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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)