)