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Magnetic dynamo over different astrophysical scales

Magnetic dynamo over different astrophysical scales. diagnostic interest (CMB). primordial (decay). seed field. Axel Brandenburg & Fabio Del Sordo (Nordita) with contributions from many others. AGN outflows MRI driven SS dynamo. galactic LS dynamo. helicity losses.

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Magnetic dynamo over different astrophysical scales

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  1. Magnetic dynamo over different astrophysical scales diagnostic interest (CMB) primordial (decay) seed field Axel Brandenburg & Fabio Del Sordo (Nordita) with contributions from many others AGN outflows MRI driven SS dynamo galactic LS dynamo helicity losses

  2. Early-Universe B-field may have been helical: 3-D decay simulations helical vs nonhelical Initial slope E~k4 Christensson et al. (2001, PRE 64, 056405) cosmological scale: ~3 cm

  3. Proposed helical decay law M. Christensson, M. Hindmarsh, A. Brandenburg: 2005, AN 326, 393 H not exactly constant Assume power law, not const H follows power law iff r=1/2; then

  4. Many astrophysical sources of turbulence are potential Examples: EW phase transition bubbles, SN explosions No dynamo action in nearly potential flows (at least not so far) a and ht are also small

  5. Nearly potential flows neither wnor B is produced, unless … Mee & Brandenburg (2006, MNRAS 370, 415)

  6. Baroclinic and battery terms Analogy between w and B eqns  even with baroclinicity Kulsrud et al. (1996)  relevance to oblique cosmological shocks degree of ionisation

  7. Alternative: Magnetisation from quasars? Poynting flux 10,000 galaxies for 1 Gyr, 1044 erg/s each Similar figure also for outflows from protostellar disc B. von Rekowski, A. Brandenburg, W. Dobler, A. Shukurov, 2003 A&A 398, 825-844

  8. To maintain equipartition-strength fields need dynamos: small-scale vs large-scale energy injection scale Wavenumber =1/scale B-scale smaller than U-scale B-scale larger than U-scale

  9. Small-scale vs large-scale dynamo

  10. Growth rate, applied to galaxy urms=10 km/s kf=2p/70 pc=0.1 pc-1 urmskf=10-6 s-1 Re=109 l=3x10-2 yr-1 (very fast) l=2x10-9 yr-1 Amplication factors: exp(lt)=104 for 5 Gr

  11. Kick-start the GBF with strong B Forced turbulence with shear and strong initial field at k=6

  12. Weaker initial field Kazantsev slope, exponential growth

  13. Reach full saturation: need helicity fluxes evidence from different simulations 3-D simulations, no mean-field modeling Forced turbulence in domain with solar-like shear Brandenburg (2005, ApJ 625, 539) Convective dynamo in a box with shear and rotation Käpylä, Korpi, Brandenburg (2008, A&A 491, 353) Only weak field if boxis closed

  14. Saturation phase: here for convection Käpylä et al (2008, A&A 491, 353) with rotation without rotation

  15. It can take some time… Rm=121, By, 5123 LS dynamo not always excited

  16. Conclusion factor 10,000 70 pc 10 pc slope 3/2 factor 500

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