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Stochastic Reconnection in Partially Ionized Gas:Progress Report

Stochastic Reconnection in Partially Ionized Gas:Progress Report. A. Lazarian (UW-Madison) Collaboration with J. Cho (UW-Madison and CITA) A.Esquivel (UW-Madison) H. Yan (UW-Madison) E. Vishniac (Johns Hopkins). Questions to address?. How does turbulence affect reconnection?

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Stochastic Reconnection in Partially Ionized Gas:Progress Report

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  1. Stochastic Reconnection in Partially Ionized Gas:Progress Report A. Lazarian (UW-Madison) Collaboration with J. Cho (UW-Madison and CITA) A.Esquivel (UW-Madison) H. Yan (UW-Madison) E. Vishniac (Johns Hopkins)

  2. Questions to address? • How does turbulence affect reconnection? • What are the properties of turbulence in partially ionized gas? • How does partial ionization change the expected reconnection rate?

  3. Motivation: Interstellar Fields • Turbulent: Re ~VL/n ~1010 >> 1 n ~ rLvth, vth < V, rL<< L

  4. Armstrong & Spangler (1995) Lazarian& Pogosyan (00) & Starnimirovic & Lazarian (01) showed Kolmogorov velocity spectrum of HI here. Slope ~ -5/3 Electron density spectrum pc AU

  5. What is the effect of Interstellar Tubrulence? • Makes boundary conditions difficult to control. • X point reconnection is not feasible unless large scale field reconfigure themselves over hundreds of parsec scales. • Fast local (e.g. X point) reconnection does not guarantee fast reconnection if the global outflow regions are narrow.

  6. Relation to Center Activities • Properties of turbulence: related to “Magnetic Chaos and Transport” • Reconnection and properties of turbulence are related to “Ion Heating”. • Reconnection is an essential part of the picture of “Dynamo” and “Angular Momentum Transport”

  7. Stochastic reconnection: The natural state of fluids is turbulence. Presence of an stochastic component of the B field. Magnetic field lines dissipate not on their entire scale length (L), but on a smaller scale (||) determined by turbulence statistics. Many simultaneous S-P reconnections. What is Stochastic Reconnection? Lazarian & Vishniac (1999)

  8. Properties of Stochastic Reconnection • Can be both fast and slow (depending on the level of turbulence) (dB!) • Allows flares of reconnection. • Depends of the properties of turbulence

  9. Partially ionzed gas:possible effects • Free diffusion of neutrals out of the current sheet. Probably not so important (Vishniac & Lazarian 1998, Heitsch & Zweibel 2003). • Turbulence is affected by damping caused by neutrals. Is it fatal?

  10. Turbulence in partially ionized gas:Theoretical expectations(from Lazarian, Vishniac & Cho 2004) • In partially ionized gas MHD turbulence does not vanish at the viscous damping scale. • Magnetic intermittency increases with decrease of the scale. • Turbulence gets resurrected at ion decoupling scale.

  11. Turbulence in Partially Ionized Gas B Viscosity is important while resistivity is not. Viscous magnetized fluid Does viscous damping scale is the scale at which MHD turbulence ends? ~0.3pc in WNM

  12. Viscosity Damped Turbulence: New Regime of MHD Turbulence E(k)~k-1 intermittent Expected: k-1 for magnetic field k-4 for kinetic energy Scale dependent intermittency Numerical testing confirms that magnetic turbulence does not die!!! Cho, Lazarian & Vishniac 2002b

  13. Resurrection of MHD Turbulence! Viscosity damped turbulence protrudes up to the scales at which neutrals decouple from ions. After that the normal MHD turbulence in ionic fluid is restored. Lazarian, Vishniac & Cho (2003) Yet to be tested with two fluid code

  14. Results: Expected Reconnection Rates for Phases of ISM(from Lazarian, Vishniac & Cho 2004) • Molecular cloud: 0.1 VA (L30/l303/2) • Dark cloud: 0.1 VT MA1/2b5/4(L30/l301/4) • Cold Neutral Medium: 0.08 VTM2b-1/2(L30/l303/2)

  15. Some Astrophysical Implications • Removal of magnetic field during star formation • Solar flares and particle acceleration

  16. Numerical Testings • Numerical testing of the stochastic reconnection idea • Further testing of the divergence of the field lines in the new regime of turbulence. • Numerical testing of the resurrection of turbulence prediction (using two fluid code).

  17. Summary • Interstellar reconnection happens in turbulent medium and on very large scales. • Turbulence and external forcing makes large scale X point not probable. • Stochastic reconnection is fast, but it may also be slow. • The research requires interaction with other directions of the Center.

  18. Compressible MHD Turbulence: Stimulating Prior Work Higdon 1984 (anisotropy in compressible MHD turbulence) Goldreich & Shridhar 1995 (incompressible MHD theory, hints about compressibility) Lithwick & Goldreich 2001 (effects of compressibility) Choice is biased by author’s preferences. Longer list is in Cho, Lazarian & Vishniac 2003.

  19. Implication 1: CR transport Big difference!!! (Kolmogorov) Scattering efficiency Fast modes From Yan & Lazarian 2002 Alfven modes are inefficient. Fast modes are efficient in spite of damping

  20. What are the scattering rates for different ISM phases? Gyroresonance TTD From Yan & Lazarian 2004 Solid line is analytical results Symbols are numerical results gyroresonance is dominant; the scattering in partially ionized media is not important.

  21. Implication 2: Dust Dynamics • Gyroresonace with fast modes is most efficient • Grains get supersonic • Grains may get aligned Grain size From Yan & Lazarian 2003

  22. Implication 3: Decay of MHD turbulence • Fast decay of MHD turbulence reported earlier is not due to coupling of compressible and incopressible motions! Incompressible MHD turbulence decays fast. tcas~k-2/3 Cascade time follows Kolmogorov scaling Inportant for star formation. Cho, Lazarian, & Vishniac 2002a

  23. Normal MHD Turbulence Viscosity damped regime Large scales Magnetic structures perpendicular to mean B. Large Scales (Small k only) Intermittency is prominent for new regime at small scales. Small Scales (Large k only) Smaller and smaller structures forming at scales smaller than the damping scale. Intermittent structures From Cho, Lazarian & Vishniac 2003

  24. Scale Dependent Intermittency Ordinary MHD New regime Cho, Lazarian & Vishniac 2003 Viscosity damped turbulence exhibits scale-dependent intermittency! Corresponds to prediction in Lazarian, Vishniac & Cho 2003

  25. Compressible MHD: formation of density structure From Cho & Lazarian 2003 Viscosity damped MHD turbulence results in a shallow spectrum of density fluctuations. Could there be a relation to tiny scale structures observed in the ISM?

  26. E(k)~k-1 kE(k)~Bl Bl2 =const=>Bl2~const, or tdiss Why E(k)~k-1? • Magnetic fluctuations evolve due to shear at the damping scale. => Cascade of magnetic energy with the fixed rate: Expect to see a lot of magnetic structure below the viscous damping scale (e.g. below 0.3pc for WNM)

  27. Genus analysis (cont.) SMC • A shift from the mean can reveal “meatball” or “Swiss cheese” topology. • Genus curve of the HI in the SMC and from compressible MHD simulations. • The SMC show a evident “Swiss cheese” topology, the simulations are more or less symmetric. • Genus are a quantitative measure of the topology, allows to test simulations & observations. MHD

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