Our Galaxy is pervaded by strong magnetic fields, which, together with the ordinary matter (made of gas and dust) and the cosmic rays, form the three basic constituents of the interstellar medium. After briefly reviewing our current observational knowledge about Galactic magnetic fields, I will address, from a theoretical point of view, the fundamental questions of their origin and their impact on the interstellar medium.


Total magnetic fields in spiral galaxies, as observed through their total synchrotron emission, are strongest (up to ≲30 µG) in the spiral arms. The degree of radio polarization is low; the field in the arms must be mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to ≲15 µG), sometimes forming “magnetic arms” parallel to the optical arms. The field structure is spiral in almost every galaxy, even in flocculent and bright irregular types which lack spiral arms. The observed large-scale patterns of Faraday rotation in several massive spiral galaxies reveal coherent regular fields, as predicted by dynamo models. However, in most galaxies observed so far no simple patterns of Faraday rotation could be found. Either many dynamo modes are superimposed and cannot be resolved by present-day telescopes, or most of the apparently regular field is in fact anisotropic random, with frequent reversals, due to shearing and compressing gas flows. In galaxies with massive bars, the polarization pattern follows the gas flow. However, around strong shocks in bars, the compression of the regular field is much lower than that of the gas; the regular field decouples from the cold gas and is strong enough to affect the flow of the diffuse warm gas. – The average strength of the total magnetic field in the Milky Way is 6µG near the sun and increases to 20–40 μG in the Galactic center region. The Galactic field is mostly parallel to the plane, except in the center region. Rotation measure data from pulsars indicate several field reversals, unlike external galaxies, but some reversals could be due to distortions of the nearby field.

Observations of magnetic fields in molecular clouds are essential for understanding their role in the evolution of dense clouds and in the star formation process. The two extreme-case models of star formation are the weak magnetic field scenario, in which turbulence drives molecular cloud evolution and therefore star formation, and the strong magnetic field scenario, with magnetic fields governing molecular cloud formation and ambipolar diffusion ultimately driving star formation. Continuum and spectral line polarization observations make it possible to infer magnetic field strengths and morphologies, and therefore to test which of the two star formation scenarios dominates. The observational techniques available for such observations – the Zeeman effect, mapping of polarized dust emission, and mapping of linearly polarized line emission – are briefly described, together with how these observations can test star formation theory. Examples of the existing observations and the current state of the testing of magnetically controlled star formation are presented. Finally, the prospects for the future of study of magnetic fields in molecular clouds and their role in the star formation process is discussed.

The diffuse (non self-gravitating) interstellar medium of the Galaxy is almost impossibly complex and diverse. Temperatures and densities range over many orders of magnitude. The magnetic field links all regimes of the gas, including cosmic rays. Shocks from supernovae and other sources buffet the medium. Into this maelstrom, theorists have ventured. Some models emphasize time-independent thermal equilibrium between hot and cold phases of the gas. Other models stress frequent dynamical events that throw much of the medium out of thermal equilibrium. Some models include magnetic field effects, others do not. Here we report on the nature of diffuse atomic gas, emphasizing observational results from a recent, extensive survey of HI emission and absorption along random lines of sight in the local diffuse medium. We find much of the warmer diffuse atomic gas is out of thermal equilibrium, yet the medium retains a clear dichotomy between warmer and cooler phases. The warmer phase comprises about half of the total diffuse atomic hydrogen gas. Magnetic fields have a median value of about 6 μG in the cold gas, insuring that their dynamical effects cannot be ignored. The conundrum of similar magnetic field strengths in diffuse gas at widely disparate densities remains as an observational fact and a challenge to explain theoretically.

The cold diffuse interstellar medium (ISM) is turbulent with supersonic, most likely transAlfvénic motions. All its tracers, including velocity, are structured down to the smallest scales accessible to observations. This structure, though, is not random and can be quantified using the statistics of its Fourier phase increments. The quantity of structure of interstellar clouds is larger than that of fractal Brownian motion (fBm) fields and of mildly compressible turbulence. We propose that this structure is a response of the medium to a general property of turbulence, the space-time intermittency of its dissipation. The bursts of turbulent dissipation, because they temporarily (over a few hundred years) and locally heat the diffuse medium up to ~103 K, irreversibly modify the subsequent thermal and chemical evolution of the gas. Preliminary models of such bursts predict chemical abundances and excitation of a subset of molecules that are in agreement with the observations. Timescales of the velocity fluctuations are short enough to locally decouple the neutrals from the ionized species in the diffuse medium and generate helical magnetic fields. Small scale fluctuations of the polarization properties of the diffuse ISM are therefore expected.

So far our understanding of the evolution of the ISM has been scanty, because of the inherent nonlinearity of all the processes involved. Modelling the interstellar medium in galaxies in a self-consistent way requires an approach that must take into account the relevant scales, which may cover several orders of magnitude (e.g., from kpc to less than 1 pc). This is a difficult task that requires the use of sophisticated numerical codes, adequate computing power, and precision input data from observations. The large range of scales can be resolved by means of adaptive mesh refinement (AMR), that is, the grids reproducing the computational domain are refined on the fly such that a minimum number of cells is needed to provide the most complete description of the flow.
In this paper we review the most recent results on 3D modeling of the interstellar medium and disk-halo interaction in a section of the Milky Way that includes the Galactic magnetic field, background heating due to starlight, self-gravity and allows for the establishment of the duty-cycle between the disk and halo (commonly known as galactic fountain) by using a grid that extends up to 10 kpc on either side of the midplane. Our simulations capture both the largest structures (e.g., superbubbles) together with the smaller ones (e.g., filaments and eddies) down to 0.625 pc. We investigate, among other things, the variability of the magnetic field in the Galactic disk and its correlation with the density, the rôle of ram pressure in the dynamics of disk gas and the relative weight of the ram, thermal and magnetic pressures, the mass distribution and the volume filling factors of the different temperature regimes in the ISM, as well as the scales at which energy is injected into the interstellar turbulence and we give an estimate for the dimension of the most dissipative structures.

Recent radio polarization observations have revealed a plethora of unexpected features in the polarized Galactic radio background that arise from propagation effects in the random (turbulent) interstellar medium. The canals are especially striking among them, a random network of very dark, narrow regions clearly visible in many directions against a bright polarized Galactic synchrotron background. There are no obvious physical structures in the ISM that may have caused the canals, and so they have been called Faraday ghosts. They evidently carry information about interstellar turbulence but only now is it becoming clear how this information can be extracted. Two theories for the origin of the canals have been proposed; both attribute the canals to Faraday rotation, but one invokes strong gradients in Faraday rotation in the sky plane (specifically, in a foreground Faraday screen) and the other only relies on line-of-sight effects (differential Faraday rotation). In this review we discuss the physical nature of the canals and how they can be used to explore statistical properties of interstellar turbulence. This opens studies of magnetized interstellar turbulence to new methods of analysis, such as contour statistics and related techniques of computational geometry and topology. In particular, we can hope to measure such elusive quantities as the Taylor microscale and the effective magnetic Reynolds number of interstellar MHD turbulence.

After a quick tour of polarization basics, I review interstellar polarization starting with its galactic distribution, and then going into molecular clouds. The properties of interstellar polarization are discussed, its wavelength dependence and the 3 parameters used to specify it. Then more recent work since approximately the last 15 years which show significant deviations from the usual behavior in the densest regions of molecular clouds is presented. Recent results for two specific regions are presented, and finally I conclude by summarizing the main points.


Observations of far-infrared (FIR) and submillimeter (SMM) polarized emission are used to study magnetic fields and dust grains in dense regions on the interstellar medium (ISM). These observations place constraints on models of molecular clouds, star-formation, grain alignment mechanisms, and grain size, shape, and composition.
 The FIR/SMM polarization is strongly dependent on wavelength. We have attributed this wavelength dependence to sampling different grain populations at different temperatures. To date, most observations of polarized emission have been in the densest regions of the ISM. Extending these observations to regions of the diffuse ISM, and to microwave frequencies, will provide additional tests of grain and alignment models.
An understanding of polarized microwave emission from dust is key to an accurate measurement of the polarization of the cosmic microwave background. The microwave polarization spectrum will put limits on the contributions to polarized emission from spinning dust and vibrating magnetic dust.


Several of the current and next-generation cosmic microwave background (CMB) experiments have polarimetric capability, promising to add to the finesse of precision cosmology. One of the contaminating Galactic foregrounds is thermal emission by dust. Since optical interstellar polarization is commonly seen, from differential extinction by aligned aspherical dust particles, it is expected that thermal emission from these grains will be polarized. Indeed, in the Galactic plane and in dark (molecular) clouds, dust emission in the infrared and submillimetre has been measured to be polarized. It seems likely that the faint diffuse cirrus emission, of more relevance to CMB experiments, will be polarized too. We discuss how well the amount of polarization of this component can be predicted, making use of what is known about optical (and infrared and ultraviolet) interstellar polarization and extinction. Some constraints on the alignment of the carrier of the dust-correlated anomalous microwave emission can be made as well.


Future cosmology space missions will concentrate on measuring the polarization of the Cosmic Microwave Background (CMB), which potentially carries invaluable information about the earliest phases of the evolution of our universe. Such ambitious projects will ultimately be limited by the sensitivity of the instrument and by the accuracy at which polarized foreground emission from our own Galaxy can be subtracted out. We present the PILOT balloon project which will aim at characterizing one of these foreground sources, the polarization of the dust continuum emission in the diffuse interstellar medium. The PILOT experiment will also constitute a test-bed for using multiplexed bolometer arrays for polarization measurements.


In this short review, I try to give a flavor of what are the motivations and difficulties of observing the Cosmic Microwave Background (CMB) polarization. After describing briefly the physics of CMB polarization, I give a snapshot of the observational status, as well as the data processing techniques developed so far. I end up describing how polarization information should constrain some key aspects of the early universe physics.


This contribution reviews the most recent experimental results obtained on the polarization anisotropy of the Cosmic Microwave Background and those anticipated in the near future. Some basic arguments for a dedicated satellite are also presented, as well as the need for a better characterization of polarized Galactic foregrounds.


Planck is designed to image the anisotropies of the Cosmic Microwave Background (CMB) over the whole sky, with unprecedented sensitivity (ΔT/T ~ 2 × 10-6) and angular resolution (~5 arcmin). Planck will provide a major source of information relevant to several cosmological and astrophysical issues, such as testing theories of the early universe and the origin of cosmic structure. The ability to measure to high accuracy the angular power spectrum of the CMB fluctuations will allow the determination of fundamental cosmological parameters with an uncertainty of order a few percent. In addition to the main cosmological goals of the mission, the Planck sky survey will be used to study in detail the very sources of emission which “contaminate” the signal due to the CMB. This will result in a wealth of information on the properties of extragalactic sources, and on the dust and gas in our own galaxy. One specific notable result will be the measurement of the polarized emission from our own Galaxy. The ability of Planck to measure polarization across a wide frequency range (30–350 GHz), with high precision and accuracy, and over the whole sky, will provide unique insight into the properties of the interstellar medium. An overview is presented of the Planck mission, its scientific objectives, the key elements of its technical design, and its current status.