Some Key Issues in Solar Plasmas

1. Some Key Issues in Solar Plasmas (Leiden, March 21, 2005) Eric Priest

2. “My ‘retirement’ approaches this summer - but is not to be taken too literally as far as I am concerned” “I have my calculations, teaching & students - and I love it”

3. “Principles of MHD” - already a hit with new generation of researchers

4. “Milo shows signs of interest in geometry - and playing with grandpa Hans - What could be nicer ? ”

5. “Some Key Issues in Solar Plasmas” CONTENT: 1. Introduction 2. Structure of the Sun 3. Sunspots 4. Corona 5. MHD - Reconnection 6. Key advances - SOHO satellite * Interior * Solar Flares and CME's * Heating Atmosphere Conclusions

6. 1. INTRODUCTION Our Sun 4. Many basic properties of Sun a mystery 1. Of great scientific interest in own right B generated ? Solar wind acceld ? 2. Influence on Earth Corona heated ? Nature sunspots ? 3. Important for Astronomy Eruptions occur ? Flare particles acceld ? -- fundamental cosmic processes Today some of progress

7. close link St Andrews - Holland Traditionally St Andrews founded 1411 Many students came from Low Countries (16/17 C) Dutch stone masons. James Gregory 1st regius prof maths at St Andrews (1668) age 30 Invented Reflecting Telescope Co-founder of Calculus

8. James Gregory • In his lab -- see meridian line -- clock designed by C Huygens (stud. at Leiden)

9. James Gregory - died 1675 (37) Discovered: -- General binomial theorem -- Taylor expansions -- Ratio test for convergence of a series -- Series for sin x and tan x -- Integral of log x and sec x -- Differentiation is inverse of integration -- How to use change of variable in integration

10. 2. Overall Structure of Sun Interior: Core (< 0.25 R0), [R0 = 700 Mm] Radiative zone, Convection zone (> 0.7 R0) Atmosphere: Photosphere (6000K), Chromosphere (104K), Corona (106K)

11. Classical Picture: static plane-parallel atmosphere - rise in T But - highly nonuniform - multi-T - strongly t-dept - plasma heating/cooling dynamically So need analytical / computl MHD (e.g., Keppens) - idealised 1D models + physical insight - sophisticated 2D & 3D - both -> understanding

12. Start with 1D atmosphere T(h) Impose small oscn at photo (Carlsson & Stein) Even 1D model of Chromosphere (B=0) tough • Need high-resolution adaptive grid to resolve shocks Similar process in flux tubes -> spicules (De Pontieu, Erdelyi)

13. Photosphere Covered with turbulent convection cells: “Granulation” (1 Mm) “Supergranulation” (15 Mm)

14. Map ofPhotospheric Magnetic Field White -- towards Black -- away from 1. around spots -- bipolar "Active Regions" 2. Tiny intense magnetic fields over whole Sun B carried to edges of supergran. cells 3. Diffl. rotation

15. Model of Flux Emergence from Interior to CoronaV Archontis, F Moreno-Insertis,K Galsgaard, A Hood 3D compressible MHD, through 108 in density

16. Model of Flux Emergence from Interior to CoronaV Archontis, F Moreno-Insertis,K Galsgaard, A Hood 3D compressible MHD, through 108 in density

17. Magnetic field lines expanding into corona Similar to TRACE images

18. Current sheet forms High-velocity jets Reconnection High temperatures

19. Amazing images at 0.1”from Swedish telescope,La Palma(G Scharmer)

20. In close-up:effects of B around each granule points, flowers, ribbons- half flux in supergran.

21. Magnetoconvection models (e.g. Bushby) [256 x 256 x 120 points] Produce many observed features of granulation Temperature Vertical Magnetic Field

22. Results depend on B through Chandra. no Q=100 -> ribbons Q=10 -> points

23. 3. SUNSPOTS Dark because cool Photosphere --> Sunspots - magnetic field (B) stops granulation Vary with 11-year cycle Vertical magnetic flux tubes “Not so simple !”

24. StunningImage(Swedish telescope)[Scharmer & van der Voort] Close-up of penumbral structure (created by B) -> new surprises:

25. Points moving along lanes; Bright flows in/out; Strange dark cores

26. (Weiss, Thomas et al) New Model Penumbra - a mixture of interlocked field lines Dark filaments- (low) held down by granule flux pumping Bright filaments- (high)

27. See below sunspot by t-distance seismology(eg Bogdan) Wave speed slower - cooler Wave speed higher - B

28. -- See at ECLIPSE of Sun 4. CORONA Temperature is million degrees Magneticfield dominates plasma -> magnetic world -- heats corona But how ?? Iran (1999) - Koutchmy

29. Can observe corona direct in x-rays/euv • Early image from Skylab • bright pts, holes, loops, • act. reg.

30. TRACE (Active region) - from above

31. TRACE - from side - intricate structure Not isolated coronal loops - plasma that is at one temp. [1.5 MK]

32. Key Discovery from SOHO/TRACE MHD WAVES in CORONA (eg Nakariakov) Periods 2-20 min, amplitudes 2-5% -> insuff. to heat corona But Coronal Seismology: B, structure, transp coeffs. 1D slabs/tubes - basis (Hans G) 2 & 3D modelling (Andries, Bogdan, Erdelyi, Goossens, Poedts, Young ….)

33. 5. Eqns of Magnetohydrodynamics But - in corona: ? H ? Correct forms for transport coeffs. need collisionless effects when l < 30 km - in photosphere: optically thick

34. Induction Equation • [B changes due to transport + diffusion] • In most of Universe Rm>>1, B frozen to plasma Except SINGULARITIES -- & large Reconnecting current sheets Resonant absorption layers Shock waves

35. In 2D, reconnecting sheets form at NULL POINTS, B = 0(e.g., Baty) • In 3D reconnection can take place at nulls or at non-null points (eg Galsgaard)

36. 5.1 3D RECONNECTION Many New Features (i) Structure of Null Point Simplest B = (x, y, -2z) 2 families of field lines through null point: Spine Field Line FanSurface

37. (ii) Topology of Fields - Complex In 2D -- Separatrix curves In 3D -- Separatrix surfaces -- intersect inSeparator

38. Note Coronal magnetic field - highly complex - many sources. 1. When constructing coronal field/ numerical expts - useful to construct skeleton (web of separatrix surfaces). 2. Understand nature bifurcations [3. For continuous sources: quasi-separatrix surfaces, quasi-separator, - no discont., but steep change in mapping grad]

39. (iii)Numerical Experiment(Linton & Priest) [3D pseudo-spectral code, 2563 modes.] Impose initial stagn-pt flow v = vA/30 Rm = 5600 Isosurfaces of B2:

40. B-Lines for 1 Tube Colour shows locations of strong Ep stronger Ep Final twist

41. 6. SOHO (Solar & Heliospheric Observatory) Launched 1995. Orbiting Sun at point in phase with Earth Observing Sun continuously for 1st time (ESA/NASA) --> 1st comprehensive view of Sun MANY NEW ADVANCES - only 3 today

42. QN. 1 -- ? Structure of Solar Interior SOHO (MDI) detected several million normal modes Measures velocity of Sun's surface at million pts/min -> frequencies: -> T(r) [agrees with model to < 1%]

43. Deduce Internal Rotation Observe: * Faster at equator -- Expect: * const. on cylinders * B generated throughout conv. zone Surprise: -- const on radial lines -- intense shear layer ? site dynamo Need build models for tacocline (see Rosner)

44. QN. 2 -- ? How Do Flares & CME’s Occur Oct-Nov, 2003- v. complex sunspot group-> largest flares + mass ejections Photosphere

45. Outer Coronafrom SOHO CME 2000 km/s (5 times faster than normal) Snow -- relc particles

46. Aurora in St Andrews Thurs

47. Overall Picture of Eruption twisted magnetic tube - erupts drives reconnection

48. (Priest and Schrijver 1999) Reconnection heats loops Continues:new loops Form Old loops cool & drain

49. Example from TRACE (171 A) 20 MK [Fe XVI] + 1 MK [Fe IX]

50. RHESSI Overlay of TRACE • Red contours: 12 – 25 keV X-ray flux. • Blue contours: 50 – 100 keV X-ray flux. • Particle acceleration: DC acceleration in sheet +Fermi in collapsing trap