The magnetism of the very quiet Sun

1. The magnetism of the very quiet Sun J. Sánchez Almeida Instituto de Astrofísica de Canarias, Spain

2. Summary • What is quiet Sun? Why is it important? • Main observational properties • Surface coverage • Degree of tangling • Magnetic field strengths • Magnetic flux and energy • Variations with the solar cycle • Origin of the QS magnetism (hints from theory) • Influence on the corona • Connection with Solar-B • Conclusions

3. What is quiet Sun? Inter-Network Network

4. angular resolution mag. @ 0.5” • sensitivity @ 20 G • VTT (obs. Teide), speckle reconstructed • Unsigned flux density  20 G Inter-Network Quiet Sun 1”x1” Domínguez Cerdeña et al. (03))

5. Why is the quiet Sun important? • Most of the (unsigned) magnetic flux and energy existing on the solar surface at any given time is in the quiet Sun. • It may play a significant role in all the physical processes pertaining to the global solar magnetic properties (dynamo, coronal heating, sources of the solar wind,…). • This role has been neglected so far. • Easy target for the new spectro-polarimeters.

6. Main observational properties Surface Coverage Between 90% at solar maximum and 99% at solar minimum (e.g., Harvery 1994)

7. observer Degree of tangling The magnetic structures of the quiet Sun are not spatially resolved. Actually, the magnetic field vector varies within scales smaller than the smallest that we can resolve Resolution Element magnetic field vector 100 km 350 km º 0.5” line of sight

8. IR spectral lines Visible spectral lines SA et al. 2003b

9. Stokes Profiles

10. Usual hypothesis Stokes V profiles observed in the Quiet Sun (SA & Lites, 2000)

11. V2 = -V1 V1+V2 = Vobs = 0  Q2 = -Q1 Q1+Q2 = Qobs = 0 polarization signals in complex (tangled) magnetic fields cancel out

12. The symbols correspond to observations of IN Quiet Sun Magnetic fields SA et al. (2003)

13. In short, due to the complex topology of the QS magnetic fields, All measurements are bound to underestimate the magnetic flux content of the QS fields . The measure of the magnetic field properties is a non-trivial issue. It involves a big deal of modeling and assumptions on the underlying atmosphere: Inversion Codes.

14. Magnetic Field Strengths Sunspots: Plage & Network regions: Inter-Network quiet Sun:

15. SA et al. 2003b

16. PDF: probability of finding a given field strength (per unit field strength). SA et al. 2003b; Socas Navarro & SA 2003 from Hanle effect from Zeeman effect

18. Most of the quiet Sun (IN) surface is covered by weak magnetic fields. more than 95% of the surface has • Magnetic fluxdensity in the form of weak and strong magnetic field strengths. Flux (B < 100 G ; Hanle signals) = 50 G = 60 % of the flux Flux (B < 500 G) = 55 G = 75 % of the flux Flux (B > 500 G) = 20 G = 25 % of the flux 75 G < B < 120 G All thepresent observationsshowing quiet Sun magnetic fieldsunderestimatethe existingfluxdensity: 75 G < B < 120 G

19. Quiet Sun (Hanle) Quiet Sun (visible + IR) AR+N data from Schrijver & Harvey, 94, SPh, 150, 1 AR + Network Network

20. Energy density in the form of weak and strong magnetic field strengths. Energy (B < 100 G ; Hanle signals) = 19 % of the mag. energy Energy (B < 500 G) = 24 % of the mag. energy Energy (B > 500 G) = 76 % of the mag. energy

21. Variation along the solar cycle Unknown, but it is one of the clear observational targets. To be achieved thanks to the new synoptic magnetograms (e.g., SOLIS www.nso.noao.edu/solis/ ). Claims on the the variation: • Variation of some 100% (Faurobert et al. 2001). Refers to the weakest fields, deduced from Hanle signals. • Noflux densityvariation along the cycle within 40% (SA 2003c). Refers to the tail of kG of the PDF • No variation of hanle signals (Trujillo-Bueno & Shukina 2003). If existing, the variations are negligible as compared to the variation observed in active regions.

22. Origin of the quiet Sun magnetism (hints from theory) Options for the origin of the quiet Sunmagneticstructures • Debris from active regions produced by the global solar dynamo • Turbulent local dynamodriven by granulation. (Petrovay & Szakaly 1993, Cattaneo 1999,…) • Turbulent global dynamo (Stein & Nordlund 2001, Schussler et al. 2003, etc.)

23. Debris from Active Regions: Unlikely ARs emerge (and so decay) at a rate of 6 x 1021 Mx day-1 IN magnetic flux > 1.2 x 1024 Mx then IN cannot decay in less than 200 days, (200 days = 1.2 x 1024 Mx / 6 x 1021 Mx day-1) otherwise they would be gone before fresh AR flux replenishes it. 200 days is too long since the IN fields vary in timescales of min … Lifetimes found in the literature are hours.

24. Turbulent local dynamo Bz Temperature 1” Cattaneo & Emonet, 2001

25. SA et al. (2003)

26. Turbulent global dynamo Unclear how to distinguish this mechanism from the local dynamo. • complex topology • no variation along the cycle • tight coupling with granular motions Stein & Nordlund 2001

28. Influence on the corona Traditionally, the influence of the IN fields on the coronal magnetic field is neglected. Argument: the magnetic field is so complex that most of the field lines close in very low loops and never reach the corona. However: • The base of the corona is very low: 2500 km (VALC) • Cancellation is often non-local (e.g., Schrijver & Title, 2002) so a fraction actually makes it to the corona. • The IN flux is so large that a small fraction makes a significant absolute contribution

29. force free extrapolations 1000 km < height < 2000 km loop height < 500 km Hoffman et al. 2003

30. prominences magnetic energy equals thermal energy density Hoffman et al. 2003

31. The topology of the network fields reaching the corona is significantly modified (in a non-trivial way) by the presence of IN fields. Schrijver & Title (2003)

32. Connections with Solar-B? IN fields would be routinely detected with normal mapping of the spectropolarimeter of SOT. (however, only the tail of kG IN fields) 0.3”  Solar B resolution

33. Studying the IN magnetism is appealing, since it allows to address basic problems of solar physics (solar dynamo, coronal structure and heating, sources of the solar wind, magnetic decay, …) Because of the complications of the magnetic field, non-trivial magnetic field measurementsare needed. Stokes profiles are needed for this task (provided complex magnetic fields are allowed for by the inversion techniques).

34. Conclusions • The quiet Sun isa component of the solar magnetism whose role has been neglected so far, but whose true role is not understood yet. • Quantitatively importantin terms of the global magnetic properties (e.g., carries more unsigned flux than all active regions at the solar max.) It occupies most of the solar surface. • Complex magnetic topology. The diagnostic of their properties based on the observed polarization is a non-trivial issue. Stokes profiles + inversion techniques needed! It is not possible to describe the magnetic field of a IN pixel with a single magnetic field vector.

35. Distribution ofmagneticfield strengths: from 0 to 1.5 kG. • (?) Magnetic Flux andmagneticenergy dominated by the tail of kG fields. • (?) fairlyeasy to detect with high angular resolution(easy to achieve by the new generation of ground based 1m-class + Adaptive Optics or space-borne magnetometers, line SOLAR-B.) • (??) No strong variation along the cycle. Refers to the tail of kG fields detected in visible magnetograms.

36. (??) Does it reach coronal heights? Probably it does in the quiet Corona • (??) Coronal heating due to nano-flares? • (??)Role within the global solar dynamo responsible for the 22 years solar cycle: passive, leading role … • (??) Responsible for thechromosphericbasal flux. • (??)Relationship with the origin and acceleration of the solar wind?