Galactic Dynamos and ISM Turbulence.

1
Galactic Dynamos and ISM Turbulence.

• Steve Cowley,UCLA/ Imperial College
• Alex Schekochihin, Imperial
• Jim McWilliams, UCLA
• Greg Hammett, Princeton
• Greg Howes, UCLA/Imperial
• Russell Kulsrud, Princeton
• Ben Chandran, Univ. Iowa
• Jason Maron, Iowa/Rochester
• Sam Taylor, Imperial

2
Outline

• Galactic Dynamos can they work? What can we do
  to improve our knowledge of this kind of dynamo?
  Experiments? Theory? Can we understand
  anything if we dont solve everything.
• ISM turbulence issues. Experiments? Theory.

3
Field in M51.
Polarization of 6cm emission. Indicates
direction of B field.
Large scale coherent Field.
Beck 2000
4
Theorists Milky Way
Vertical Field in Center B 1mG
Horizontal Field in Disc B 3?G
V
V
Scale Rotation time
Supernova explosion.
Scale Eddy turnover time
5
Observed Density Fluctuations -- Small Scale
Turbulence.

• A spectrum of density fluctuations is inferred
  from various radio scattering observations. It
  has the classic k-5/3 spectrum from above the
  mean free path to gyro-radius scales. This is
  more than a whole talk in itself.
• It is believed to be evidence of an MHD turbulent
  cascade. Goldreich, Shridhar, Maron, Vishniac,
  Lithwick, Lazarian,Boldyrev, . Etc.

6
Galactic Dynamo -- The Usual Mechanism - Almost
Parker 1956
Coriolis Twist
7
The Usual Mechanism - Almost
Parker 1956
Rotation Rate
w
Rotation Rate
8
Velocity Spectrum -- with no B field
Kolmogorov 1941.
2
V
14
l
5
10
m


k
?
Viscous scale
-
5
/
3
k
Viscous eddy Turnover.
Helical part of spectrum
Smaller eddies turnover faster.
k
1
1
1
L
l
l
G
0
?
9
The Large Prandtl Number Case Galaxies, Clusters
etc.

• Magnetic Prandtl number Pr ??? 1012 in
  the ISM. THIS IS VERY DIFFERENT FROM THE SUN AND
  EARTH WHERE Pr ltlt 1.
• On the turnover time of the viscous eddies the
  seed field grows. The field develops structure
  below the viscous scale down to the resistive
  scale l? Pr -1/2 l?

l
?
10
Stage 1. Small Scale Dynamo
æ
k
5
(ln
k
)
3
g
t
ö
3
/
2
2
ç
2
B
(
k
,
t
)

exp
-

2
V
è
ø
1
/
2
t
4
g
t
4
k
Resistive scale Is very small-
6
l

10
m
R
1
1
1
1
L
l
l
l
G
0
D
R
11
Large Pr Dynamo Numerical.
Schekochihin, SCC, McWilliams, Maron.
12
Energy spectra Simulation and Theory
13
Stage 2. The First Inverse Cascade?
Perhaps an approach to equipartition? I like
to think of untangling the small scales and
building the large scale field.
k
14
No Numerical evidence for equipartition. No
Goldrich-Sridhar.
Stirring scale still folding Field lines.
Folded field lines - small scale energy.
Alfven waves on folded field lines?
15
Folded Field saturated state.
16
Stage 3. The Inverse Cascade?
17
Helicity Will Save Us?
Coriolis Twist
18
Helicity Will Save Us?
19
Can the Helical Dynamo work when ltB2gt ltv2gt and
Helicity is small?

• One assumption is yes -- it is common to use the
  ?-? dynamo equations for the disk even though
  there is no kinematic regime for the helical
  galactic dynamo.
• One old assumption was that the homogeneous
  turbulence acts like a turbulent resistivity on
  ltBgt and ltB2gt. It doesnt of course at high Pr.
• Jason Maron and Eric Blackman looked at this in a
  homogeneous box and got growth -- the resolution
  was unavoidably low and Pr 1.
• This is a significant question. I expect the
  dynamo to be suppressed but ??

20
Better simulations?

• Need geometry -- Greg Howes is doing disc
  simulations. Buoyancy, shear. Galactic MRI?
• Extra physics Ambipolar drift, tensor viscosity.
  
• Inhomogeneous ISM, clouds etc.
• RESOLUTION.
• I would be progress just to get the homogeneous
  isotropic Pr1dynamo and steady state field
  sorted out. (the MHD Kolmogorov problem)

21
Sodium experiments?

• Prlt1 small scale dynamo may not exist -- see
  Schekochihin submitted to PRL (see XXX). For
  sodium Pr 10-4.
• Only the helical dynamo.
• Relevant for the sun and geodynamo.
• Could try to increase sodium viscosity to reduce
  Re. (temperature control fillings etc.)

22
Plasma Prgt1 Dynamo experiments?

• Is the RFP dynamo relevant -- perhaps it is for
  sustainment.
• It would be nice to have a flow driven plasma
  dynamo experiment.
• I dont have a clue how to do that.