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Origin & Evolution of Magnetic Fields
Ellen Zweibel
zweibel@astro.wisc.edu

Depar tments of Astronomy & Physics University of Wisconsin, Madison and Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas
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Importance of the Problem
A cosmological problem. Exotic origin in the early Universe, or standard plasma physics? Top-down or bottom-up process? Role in formation of the first galaxies and stars? Past and present sites of cosmic ray acceleration and propagation? Interplay with ISM studies Structure of galactic fields on large & small scales? How do fields affect turbulence? How is field transpor ted, what determines B - relation? What effects do fields have on star formation?
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The Plan of This Talk
Primordial fields Intergalactic & intracluster fields What must be explained about galactic fields Effects of fields on turbulence & vice versa The future

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Primordial Fields
Bounds on magnetic energy density from Big Bang nucleosynthesis Signatures in CMB polarization effects on oscillations heating by magnetic energy dissipation

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Intergalactic & Intracluster Fields
Only upper limits on a pervasive intergalactic field Evidence for turbulent amplification of cluster fields Interaction of magnetic fields & cosmic rays with AGN Are accretion shocks magnetized & how far back? How far back is Faraday rotation in damped Ly systems detectable?

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Galactic Fields I
Reasonable expectations: Azimuthally oriented on large scales Turbulent component in rough equipar tition with kinetic turbulence down to subviscous scales Strong swept up fields in shells Randomly directed ver tical fields along chimney walls & winds, if present Stronger fields in denser gas
Based on plain vanilla MHD, not dynamo theory

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Galactic Fields II
What do we see? Azimuthally oriented on large scales? Yes Large scale azimuthally directed field with some reversals Turbulent component in rough equipar tition with kinetic turbulence? Yes Detailed small scale structure unknown; B possibly > B . Strong swept up fields in shells? Yes Randomly directed ver tical fields along chimney walls & winds? Some Stronger fields in denser gas? Not in diffuse ISM, possibly in molecular gas.
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We Also Know...
Li, Be, B found in oldest halo stars cosmic ray spallation magnetic field in parent gas Due to infall, outflow(?), bir th & death of stars, ISM turns over on 109 yr timescales. Best evidence for dynamo as opposed to primordial origin.

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What Must be Explained
Directional coherence of in plane field over at least few kpc scales Is there a coherent ver tical field? Maintenance of field in steady state in rapidly turning over interstellar gas That a field was apparently in place at moderate redshift Light elements energy density Faraday rotation coherence

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Thumbnails of Theory I
Magnetogenesis by the Biermann Battery: fastest in l small objects - rlL cs t . Create 10-18 G fields in cosmological shocks & ionization fronts, stronger fields in accretion disks & stars. Basic ingredients of dynamo theory: Exponentially fast stretching of field by chaotic flow turbulence Large scale flow or other preferred direction to impose order Fast reconnection of small scale field & provide irreversibility

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Thumbnails II
Special features of galactic dynamos: Fast diffusion of field from many small sources Fast incorporation of magnetic field into unmagnetized or undermagnetized material Escape of field, with or without accompanying wind

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Dynamo Processes & ISM Studies
Mapping large scale component characterize field fluctuations: spectrum, isotropy ver tical structure, halo fields, fields in HVCs fieldstrength - density relation Is there a Galactic wind? Is the ISM chemically well mixed? Is there evidence for magnetic reconnection?

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Example: Shear Layers

The magnetized Kelvin-Helmholtz instability generates rich magnetic structure which depends on resistivity; the left panel is 10в more resistive than the right panel & 1012 в more resistive than the ISM (Palotti et al. submitted to ApJ).
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Velocity Diffusion

Small scale turbulence causes a shear layer to broaden and relax, but in a resistivity dependent way (Palotti et al. submitted to ApJ).
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Magnetized Turbulence I
Is there a direct cascade? Energy injection at large scales, nonlinear transfer to intermediate scales, dissipation at smallest scales? Energy sources: supernovae, instabilities, infall Dissipation sources: viscosity, ion-neutral friction, radiative losses, resistivity, collisionless processes.

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Magnetized Turbulence II
Is there a direct cascade in molecular clouds and if so, what drives it? Is there an inverse cascade which creates flows and/or magnetic field on larger scales? Is there strong spatial intermittency strong localized dissipation? Is the power spectrum anisotropic?

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The Future
Observations: Mapping in-plane and ver tical structure of B Zeeman mapping of more molecular clouds Faraday rotation at moderate redshift Spectroscopic probes of a Galactic wind Theory: Grapple with diffusivity/resolution dependence; develop scalings Improve subgrid modelling Better post-processing of ISM simulations; include chemistry, radiative transfer, predict line profiles & dissipation scales
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