Magnetic dynamo over different astrophysical scales

1
Magnetic dynamo over different astrophysical
scales
diagnostic interest (CMB)
primordial (decay)
seed field
AGN outflows MRI driven SS dynamo
galactic LS dynamo
helicity losses

• Axel Brandenburg Fabio Del Sordo (Nordita)
• with contributions from many others

2
Early-Universe B-field may have been helical 3-D
decay simulations
helical vs nonhelical
Initial slope Ek4
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 r1/2 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 w nor B is produced, unless
Mee Brandenburg (2006, MNRAS 370, 415)
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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 AA 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
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Growth rate, applied to galaxy
urms10 km/s kf2p/70 pc0.1 pc-1 urmskf10-6
s-1 Re109 l3x10-2 yr-1 (very fast)
l2x10-9 yr-1
Amplication factors exp(lt)104 for 5 Gr
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Kick-start the GBF with strong B
Forced turbulence with shear and strong initial
field at k6
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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, AA
491, 353)
Only weak field if box is closed
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Saturation phase here for convection
Käpylä et al (2008, AA 491, 353)
with rotation
without rotation
15
It can take some time
Rm121, By, 5123
LS dynamo not always excited
16
Conclusion
factor 10,000
70 pc
10 pc
slope 3/2
factor 500