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Magnetic field excitation in galaxies
Magnetic field excitation in galaxies
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OutlineOutline
The problem of large scale fields Large scale flows Meanfield dynamos (alpha effect due to …) The role of cosmic rays MRI
The problem of large scale fields Large scale flows Meanfield dynamos (alpha effect due to …) The role of cosmic rays MRI
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ObservationsObservationsLarge scale magnetic fieldsLarge scale magnetic fields
Axisymmetric mode dominatesAxisymmetric mode dominates
Large pitch angleLarge pitch angle
Strongly (anti) correlated with optical armsStrongly (anti) correlated with optical arms
Strength is compatible to the small scale componentStrength is compatible to the small scale component
Seldomly reversalsSeldomly reversals
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The problemThe problem A lot of processes inject small scale fields in the A lot of processes inject small scale fields in the ISM ISM
stellar dynamos combined with windsstellar dynamos combined with winds shock-turbulenceshock-turbulence plasma instabilitiesplasma instabilities small scale dynamossmall scale dynamos
A lot of processes inject small scale fields in the A lot of processes inject small scale fields in the ISM ISM
stellar dynamos combined with windsstellar dynamos combined with winds shock-turbulenceshock-turbulence plasma instabilitiesplasma instabilities small scale dynamossmall scale dynamos
No problem to explain the No problem to explain the magnetic field energy in galaxiesmagnetic field energy in galaxies
But which processes bring But which processes bring coherent magnetic fields on a coherent magnetic fields on a scale of several kpc?scale of several kpc?
No problem to explain the No problem to explain the magnetic field energy in galaxiesmagnetic field energy in galaxies
But which processes bring But which processes bring coherent magnetic fields on a coherent magnetic fields on a scale of several kpc?scale of several kpc?
Balsara et al. 2004Balsara et al. 2004
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How to do ?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
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
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Large scale flowLarge scale flow
Rotation without diffusivity (ideal MHD) (50 rotations) In a Hubble time amplification by a factor of
10 Strong seed field is neccassary (10-7G) on
large scale Pitch angle would be less than 1o
A lot of radial reversals Adding turbulent diffusion one needs a source
for Br
Rotation without diffusivity (ideal MHD) (50 rotations) In a Hubble time amplification by a factor of
10 Strong seed field is neccassary (10-7G) on
large scale Pitch angle would be less than 1o
A lot of radial reversals Adding turbulent diffusion one needs a source
for Br
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Large scale flowLarge scale flow Br from nonuniform radial motions in spirals
reversals or positive pitch angles
Br from nonuniform radial motions in spirals
reversals or positive pitch angles
Otmianowska-Mazur & Chiba 1995Otmianowska-Mazur & Chiba 1995
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Inverse cascadeInverse 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
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-effectMean field language: Alpha-effect
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BBBu
Et
B
curl T
curl
curl
Parker ´55
Steenbeck, Krause &
Rädler ´66
BC: curl B = 0 outside
Dynamo TheoryDynamo Theory
div B = 0
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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
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 fieldstationary axisymmetric quadrupolar field field strength several field strength several G G pitch angle about 20pitch angle about 20oo
maximal field strength at maximal maximal field strength at maximal
stationary axisymmetric quadrupolar fieldstationary axisymmetric quadrupolar field field strength several field strength several G G pitch angle about 20pitch angle about 20oo
maximal field strength at maximal maximal field strength at maximal
Dynamo TheoryDynamo Theory
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Mean field dynamoMean field dynamo
Dynamo numbers:Dynamo numbers:
C= H -
1
C= H -
1
C= H2 -
1
C= H2 -
1
Pitch angle:Pitch angle: tan p =
CC
tan p =
CC
Independent of Independent of
But with quenchingBut with quenching tan p = C
critC= 0.4
with km kpc s-1
p=20o
tan p = Ccrit
C= 0.4
with km kpc s-1
p=20o
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Single supernova explosions Ferrière, 1998
Single supernova explosions Ferrière, 1998 Pitch angle : 0.1o - 1oPitch angle : 0.1o - 1o
Turbulence from SNTurbulence from SN
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Super Bubbles Ferrière, 1998
Super Bubbles Ferrière, 1998 Pitch angle : 1o - 30oPitch angle : 1o - 30o
Turbulence from SNTurbulence from SN
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Mean flowMean flow
Rotation law:Rotation law: = (1+(r/rn)-
1/n
= (1+(r/rn)-
1/n
(n=2)(n=2)
r=2r=2 r=5r=5
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Density wave flowsDensity wave flows
Model: Brandt law of rotation, rand C=41, C=4.7Model: Brandt law of rotation, rand C=41, C=4.7
ur=u0cos(mp(pt) + K r/Rmax) f(r)ur=u0cos(mp(pt) + K r/Rmax) f(r)
u=u0sin(mp(pt) + K r/Rmax) f(r)u=u0sin(mp(pt) + K r/Rmax) f(r)
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Density wave flowsDensity wave flows
2D stationary density wave flows disturb the dynamo2D stationary density wave flows disturb the dynamo
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Pitch angle is maximal at field minimumPitch angle is maximal at field minimum
Density wave flowsDensity wave flows
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MHD instabilitiesMHD instabilitiesSupernova explosions provide enough energy for the turbulence in the ISM
Supernova explosions provide enough energy for the turbulence in the ISM
Magnetic field instabilities contribute to the turbulenceMagnetic field instabilities contribute to the turbulence
MRI in galaxiesMRI in galaxies
Parker instability together with cosmic raysParker instability together with cosmic rays
Observational hint: Turbulence in low star forming regionsObservational hint: Turbulence in low star forming regions
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Parker instability Parker instability Hanasz et al. 2002Hanasz et al. 2002
Parker instability Parker instability
largescale Br largescale Br
Rotation Rotation coalesence coalesence
largescale B largescale B
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Cosmic rays and Parker instabilityCosmic rays and Parker instability
Hanasz et al. 2004Hanasz et al. 2004
SN-explosions with cosmic raysSN-explosions with cosmic rays
Fast diffusion of cosmic raysFast diffusion of cosmic rays
+ Parker instability+ Parker instability
Fast expansion of pressure disturbancesFast expansion of pressure disturbances
More effective creation of BrMore effective creation of Br
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Cosmic rays and Parker instabilityCosmic rays and Parker instability
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Magneto rotational instability (MRI)Magneto rotational instability (MRI)
Dziourkevitch 2004Dziourkevitch 2004
Weak magnetic fieldWeak magnetic field
Va < H Va < H
is unstableis unstable
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MRIMRI
Dziourkevitch 2004Dziourkevitch 2004
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MRIMRI Dziourkevitch 2004Dziourkevitch 2004
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MRIMRI Dziourkevitch 2004Dziourkevitch 2004
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MRIMRI Dziourkevitch 2004Dziourkevitch 2004
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MRIMRI
Dziourkevitch 2004Dziourkevitch 2004
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Open problemsOpen problems
Turbulence in the ISM – simulations in the boxTurbulence in the ISM – simulations in the box
Simulation with SuperbubblesSimulation with Superbubbles
Role of magnetic instabilitiesRole of magnetic instabilities
Multicomponent gas - role of hot phaseMulticomponent gas - role of hot phase
External temporal gravitational disturbancesExternal temporal gravitational disturbances
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Open problemsOpen problems
Up to which scale are magnetic fields dynamical importantUp to which scale are magnetic fields dynamical important
What is the influence of a more complicated mean flow in a high Reynolds number regime
What is the influence of a more complicated mean flow in a high Reynolds number regime
What determines the correlation between optical arms and magnetic arms
What determines the correlation between optical arms and magnetic arms
How important is the galaxy evolution for magnetic field propertiesHow important is the galaxy evolution for magnetic field properties
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Questions to ObserversQuestions to Observers
Can we see coherent structures on the large scale during galaxy formation process
Can we see coherent structures on the large scale during galaxy formation process
Detailed observations of smaller scales 3D field topology in SNR
Detailed observations of smaller scales 3D field topology in SNR
Correlation to rotation curves and spiral flowsCorrelation to rotation curves and spiral flows
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Thank youThank you
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