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Magnetic field excitation in galaxies

Magnetic field excitation in galaxies. Outline. The problem of large scale fields Large scale flows Meanfield dynamos (alpha effect due to …) The role of cosmic rays MRI. Observations. Large scale magnetic fields. Strength is c ompatible to the small scale component.

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Magnetic field excitation in galaxies

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  1. Magnetic field excitation in galaxies

  2. Outline • The problem of large scale fields • Large scale flows • Meanfield dynamos (alpha effect due to …) • The role of cosmic rays • MRI

  3. Observations Large scale magnetic fields Strength is compatible to the small scale component Axisymmetric mode dominates Seldomly reversals Large pitch angle Strongly (anti) correlated with optical arms

  4. The problem • A lot of processes inject small scale fields in the ISM • stellar dynamos combined with winds • shock-turbulence • plasma instabilities • small scale dynamos • No problem to explain the magnetic field energy in galaxies • But which processes bring coherent magnetic fields on a scale of several kpc? Balsara et al. 2004

  5. How to do ? • large scale flows amplifying a primordial field • inverse turbulent cascade • formation of stronger fields during galaxy mergers and a more rapid decay of small scale structures

  6. Large scale flow Rotation without diffusivity (ideal MHD) (50 rotations) • In a Hubble time amplification by a factor of 10 • Strong seed field is neccassary (10-7G) on large scale • Pitch angle would be less than 1o • A lot of radial reversals • Adding turbulent diffusion one needs a source for Br

  7. Large scale flow • Brfrom nonuniform radial motions in spirals • reversals or positive pitch angles Otmianowska-Mazur & Chiba 1995

  8. Inverse cascade • inverse cascade in the turbulence • concept of the turbulent dynamo • still no simulations for a global galaxy possible • Questions about early saturation (catastrophic quenching) • turbulence in the multiphase ISM Mean field language: Alpha-effect

  9. Dynamo Theory Parker ´55 Steenbeck, Krause& Rädler ´66 div B = 0 BC: curl B = 0 outside

  10. Dynamo Theory • Differential rotation max = 50 Gy-1 • turbulence defined by u´ and  • u´= 10 km s-1 and  = 0.01 Gy •  = 1 kpc2 Gy-1 and  = 10 km s-1 • induction equation with -quenching • stationary axisymmetric quadrupolar field • field strength several G • pitch angle about 20o • maximal field strength at maximal 

  11. Mean field dynamo Dynamo numbers: Ca=a H h-1 CW=W0 H2h-1 tan p = ÖCa / CW Pitch angle: Independent of h tan p = ÖCacrit/ CW = 0.4 h/h0 with h0 = 1 km kpc s-1 p=20o But with quenching

  12. Turbulence from SN Single supernova explosions Ferrière, 1998 Pitch angle : 0.1o - 1o

  13. Turbulence from SN Super Bubbles Ferrière, 1998 Pitch angle : 1o - 30o

  14. Mean flow W=W0 (1+(r/rW)n)-1/n Rotation law: (n=2) rW=2 rW=5

  15. Density wave flows Model: Brandt law of rotation, rW=3 and CW=41, Ca=4.7 ur=cu0cos(mp(f - Wpt) + K r/Rmax) f(r) uf=u0sin(mp(f - Wpt) + K r/Rmax) f(r)

  16. Density wave flows 2D stationary density wave flows disturb the dynamo

  17. Density wave flows Pitch angle is maximal at field minimum

  18. MHD instabilities Supernova explosions provide enough energy for the turbulence in the ISM Magnetic field instabilities contribute to the turbulence Parker instability together with cosmic rays MRI in galaxies Observational hint: Turbulence in low star forming regions

  19. Parker instability Hanasz et al. 2002 Parker instability Rotation coalesence largescale B largescale Br

  20. Cosmic rays and Parker instability SN-explosions with cosmic rays Fast diffusion of cosmic rays + Parker instability Fast expansion of pressure disturbances More effective creation of Br Hanasz et al. 2004

  21. Cosmic rays and Parker instability

  22. Magneto rotational instability (MRI) Weak magnetic field Va <  H is unstable Dziourkevitch 2004

  23. MRI Dziourkevitch 2004

  24. MRI Dziourkevitch 2004

  25. MRI Dziourkevitch 2004

  26. MRI Dziourkevitch 2004

  27. MRI Dziourkevitch 2004

  28. Open problems Turbulence in the ISM – simulations in the box Simulation with Superbubbles Multicomponent gas - role of hot phase Role of magnetic instabilities External temporal gravitational disturbances

  29. Open problems Up to which scale are magnetic fields dynamical important What is the influence of a more complicated mean flow in a high Reynolds number regime How important is the galaxy evolution for magnetic field properties What determines the correlation between optical arms and magnetic arms

  30. Questions to Observers Detailed observations of smaller scales 3D field topology in SNR Correlation to rotation curves and spiral flows Can we see coherent structures on the large scale during galaxy formation process

  31. Thank you

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