The Solar Dynamo Saga: Chapter 11

1. The Solar Dynamo Saga:Chapter 11 Dr. David Hathaway NASA Marshall Space Flight Center 2009 August 15 Huntsville Hamfest

2. Outline • Sunspots, space weather, and climate • Solar cycle characteristics • Solar dynamo basics (40s and 50s) • Solar dynamo models (60s to Present) • Dynamo dilemmas (Starting in the 70s) • The role of the meridional circulation • Conclusions

3. Sunspots, Space Weather, and Climate(why should YOU care)

4. Sunspots Sunspots are dark (and cooler) regions on the surface of the Sun. They have a darker inner region (the Umbra) surrounded by a lighter ring (the Penumbra). Sunspots usually appear in groups that form over hours or days and last for days or weeks. These early sunspot observations indicated that the Sun rotates once in about 27 days.

5. Solar Activity Solar Flares, Prominence Eruptions, and Coronal Mass Ejections are all forms of solar activity. Each can occur on its own but they often occur together as in “The Bastille Day Event.”

6. Space Weather Space weather refers to conditions on the Sun and in the space environment that can influence the performance and reliability of space-borne and ground-based technological systems, and can endanger human life or health.

7. Effects of Solar Activity: On Radio Wave Propagation Variations in ionizing radiation (UV, EUV, X-rays) from the Sun, as well as solar induced changes to the Earth’s magnetosphere, alter the ionosphere – changing the Maximum Usable Frequency for high frequency radio communications.

8. HF Communication only Effects of Solar Activity: On Airline Operations • Polar flights departing from North America use VHF (30-300 MHz) comm or Satcom with Canadian ATCs and Arctic Radio. • Flights rely on HF (3 – 30 MHz) communication inside the 82 degree circle. • Growth: Airlines operating China-US routes goes from 4 to 6 and then number of weekly flights goes from 54 to 249 over the next 6-years.

9. Total Irradiance and Climate The 0.1% change in the Total Solar Irradiance seen over the last three solar cycles only produces a 0.1° C temperature change in climate models. However, the Sun seems to have a bigger impact. Two other mechanisms (besides direct forcing by the Total Solar Irradiance variations) are under study: 1) solar ultraviolet and extreme ultraviolet variability and 2) Cosmic Ray modulation on cloud cover.

10. Solar Cycle Characteristics(that dynamo theories should reproduce)

11. Sunspots! Sunspots are cooler and darker regions on the surface of the Sun. Typical sunspots are about the size of Earth.

12. The Sunspot Cycle The average cycle lasts 131±14 months and has a smoothed sunspot number maximum of 114±40.

13. Equatorward Drift & Cycle Overlap Sunspots appear in two bands on either side of the equator. These bands drift toward the equator as the cycle progresses and cycles overlap by 2-3 years at minimum.

14. Active Latitude Position The equatorward drift rate is not constant. The drift rate slows as the active latitude approaches the equator. Bigger cycles have faster drift rates.

15. Active Latitude Width The width of the active latitude band increases from cycle minimum to cycle maximum and then decreases after the equatorward edge approaches the equator. Bigger cycles have wider active latitude zones.

16. It’s Magnetic

17. Hale’s Magnetic Polarity Law The magnetic polarity of the sunspots in active regions switches from one hemisphere to the other and from one cycle to the next.

18. Sunspot Group Tilt- Joy’s Law Sunspot groups are tilted with the leading spots closer to the equator than the following spots. This tilt increases with latitude.

19. Polar Field Reversals The magnetic polarities of the Sun’s poles reverse from one cycle to the next at about the time of sunspot cycle maximum.

20. Magnetic Cycle in Motion Notice the Differential Rotation – flow to the right (faster than average rotation) near the equator and flow to the left (slower than average rotation) near the poles – and Meridional Flow from the equator towards the poles.

21. The Surface Evidence The evolution of the Sun’s surface magnetic field pattern must be matched with any viable dynamo model.

22. What about the Interior? We will return to this later!

23. Solar Dynamos

24. Dynamo? • Cowling (1934) “Anti-Dynamo Theorem” – no dynamo action from axisymmetric flows. • Cowling (1945) Decay time for “fossil” field (1010 years) incompatible with 11-year cycle – must have dynamo activity. • Elsassar (1946) & Bullard (1949) – dynamo action from non-axisymmetric flows. • Parker (1955) Dual effects of differential rotation and cyclonic motions give oscillatory mean field with traveling dynamo waves.

25. Babcock (1961) a) Dipolar field at cycle minimum threads through a shallow layer below the surface. b) Differential rotation shears out this poloidal field to produce a strong toroidal field (first at the mid-latitudes then progressively lower latitudes). c) Buoyant fields erupt through the photosphere giving Hale’s polarity law and Joy’s Tilt. d) Meridional flow away from the mid-latitudes gives reconnection at the poles and equator.

26. Leighton (1969) • Supergranules (which were “discovered” by Leighton, Noyes, & Simon in 1962) provide diffusive transport of 104 km2/s in the photosphere (no meridional flow required). • A rotation rate increasing inward is required for equatorward drift of the active latitudes. • A numerical model for the depth and longitude averaged magnetic field can reproduce many observed aspects – provided several adjustable parameters are set.

27. αΩ Dynamos • Toroidal field, Bɸ, is generated from poloidal field by the Omega effect – shearing by differential rotation - Ω. • Poloidal field, Aɸ, is generated from toroidal field by the alpha effect – cyclonic flows – α~v·v. • A dynamo wave travels along isorotation surfaces with frequency ~ αΩ

28. 3D-MHD Dynamos • Gilman & Miller (1981) “Dynamically consistent nonlinear dynamos driven by convection in a rotation spherical shell” • Glatzmeier (1985) “Numerical simulations of stellar convective dynamos. II. Field propagation in the convection zone” • Oscillatory dynamos were produced • The dynamo waves moved poleward • The periods were too short by 10x

29. Dynamo Dilemma #1 Equatorward propagation could be achieved if the rotation rate increased inward across the convection zone. However, the α-effect in the bulk of the convection zone is orders of magnitude too large and gives short period (2-year instead of 22-year) dynamos no matter how the internal rotation rate varies. Rapid twisting motions produced very short solar cycles.

30. Dynamo Dilemma #2 The magnetic field produced in the convection zone should be buoyant and rise rapidly to the surface (Parker, 1975). Buoyant magnetic fields can’t stay down long enough to get amplified.

31. Dynamo Dilemma #3 Observed (Helioseismology) Hydrodynamic Model Kinematic Model The internal rotation profile determined with helioseismic methods shows shear layers at the top and bottom of the convection zone with nearly constant rotation rate in between – unlike the rotation profiles produced in the hydrodynamic models or assumed in the kinematic dynamo models. The Sun’s internal rotation didn’t match ANY of the models.

32. Interface Dynamos All three dilemmas could be circumvented if the dynamo action took place at the base of the convection zone. Parker (1975) made this suggestion early on to solve the magnetic buoyancy dilemma. DeLuca & Gilman (1986) produced dynamo models in which the overshooting convective motions act on the magnetic field and could produce longer period dynamo oscillations. The dynamics of flux tubes rising rapidly through a rotating convection zone dramatically reduces the twisting otherwise produced by the convection itself.

33. Interface Dynamo Waves The shear layer at the base of the convection zone has rotation rate decreasing inward at latitudes below about 30º and increasing inward at higher latitudes. This gives two dynamo waves – one moving toward the equator below 30º and another moving poleward at higher latitudes. Dynamo Dilemma # 4 – there isn’t any evidence for a poleward wave.

34. Flux Transport Dynamos Dikpati & Choudhuri (1994, 1995) proposed flux transport dynamos like those of Babcock and Leighton but with key differences. 1) The Ω-effect is in the shear layer at the base of the convection zone. 2) The α-effect is produced by the Coriolis force on rising flux tubes. 3) A poleward meridional flow at the surface facilitates polar field reversals. The return flow at the base of the convection zone gives the equatorward drift of the active latitudes and the 22-year period of the magnetic cycle. The deep Meridional Flow gives only equatorward motions and is slow enough to give 11-year sunspot cycle.

35. The Meridional Flow A poleward meridional flow of about 12 m/s is observed at the surface. This poleward flow is also observed below the surface with helioseismology.

36. The Deep Return Flow The poleward surface flow must sink inward in the polar regions and return to the equator at some depth. If the flow reverses at a point half way through the convection zone then a 12 m/s flow at the surface gives a 1 m/s flow at the base of the convection zone. Nandy & Choudhuri (2002) Braun & Fan (1998)

37. A Flux Transport Dynamo Dikpati & Charbonneau (1999) investigated the characteristics of these dynamos and their sensitivity to changes in flow parameters. The meridional flow speed controls both the dynamo period and its strength. In/CCW Out/CW

38. Dikpati & Charbonneau (1999) The Dikpati & Charbonneau (1999) model produced 20 year cycles with a latitude drift that slowed as it approached the equator and polar fields that reversed at cycle maximum. The cycle period was inversely proportional to the meridional flow speed. Fast meridional flow gives strong polar fields. The model has a “memory” of about 2 sunspot cycles.

39. Corroborating Evidence Hathaway, Nandy, Wilson, & Reichmann (2003, 2004) studied the equatorward drift of the active latitudes from 128 years of sunspot data. 1) There was no evidence of a poleward moving dynamo wave. 2) The drift rate slows as the activity band approaches the equator. 3) There is a positive correlation between the speed of the drift in cycle N and the amplitude of the N+2 cycle.

40. Dikpati Dynamo Prediction Cycle 24 Prediction ~ 180 ± 15 (with slow flow: late and ~ 165 ± 15 Dikpati, de Toma & Gilman (2006) fed sunspot areas and positions into their numerical model for the Sun’s dynamo and reproduced the amplitudes of the last eight cycles with unprecedented accuracy (RMS error < 10).

41. Meridional Flow Variations The strength of the meridional flow shows variability. The delayed start of Cycle 24 in the Dikpati prediction was based on the slowdown reported by Basu & Antia (2003). In Dikpati’s model this gives a long cycle and weak polar fields at the end of Cycle 23. Hathaway (1996) Basu & Antia (2003)

42. Flow Speed and Polar Fields Flux Transport Dynamos produce weak polar fields and long cycles when the meridional flow is weak. From SURYA code of Choudhuri et al.

43. Cycle 24 Minimum We see weak polar fields (about half as strong as in previous cycles) and a long Cycle 23 (12 years instead of 11).

44. Meridional Flow Variability With Summer Intern Lisa Rightmire (Univ. Memphis), I have measured the meridional motion of the weak field magnetic elements by cross-correlation full disk magnetograms. The results for 1996-2002 are consistent with the strength and variations seen by Basu & Antia (2003) but they show a return to even faster flow from 2003-2009. Dynamo Dilemma # 5 – the faster flow after 2002 SHOULD have made stronger polar fields and a shorter cycle.

45. Previous Cycle Variability Komm,Howard, & Harvey (1993) made similar measurements from Kitt Peak daily magnetograms and found fast flow at minimum that slows down at maximum. The slow down in the meridional flow from 1996 to 2002 represented the normal solar cycle variation in the meridional flow and cannot explain the weak polar fields!

46. Cycle 23 Latitude Drift The equatorward drift of the active latitudes during cycle 23 looks very average – neither slow nor fast and its variations are not in sync with the surface flows.

47. Where is Cycle 24? Cycle 24 is now underway. Almost all the sunspots since September of 2008 have been Cycle 24 sunspots. The smoothed sunspot number went through a minimum in November 2008.

48. Where is Cycle 24 Going? With the exception of the Flux Transport Dynamo predictions, all indications are that Cycle 24 will be a weak sunspot cycle.

49. Conclusions • A magnetic dynamo is required to explain the 11-year solar activity cycle. • The basic processes of the Ω-effect, the α-effect, and transport by meridional flow and cellular convection must be at play. • Models relying on deep meridional flow seemed to be the most appropriate. • But … Measurements of meridional flow variations do not support these models. • Solar Cycle 24 has begun but is expected to be very weak.