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Deep roots of solar activity
Michael Thompson
University of SheffieldSheffield, U.K.
michael.thompson@sheffield.ac.uk
With thanks to:
Alexander Kosovichev, Rudi Komm, Steve Tobias
Connections between the solar interior and solar activity
• Magnetic field generation• Field emergence and evolution
Active regions Magnetic carpet
• Sub-photospheric flowsCause-and-effect between solar interior and eruptive events contributing to solar activity: flares, coronal mass ejections
Solar StructureSolar Interior
1. Core2. Radiative Interior3. (Tachocline)4. Convection Zone
Visible Sun
1. Photosphere2. Chromosphere3. Transition Region4. Corona5. (Solar Wind)
•amplitude variations of a factor of 3
•length 8-15 yr
•mean 11.1 yr
•asymmetric rise-decline (strongest for high-amplitude cycles)
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Observations Solar
• Solar cycle not just visible in sunspots• Solar corona also modified as cycle progresses.• Weak polar magnetic field has mainly one polarity at each pole a nd
two poles have opposite polarities• Polar field reverses every 11 years – but out of phase with the
sunspot field.
• Global Magnetic field reversal.
Longitudinally averagedphotospheric magnetic field
Coronal heating and the magnetic carpet
• Small-scale reconnection may play a large role in heating the corona, with magnetic energy being released as heat.
• SOHO observations have led to the concept of the magnetic carpet, with small-scale flux being renewed every 14 hours.
• Work in St Andrews indicates that only a small fraction (a few per cent) of flux tubes reach the corona. Reconnection amongst the tangle of low-lying field lines may heat the feet of the overlying loop structures.
The large-scale coronal magnetic field
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Evolution of the coronal magnetic field
Coronal loops observed by TRACE satellite
Theoretical pictureSunspot pairs are believed to be formed by the instability of a magnetic field generated deep within the Sun.
Flux tube rises and breaks through the solar surface forming active regions.
This instability is known asMagnetic Buoyancy.
It is also important in Galaxies andAccretion Disks and Other Stars.
Wissink et al (2000)
Stressed magnetic fields
• The high conductivity of the photospheric plasma means that the field is frozen in and must move with the plasma.
• Convective motions in the photosphere move the footpoints of magnetic loops, causing the field to get contorted and storing up energy.
• If the field is sufficiently contorted, even a little diffusivity allows the field to jump abruptly into a lower-energy state “reconnection”. This can be a common explanation of such spectacular events as eruptions of prominences, solar flares and coronal mass ejections.
• The energy is released as kinetic energy and heat.
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• Also new techniques such as time-distance helioseismology : make subsurface inferences from measured wave travel times between points on the Sun’s surface
Helioseismology
• Measure mode properties ?; A, G; line-shapesEigenfunctions / spherical harmonics
• Frequencies ?nlm(t) depend on conditions in solarinterior determining wave propagation
• ?nlm – degeneracy lifted by rotation and by structural asphericities and magnetic fields
• Inversion provides maps such as of c and ? and rotation andwave-speed asphericities
Spherical harmonics
• Observe Sun oscillating simultaneously in morethan a million modes – acoustic waves.
Solar Internal Rotation• Helioseismology shows the
internal structure of the Sun.
• Surface Differential Rotation is maintained throughout the Convection zone
• Solid body rotation in the radiative interior
• Thin matching zone of shear known as the tachocline at the base of the solar convection zone (just in the stable region).
Radial cuts through inferred rotation profile of the solar interior(at latitudes indicated)
Zonal flowsat 1 Mm and 7 Mm depth(note torsional oscillation)
MeridionalMeridional circulationcirculation
Meridional flowsMostly poleward but with transient counter-cell in northern hemisphere
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Large and Small-scale dynamosLARGE SCALELARGE SCALE
SunspotsButterfly Diagram11-yr activity cycleCoronal Poloidal FieldSystematic reversalsPeriodicities------------------------------Field generation on scales
> LTURB
SMALL SCALESMALL SCALE
Magnetic CarpetField Associated with
granular and supergranular convection
Magnetic network
---------------------------------Field generation on scales
~ LTURB
The alpha-omega dynamo
Alternative Mechanisms for Producing Poloidal Field
• Poloidalfield generated by magnetic buoyancy instability in connection with rotation or shear– Either the instability of (thin) magnetic flux tubes– Or more likely the instability of a layer of magnetic
field (e.g. Brummell)
• Joint Instability of field and differential rotation in the tachocline (Gilman, Dikpati etc)– Produces a mean flow with a net helicity
• Decay and dispersion of tilted active regions at the solar surface (Babcock-Leighton mechanism)
Interface Dynamo scenario• The dynamo is thought to
work at the interface of the convection zone and the tachocline.
• The mean toroidal (sunspot field) is created by the radial diffentialrotation and stored in the tachocline.
• And the mean poloidalfield (coronal field) is created by turbulence (or perhaps by a dynamic α-effect) in the lower reaches of the convection zone
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Interface Dynamo scenario• PROS
– The radial shear provides a natural mechanism for generating a strong toroidal field
– The stable stratification enables the field to be stored and stretched to a large value.
– As the mean magnetic field is stored away from the convection zone, the α -effect is not suppressed
– Separation of large and small-scale magnetic helicity
• CONS– Relies on transport of flux to and
from tachocline – how is this achieved?
– Delicate balance between turbulent transport and fields.
Flux Transport Scenario• Here the poloidal field is
generated at the surface of the Sun via the decay of active regions with a systematic tilt (Babcock-Leighton Scenario) and transported towards the poles by the observed meridional flow
• The flux is then transported by a conveyor belt meridional flow to the tachocline where it is sheared into the sunspot toroidal field
• No role is envisaged for the turbulent convection in the bulk of the convection zone.
Flux Transport Scenario• PROS
– Does not rely on turbulent α -effect therefore all the problems of α -quenching are not a problem
– Sunspot field is intimately linked to polar field immediately before.
• CONS– Requires strong meridional
flow at base of CZ of exactly the right form
– Relies on existence of sunspots for dynamo to work (cf Maunder Minimum)
Sunspot structure and dynamics
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Observations of emerging active region by time-distancehelioseismology
magnetogram
Sound-speed perturbation(~1 km/s: 300 K or 3000 G)
460 Mm
18 M
m
AR 10488
AR 10486
AR 10484
Subphotospheric imaging of active regions
Evolution of AR 10486-488: October 24 – November 2, 2003
Sound-speed map and magnetogram of AR 10486 on October 25, 2003, 4:00 UT(depth of the lower panel: 45 Mm)
AR 10486
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Sound-speed map and magnetogram of AR 10486 on October 26, 2003, 12:00 UTAR 10488 is emerging
AR 10486 AR 10488
Emergence of AR 10488, October 26, 2003, 20:00 UT
AR 10488
Emergence of AR 10488, October 27, 2003, 4:00 UT
AR 10488
Growth and formation of sunspots of AR 10488, October 29, 2003, 4:00 UT
AR 10488
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Growth and formation of sunspots of AR 10488, October 31, 2003, 12:00 UT
AR 10488
Cut in East-West direction through both magnetic polarities, showing a loop-like structurebeneath AR 10488, October 30, 2003, 20:00 UT
AR 10488
View from the top through the semi -transparent magnetogram, October 30, 2003, 20:00 UT. The lower panel is 16 Mm deep.
AR 10488
Sunspot dynamics associated with flares and CME
• Magnetic field topology and magnetic stresses in the solar atmosphere are likely be controlled by motions of magnetic fluxfootpoints below the surface However, the depth of these motions is unknown.
• Time-distance helioseismology provides maps ofsubphotospheric flows and sound-speed structures, which can be compared with photosphericmagnetic fields and X-ray data.
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Sub-photospheric flow maps and photospheric magnetograms during X10 flare
Sub-photospheric flow maps and photospheric magnetograms during X10 flare
Energyrelease site
SSW and Active Complex 9393
7 Mm7 Mm
16 Mm16 Mm
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Flows near and beneath active region Apr 2001
SOHO 14 - GONG 2004
Kinetic Kinetic helicityhelicity
SOHO 14 - GONG 2004
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SOHO 14 - GONG 2004 SOHO 14 - GONG 2004
Variability in and near tachocline
Howe et al. 2000
1996 2002
Variations in O ( r , ? ; t )1.3-yr variations in inferred rotation rate at lowlatitudes above and beneath tachocline
Signature of dynamo field evolution?Radiative interior also involved in solar cycle?
Link between tachocline and 1.3/1.4-y rvariations in
• solar wind, • aurorae,• solar mean magnetic field ?
Boberg et al. 2002
Solar mean magnetic field
1975 2000
Wavelet analysis of the Sun’s mean photospheric magnetic field:prominent periods are the rotation period and its 2nd harmonic, and the 1.3/1.4-yr period
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Imaging of active regions on the far-side of the Sun using“acoustic holography”– before rotation brings them to the Earth-side.
FarFar--side imagingside imagingConclusions
• Field generation: probably large- and small-scale dynamos. Poloidal field generation still somewhat open. General consensus for large-scale dynamo sited in tachocline, but flux-transport dynamo also possible.
• Helioseismology gives new views of field emergence and subsurface structures and flows.
• Good prospects for now-casting of subsurface flows and active-region structures with helioseismology for space-weather studies.
• SOHO has given data of the highest quality for solar studies. This will continue with new missions such as Solar-B, STEREO and …
Solar Dynamics Observatory (2008)
Solar Dynamics Observatory: Helioseismic and Magnetic Imager1.B – Solar Dynamo
1.C – Global Circulation
1.D – Irradiance Sources
1.H – Far-side Imaging
1.F –Solar Subsurface Weather
1.E – Coronal Magnetic Field
1.I –Magnetic Connectivity
1.J – Sunspot Dynamics
1.G –Magnetic Stresses
1.A – Interior Structure
NOAA 9393
Far-s ide
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HMI Science Analysis Plan
Magnetic Shear
Tachocline
Differential Rotation
Meridional Circulation
Near-Surface Shear Layer
Activity Complexes
Active Regions
Sunspots
Irradiance Variations
Flare Magnetic Configuration
Flux Emergence
Magnetic Carpet
Coronal energetics
Large-scale Coronal Fields
Solar Wind
Far-side Activity Evolution
Predicting A-R Emergence
IMF Bs Events
Brightness Images
GlobalHelioseismology
Processing
Local Helioseismology
Processing
Version 1.0w
Filtergrams
Line-of-sightMagnetograms
Vector Magnetograms
DopplerVelocity
ContinuumBrightness
Line-of-SightMagnetic Field Maps
Coronal magneticField Extrapolations
Coronal andSolar wind models
Far-side activity index
Deep-focus v and csmaps (0-200Mm)
High-resolution v and csmaps (0-30Mm)
Carrington synoptic v and c smaps (0-30Mm)
Full-disk velocity, v(r,T,F),And sound speed, cs(r,T,F),
Maps (0-30Mm)
Internal sound speed,cs(r,T) (0<r<R)
Internal rotation O(r,T)(0<r<R)
Vector MagneticField Maps
Science ObjectiveData ProductProcessing
Observables
HMI Data
TURBULENT CONVECTION
ROTATION
STRONG LARGESCALE SUNSPOT
FIELD <BT>S. Tobias
TURBULENT CONVECTION
ROTATION
STRONG LARGESCALE SUNSPOT
FIELD <BT>
DIFFERENTIALROTATION Ω
MERIDIONAL CIRCULATION Up
S. Tobias
TURBULENT CONVECTION
ROTATION
STRONG LARGESCALE SUNSPOT
FIELD <BT>
DIFFERENTIALROTATION Ω
MERIDIONAL CIRCULATION Up
Reynolds Stress<u’ i u’j> Λ-effect
S. Tobias
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TURBULENT CONVECTION
ROTATION
STRONG LARGESCALE SUNSPOT
FIELD <BT>
DIFFERENTIALROTATION Ω
MERIDIONAL CIRCULATION Up
HELICAL/CYCLONICCONVECTION u’
LARGE-SCALEMAG FIELD <B>
Reynolds Stress<u’ i u’j> Λ-effect
Ω-effect
S. Tobias
TURBULENT CONVECTION
ROTATION
STRONG LARGESCALE SUNSPOT
FIELD <BT>
DIFFERENTIALROTATION Ω
MERIDIONAL CIRCULATION Up
HELICAL/CYCLONICCONVECTION u’
SMALL-SCALE MAG FIELD b’
LARGE-SCALEMAG FIELD <B>
Reynolds Stress<u’ i u’j>
Turbulent EMF
E = <u’ x b’>
Ω-effect
Λ-effect
Turbulentamplification of<B>
α,β,γ-effect
S. Tobias
TURBULENT CONVECTION
ROTATION
STRONG LARGESCALE SUNSPOT
FIELD <BT>
DIFFERENTIALROTATION Ω
MERIDIONAL CIRCULATION Up
HELICAL/CYCLONICCONVECTION u’
SMALL-SCALE MAG FIELD b’
LARGE-SCALEMAG FIELD <B>
Reynolds Stress<u’ i u’j>
Turbulent EMF
E = <u’ x b’>
Ω-effect
Λ-effect
Turbulentamplification of<B>
α,β,γ-effect Small-scaleLorentz forceα-quenching
MaxwellStresses
Λ-quenching
Malkus-Proctoreffect
Large-scaleLorentz force
S. Tobias
Simulations of turbulent pumping of magnetic field fromconvection zone into stablelayer beneath. Tobias et al. (1998)
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