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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