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CORONAL HEATING

CORONAL HEATING. (Space Climate School, Saariselka, March, 2009) Eric Priest (St Andrews). 1. Introduction - The Corona (Eclipse). Skylab -- X-ray telescope. Coronal holes -- loops -- X-ray bright points. Yohkoh (5 arcsec). A dynamic magnetic world - subtle interactions B & plasma.

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CORONAL HEATING

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  1. CORONAL HEATING (Space Climate School, Saariselka, March, 2009)Eric Priest (St Andrews)

  2. 1. Introduction- The Corona (Eclipse)

  3. Skylab -- X-ray telescope Coronal holes -- loops -- X-ray bright points

  4. Yohkoh(5 arcsec) A dynamic magnetic world - subtle interactions B & plasma

  5. Hinode (1 arcsec) Stunning detail on structure & dynamics (see Tsuneta) How is corona heated?

  6. Waves or reconnection? - Space Obsns: • Low-freq. wavesin loops [TRACE]-too weak to heat • High-freq. waves [UVCS]-- ?? heat outer corona • Hinode -- Chromospheric Spicules swaying (straw, prairy) Hansteen, Suematsu --?? Solar wind/ coronal heating

  7. 2. Reconnection - most likely in low corona Quiet Sun: [XRT on Hinode, Tsuneta, golub] Many brightenings X-ray bright points - above emerging and/or cancelling fields in photosphere [30-sec cadence, 12-hour duration]

  8. Hinode XRT - active region (Schmeltz et al, 2009) Observations inside white region Differential emission measure Normal active region emission at 3 MK Plus peak at 20 MK (?nanoflares)

  9. Parker’s classical Nanoflare Model by braiding (1972) Initial B uniform / motions braiding

  10. Numerical Experiment(Galsgaard) Braiding --> Current sheets grow --> turb. recon.

  11. 3. Coronal Tectonics Model (development of Parker’s model) 3.1 Effect “Magnetic Carpet” Magnetic sources in surface are concentrated

  12. Flux Sources Highly Dynamic Magnetogram movie (white +ve , black -ve) • Flux emerges ... cancels • Reprocessed very quickly (14 hrs !!!)

  13. Many Sources--> Corona has Complex Topology In 2D -- Separatrix curves In 3D -- Separatrix surfaces

  14. In 2D,reconnection at X In 3D,reconnection atseparator In complex fields we form the SKELETON-- set separatrices

  15. 3.3 “Simple” binary interaction of 2 photospheric sources (Haynes et al) - and + sources in overlying B. • Separatrix surfaces. Move sources & watch Interaction • flux tube joining sources Separator

  16. 2 separators 5separators Cross-sections of Separatrix Surfaces Separatrix surfaces(positive, negative) & Separators ( ) Number of separators: X

  17. Life of Magnetic Flux in Surface • (a) 50%? flux in Quiet Sun emerges as ephemeral regions [1 per 8 hrs per supergran, 3 x 1019 Mx] • (b) Each pole migrates to boundary (4 hours), fragments --> 10 "network elements" (3x1018 Mx) • (c) -- move along boundary (0.1 km/s) -- cancel

  18. From observed magnetograms - construct coronal field lines • each source • connects to 8 others Time for all field lines to reconnect only 1.5 hours (Close et al) • much more tectonics heating low down where field is more complex than higher up

  19. Coronal Tectonics Model (updated version of Parker nanoflare/topological dissipation) • (Priest, Heyvaerts & Title) • Each "Loop" --> surface in many sources • Flux from each source separated by separatrix surfaces • As sources move --> J sheets on separatrices & separators --> Reconnect --> Heat • Corona filled w. myriads of J sheets, heatingimpulsively

  20. Fundamental Flux Units not Network Elements • Intense tubes (B -- 1200 G, 100 km, 3 x 1017 Mx) • Each network element -- 10 intense tubes • Single ephemeral region (XBP) -- 100 sources 800 seprs, 1600 sepces • Each TRACE • Loop -- 10 finer loops 80 seprs, 160 sepces

  21. TRACE Loop Reaches to surface in many footpoints. Separatrices form web in corona

  22. Corona - Myriads Different Loops Each flux element --> many neighbours But in practice each source has 8 connections

  23. Results • Heating uniform along separatrix • Elementary (sub-telc) tube heated uniformly • But 95% photc. flux closes low down in carpet • -- remaining 5% forms large-scale connections • --> Carpet heated more than large-scale corona • So unresolved observations of coronal loops • --> Enhanced heat near feet in carpet • --> Upper parts large-scale loops heated uniformly & less strongly

  24. 4. If reconnection heats coronaat many sheets, 1. How does energy spread out ? -- conduction along B -- reconnection jets -- waves across B 2.If reconnection time-dependent, how much energy liberated locally/globally? Simple model problem [Longcope & Priest]

  25. Magnetic field of Current Sheet in X At large r,B = B0 + B1 (line current), Lots of energy far from CS

  26. Suppose sheet reconnects Current (I) dissipates Local process but has global consequences: Decrease I --> B must change at large distances How ??

  27. Model for effect of reconnection Linearize about X-point B0 : Assume B1 @ t=0 is due to current sheet current diffuses i.e. reconnection  is “turned on”

  28. (r) I Expect: I0 r r D Combine equations: Put = twice current enclosed in r wave diffusion

  29. I0 R I0 r wave diffusion (i)Large r (wave) limit:when >> I(R,t)=I0-F(t-R) (ii)Small r (diffusive) limit: NB --> 0 at origin as t increases

  30. Numerical Solution I(r) Wave solution R Transition: diffusive to wave solution Diffusive solution t

  31. Sheath of Current propagates out In wake of sheath a flow, assocd with But flow near X does not disappear -- it slowly increases ! EV increasing t

  32. At large t Resolving the Paradox - 3rd regime = Advection diffusion Peak in j remains at X and produces a steady E (indep of )

  33. 5. Summary Coronal tectonics -- updated version of Parker braiding Response to enhanced  in current sheet (CS) during coronal tectonics: (i) Diffusion spreads CS out (ii) Wave carries current out at vA - as sheath (iii) Peak in j at X remains --> steady E independent of i.e. fast • Most magnetic energy is converted into • kinetic energy in wave -- • may later dissipate. • Coronal heating -- reconnection + wave

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