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Heating the Corona and Driving the Solar Wind

Heating the Corona and Driving the Solar Wind. A. A. van Ballegooijen Smithsonian Astrophysical Observatory Cambridge, MA. Coronal Heating. The corona has a multi-thermal structure:. 171 Å. 195 Å. 284 Å. TRACE 1998 May 19,20 (Brickhouse & Schmelz 2006). Coronal Heating.

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Heating the Corona and Driving the Solar Wind

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  1. Heating the Corona and Driving the Solar Wind A. A. van Ballegooijen Smithsonian Astrophysical Observatory Cambridge, MA HMI/AIA Science Team Mtg.

  2. Coronal Heating The corona has a multi-thermal structure: 171 Å 195 Å 284 Å TRACE 1998 May 19,20 (Brickhouse & Schmelz 2006) HMI/AIA Science Team Mtg.

  3. Coronal Heating Differential Emission Measure: Schmelz et al. (2001) Schmelz & Martens (2006) HMI/AIA Science Team Mtg.

  4. Coronal Heating Energy release occurs impulsively. There is a power-law distribution of flare energies: From: Aschwanden & Parnell (2002) HMI/AIA Science Team Mtg.

  5. Coronal Heating • AIA: • Wide temperature coverage allows to determine DEM(T). • Characterize spatial variability of emission as function of T. • Derive number of structures N(T) along LOS, compare with • prediction from current-heating model (e.g., Gudiksen et al.). • TRACE 284 observations suggest N >> 1 for T = 2 – 3 MK. • Measure filling factorsf(T) (requires density diagnostics). • Isolate individual nanoflares from background loops. • Study time evolution of events, especially the heating phase. • Statistics, e.g. frequency distributions of flare energies. HMI/AIA Science Team Mtg.

  6. Coronal Heating Loops are anchored in the photosphere. Source of energy for coronal heating lies in convection zone: HMI/AIA Science Team Mtg.

  7. Coronal Heating Magneto-convection creates “flux tubes” that fan out with height and merge in the chromosphere: From: Spruit (1983) HMI/AIA Science Team Mtg.

  8. Coronal Heating • Interaction of flux tubes with turbulent convection creates disturbances that propagate upward along field lines: • Periodic motions generate tube waves (e.g., kink modes) • that become MHD waves in the chromosphere/corona. • Random displacements of photospheric footpoints produce • field-aligned electric currents (quasi-static disturbances) • in coronal loops. • Dissipation of these disturbances in the chromosphere/corona generally involves the formation of small-scale structures. HMI/AIA Science Team Mtg.

  9. Wave Heating • Slow-mode waves: • Steepen into shocks and dissipate via compressive viscosity. • Important for chromospheric heating. • Strong coupling between longitudinal and transverse modes • at β = 1 surface (Bogdan et al 2003; Hasan et al 2005): HMI/AIA Science Team Mtg.

  10. Wave Heating Slow-mode shocks can form inside flux tubes even for small transverse motions (~1 km/s) at the base of the photosphere: HMI/AIA Science Team Mtg.

  11. Wave Heating • Alfven waves: • Flux tubes in intergranular lanes are shaken transversely to • generate kink-mode waves. • Above the height where flux tubes merge, kink waves are • transformed into Alfven waves: From: Cranmer & van B (2005) HMI/AIA Science Team Mtg.

  12. Wave Heating • Alfven waves: • Can undergo phase-mixing and resonant absorption due to • transverse variations in Alfven speed (e.g., Davila 1987) or • braided fields (Similon & Sudan 1989). • Alfven wave pressure is an important driver of solar wind • (e.g., Leer, Holzer & Fla 1982; Hu et al 2003). • Wave reflection produces inward propagating Alfven waves. • Nonlinear interactions between counter-propagating waves • produce turbulent cascade (Matthaeus et al 1999). HMI/AIA Science Team Mtg.

  13. Wave Heating Alfven-wave amplitudes for different outer-scale lengths Λ of the turbulence (Cranmer & van Ballegooijen 2005): HMI/AIA Science Team Mtg.

  14. Wave Heating • AIA: • Search for waves and oscillations in all AIA passbands. • High cadence allows study of high-frequency waves. • Search for Alfven waves: track transverse motion of features • in closed and open fields. • Study evolution of coronal structures on quiet Sun: • Does reconnection in “magnetic carpet” produce waves that • can drive the solar wind? HMI/AIA Science Team Mtg.

  15. Current Heating Field-aligned electric currents: Required current density in active-region loops, assuming classical resistivity: [erg s-1 cm-3] [esu] This would require very thin current sheets: ΔB = 100 G over a distance δ = 0.4 km. HMI/AIA Science Team Mtg.

  16. Current Heating Formation of current sheets in closed loops subject to random footpoint motions: a) Spontaneous formation of “tangential discontinuities” by twisting or braiding of discrete flux tubes (Parker 1972, 1983): HMI/AIA Science Team Mtg.

  17. Current Heating b) More gradual cascade of magnetic energy occurs when footpoint mappings are continuous functions of position (van Ballegooijen 1985, 1986; Craig & Sneyd 2005): HMI/AIA Science Team Mtg.

  18. Current Heating • Dissipation of field-aligned electric currents: • Energy is released via magnetic reconnection. • Reconnection occurs impulsively in nanoflares (Parker 1988) • perhaps via resistive instabilities (e.g., Galeev et al. 1981). • Strands undergo continual heating and cooling; • observed coronal loops have an unresolved multi-thermal • structure (Cargill & Klimchuk 1997, 2004). • Reconnection likely involves particle acceleration. • Thermalization of energetic particles may occur away from • reconnection site (e.g., at loop footpoints). HMI/AIA Science Team Mtg.

  19. Current Heating How much energy is available for heating? Poynting flux at coronal base (L = loop length): where q = 0.1 – 1.0 and Dcor is random-walk diffusion const. Flux tube spreading amplifies rotational motions: [erg s-1 cm-2] consistent with observed scaling (Schrijver et al. 2004). HMI/AIA Science Team Mtg.

  20. Current Heating Numerical simulations of current-sheet formation and heating: Mikic et al (1989) Hendrix & van Hoven (1996)  Galsgaard & Nordlund (1996) HMI/AIA Science Team Mtg.

  21. Current Heating Large-scale simulation of active region driven by convective motions (Gudiksen & Nordlund 2005): HMI/AIA Science Team Mtg.

  22. Current Heating Parallel electric currents, , at various heights (Gudiksen & Nordlund 2005): 0.0 Mm 3.0 Mm 5.6 Mm HMI/AIA Science Team Mtg.

  23. Current Heating • AIA: • Search for twisting and braiding of loops at all T. • Search for evidence of small-scale reconnection. • Relate the observed coronal structures to magnetic structure • predicted by extrapolation of photospheric field: • Does heating occur in sheets located at separatrix surfaces? • Determine how average heating depends on loop parameters • (B, L, …). • Determine how heating varies along loops. • Evidence for energetic particles? • Compare with theories coronal heating. HMI/AIA Science Team Mtg.

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