Solar Structure. Connections between the solar interior and solar activity. Deep roots of solar activity

Transcription

1 Deep roots of solar activity Michael Thompson University of Sheffield Sheffield, U.K. 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 flows Cause-and-effect between solar interior and eruptive events contributing to solar activity: flares, coronal mass ejections Solar Structure Solar Interior 1. Core 2. Radiative Interior 3. (Tachocline) 4. Convection Zone Visible Sun 1. Photosphere 2. Chromosphere 3. Transition Region 4. Corona 5. (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) 1

2 Observations Solar Longitudinally averaged photospheric magnetic field 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 and two poles have opposite polarities Polar field reverses every 11 years but out of phase with the sunspot field. Global Magnetic field reversal. Coronal heating and the magnetic carpet The large-scale coronal magnetic field 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. 2

3 Evolution of the coronal magnetic field Coronal loops observed by TRACE satellite This instability is known as Magnetic Buoyancy. Theoretical picture Sunspot pairs are believed to be formed by the instability of a magnetic field generated deep within the Sun. It is also important in Galaxies and Accretion Disks and Other Stars. Flux tube rises and breaks through the solar surface forming active regions. 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. Wissink et al (2000) 3

4 Helioseismology Observe Sun oscillating simultaneously in more than a million modes acoustic waves. Measure mode properties?; A, G; line-shapes Eigenfunctions / spherical harmonics Frequencies? nlm (t) depend on conditions in solar interior determining wave propagation? nlm degeneracy lifted by rotation and by structural asphericities and magnetic fields Spherical harmonics Inversion provides maps such as of c and? and rotation and wave-speed asphericities Also new techniques such as time-distance helioseismology : make subsurface inferences from measured wave travel times between points on the Sun s surface 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) Meridional flows Mostly poleward but with transient counter-cell in northern hemisphere Meridional circulation Zonal flows at 1 Mm and 7 Mm depth (note torsional oscillation) 4

5 Large and Small-scale dynamos The alpha-omega dynamo LARGE SCALE Sunspots Butterfly Diagram 11-yr activity cycle Coronal Poloidal Field Systematic reversals Periodicities Field generation on scales > L TURB SMALL SCALE Magnetic Carpet Field Associated with granular and supergranular convection Magnetic network Field generation on scales ~ L TURB 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 diffential rotation and stored in the tachocline. And the mean poloidal field (coronal field) is created by turbulence (or perhaps by a dynamic a- effect) in the lower reaches of the convection zone 5

6 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 a-effect is not suppressed Separation of large and smallscale 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 a- effect therefore all the problems of a-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 6

7 Observations of emerging active region by time-distance helioseismology magnetogram Subphotospheric imaging of active regions AR Mm AR Sound-speed perturbation (~1 km/s: 300 K or 3000 G) 460 Mm Evolution of AR : October 24 November 2, 2003 Sound-speed map and magnetogram of AR on October 25, 2003, 4:00 UT (depth of the lower panel: 45 Mm) AR

8 Sound-speed map and magnetogram of AR on October 26, 2003, 12:00 UT is emerging Emergence of, October 26, 2003, 20:00 UT AR Emergence of, October 27, 2003, 4:00 UT Growth and formation of sunspots of, October 29, 2003, 4:00 UT 8

9 Growth and formation of sunspots of, October 31, 2003, 12:00 UT Cut in East-West direction through both magnetic polarities, showing a loop-like structure beneath, October 30, 2003, 20:00 UT View from the top through the semi-transparent magnetogram, October 30, 2003, 20:00 UT. The lower panel is 16 Mm deep. 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 of subphotospheric flows and sound-speed structures, which can be compared with photospheric magnetic fields and X-ray data. 9

10 Sub-photospheric flow maps and photospheric magnetograms during X10 flare Sub-photospheric flow maps and photospheric magnetograms during X10 flare SSW and Active Complex Mm Energyrelease site 16 Mm 10

11 Flows near and beneath active region Apr 2001 Kinetic helicity SOHO 14 - GONG 2004 SOHO 14 - GONG

12 SOHO 14 - GONG 2004 SOHO 14 - GONG 2004 Variability in and near tachocline 1.3-yr variations in inferred rotation rate at low latitudes above and beneath tachocline Signature of dynamo field evolution? Radiative interior also involved in solar cycle? Variations in O ( r,?; t ) Wavelet analysis of the Sun s mean photospheric magnetic field: prominent periods are the rotation period and its 2 nd harmonic, and the 1.3/1.4-yr period Solar mean magnetic field Link between tachocline and 1.3/1.4-yr variations in solar wind, aurorae, solar mean magnetic field? Howe et al Boberg et al

13 Imaging of active regions on the far-side of the Sun using acoustic holography before rotation brings them to the Earth-side. Far-side imaging Conclusions 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 Imager 1.B Solar Dynamo 1.J Sunspot Dynamics 1.C Global Circulation 1.I Magnetic Connectivity 1.A Interior Structure 1.D Irradiance Sources 1.H Far-side Imaging 1.E Coronal Magnetic Field NOAA 9393 Farside 1.G Magnetic Stresses 1.F Solar Subsurface Weather 13

14 HMI Data Filtergrams Observables Doppler Velocity Line-of-sight Magnetograms Vector Magnetograms Continuum Brightness Processing Global Helioseismology Processing Local Helioseismology Processing HMI Science Analysis Plan Data Product Internal rotation O(r,T) (0<r<R) Internal sound speed, c s(r,t) (0<r<R) Full-disk velocity, v(r,t,f), And sound speed, c s(r,t,f), Maps (0-30Mm) Carrington synoptic v and c s maps (0-30Mm) High-resolution v and c s maps (0-30Mm) Deep-focus v and c s maps (0-200Mm) Far-side activity index Line-of-Sight Magnetic Field Maps Vector Magnetic Field Maps Coronal magnetic Field Extrapolations Coronal and Solar wind models Brightness Images Science Objective Tachocline Meridional Circulation Differential Rotation Near-Surface Shear Layer Activity Complexes Active Regions Sunspots Irradiance Variations Magnetic Shear 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 Version 1.0w S. Tobias TURBULENT CONVECTION STRONG LARGE SCALE SUNSPOT FIELD <B T > ROTATION TURBULENT CONVECTION ROTATION TURBULENT CONVECTION ROTATION Reynolds Stress <u i u j > L-effect DIFFERENTIAL ROTATION W MERIDIONAL CIRCULATION U p DIFFERENTIAL ROTATION W MERIDIONAL CIRCULATION U p S. Tobias STRONG LARGE SCALE SUNSPOT FIELD <B T > S. Tobias STRONG LARGE SCALE SUNSPOT FIELD <B T > 14

15 TURBULENT CONVECTION ROTATION TURBULENT CONVECTION ROTATION Reynolds Stress <u i u j > L-effect Reynolds Stress <u i u j > L-effect HELICAL/CYCLONIC CONVECTION u HELICAL/CYCLONIC CONVECTION u SMALL-SCALE MAG FIELD b DIFFERENTIAL ROTATION W MERIDIONAL CIRCULATION U p Turbulent amplification of <B> Turbulent EMF E = <u x b > a,b,g-effect DIFFERENTIAL ROTATION W MERIDIONAL CIRCULATION U p LARGE-SCALE MAG FIELD <B> W-effect LARGE-SCALE MAG FIELD <B> W-effect S. Tobias STRONG LARGE SCALE SUNSPOT FIELD <B T > S. Tobias STRONG LARGE SCALE SUNSPOT FIELD <B T > TURBULENT CONVECTION ROTATION HELICAL/CYCLONIC CONVECTION u SMALL-SCALE MAG FIELD b Reynolds Stress <u i u j > L-effect Maxwell Stresses L-quenching Turbulent amplification of <B> S. Tobias Turbulent EMF E = <u x b > a,b,g-effect LARGE-SCALE MAG FIELD <B> Small-scale Lorentz force a-quenching DIFFERENTIAL ROTATION W STRONG LARGE SCALE SUNSPOT FIELD <B T > Large-scale Lorentz force MERIDIONAL CIRCULATION U p Malkus-Proctor effect W-effect Simulations of turbulent pumping of magnetic field from convection zone into stable layer beneath. Tobias et al. (1998) 15