We discuss current observational and theoretical knowledge of magnetic fields, especially the large-scale structure in the disks and halos of spiral galaxies. Among other topics, we consider the enhancement of global magnetic fields in the interarm regions, magnetic spiral arms, and representations as superpositions of azimuthal modes, emphasizing a number of unresolved questions. It is argued that a turbulent hydromagnetic dynamo of some kind and an inverse cascade 1 Now at Department of Mathematics and Statistics, University of Newcastle upon Tyne, NE1 7RU, United Kingdom. 155 0066-4146/96/0915-0155$08.00 156 BECK ET AL of magnetic energy gives the most plausible explanation for the regular galactic magnetic fields. Primordial theory is found to be unsatisfactory, and fields of cosmological origin may not even be able to provide a seed field for a dynamo. Although dynamo theory has its own problems, the general form of the dynamo equations appears quite robust. Finally, detailed models of magnetic field generation in galaxies, allowing for factors such as spiral structure, starbursts, galactic winds, and fountains, are discussed and confronted with observations.

LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

Faraday rotation and depolarization of synchrotron radio emission are considered in a consistent general approach, under conditions typical of spiral galaxies, i.e. when the magneto‐ionic medium and relativistic electrons are non‐uniformly distributed in a layer containing both regular and fluctuating components of magnetic field, thermal electron density and synchrotron emissivity. We demonstrate that non‐uniformity of the magneto‐ionic medium along the line of sight strongly affects the observable polarization patterns. The degree of polarization p and the observed Faraday rotation measure RM are very sensitive to whether or not the source is symmetric along the line of sight. The RM may change sign in a certain wavelength range in an asymmetric slab even when the line‐of‐sight magnetic field has no reversals. Faraday depolarization in a purely regular magnetic field can be much stronger than suggested by the low observed rotation measures. A twisted regular magnetic field may result in p increasing with λ— a behaviour detected in several galaxies.  We derive expressions for statistical fluctuations in complex polarization and show that random fluctuations in the degree of polarization caused by Faraday dispersion are expected to become significantly larger than the mean value of p at λ ≳ 20 − 30 cm. We also discuss depolarization arising from a gradient of Faraday rotation measure across the beam, both in the source and in an external Faraday screen. We briefly discuss applications of the above results to radio polarization observations.  We discuss how the degree of polarization is affected by the scaling of synchrotron emissivity ɛ with the total magnetic field strength B. We derive formulae for the complex polarization at λ → 0 under the assumption that ɛ ∝ B2B2⊥, which may arise under energy equipartition or pressure balance between cosmic rays and magnetic fields. The resulting degree of polarization is systematically larger than for the usually adopted scaling ɛ ∝ B2⊥; the difference may reach a factor of 1.5.

Abstract. The commonly used classical equipartition or minimum-energy estimate of total magnetic fields strengths from radio synchrotron intensities is of limited practical use because it is based on the hardly known ratio K of the total energies of cosmic ray protons and electrons and also has inherent problems. We present a revised formula, using the number density ratio K for which we give estimates. For particle acceleration in strong shocks K is about 40 and increases with decreasing shock strength. Our revised estimate for the field strength gives larger values than the classical estimate for flat radio spectra with spectral indices of about 0.5-0.6, but smaller values for steep spectra and total fields stronger than about 10 µG. In very young supernova remnants, for example, the classical estimate may be too large by up to 10×. On the other hand, if energy losses of cosmic ray electrons are important, K increases with particle energy and the equipartition field may be underestimated significantly. Our revised larger equipartition estimates in galaxy clusters and radio lobes are consistent with independent estimates from Faraday rotation measures, while estimates from the ratio between radio synchrotron and X-ray inverse Compton intensities generally give much weaker fields. This may be explained e.g. by a concentration of the field in filaments. Our revised field strengths may also lead to major revisions of electron lifetimes in jets and radio lobes estimated from the synchrotron break frequency in the radio spectrum.

(Abridged) We use new multi-wavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on scales from $200\pc$ to several $\kpc$. Interferometric and single dish data are combined to obtain new maps at \wwav{3}{6} in total and polarized emission, and earlier \wav{20} data are re-reduced. We compare the spatial distribution of the radio emission with observations of the neutral gas, derive radio spectral index and Faraday depolarization maps, and model the large-scale variation in Faraday rotation in order to deduce the structure of the regular magnetic field. We find that the \wav{20} emission from the disc is severely depolarized and that a dominating fraction of the observed polarized emission at \wav{6} must be due to anisotropic small-scale magnetic fields. Taking this into account, we derive two components for the regular magnetic field in this galaxy: the disc is dominated by a combination of azimuthal modes, $m=0+2$, but in the halo only an $m=1$ mode is required to fit the observations. We disuss how the observed arm-interarm contrast in radio intensities can be reconciled with evidence for strong gas compression in the spiral shocks. The average arm--interam contrast, representative of the radii $r>2\kpc$ where the spiral arms are broader, is not compatible with straightforward compression: lower arm--interarm contrasts than expected may be due to resolution effects and \emph{decompression} of the magnetic field as it leaves the arms. We suggest a simple method to estimate the turbulent scale in the magneto-ionic medium from the dependence of the standard deviation of the observed Faraday rotation measure on resolution. We thus obtain an estimate of $50\pc$ for the size of the turbulent eddies.Comment: 21 pages, 18 figures (some at lower resolution than submitted version), accepted for publication in MNRA

Context. The spiral galaxy NGC 6946 hosts magnetic spiral arms, highly aligned magnetic fields between the gas/optical arms. Aims. The origin of the magnetic phenomena and their relation to the interstellar gas are investigated. Methods. NGC 6946 was observed in total intensity and linear polarization in five radio bands between 3 cm and 21 cm. Maps of spectral index, Faraday rotation and depolarization were derived. Results. At the inner edge of the inner gas spiral arm the ordered magnetic field is only mildly compressed and turns smoothly, to become aligned along the gas arm. Hence the field is not shocked and is probably connected to the warm, diffuse gas. At larger radii, two bright magnetic arms between the optical arms are visible in polarized intensity. The field in the northern magnetic arm is almost totally aligned. Faraday rotation measures (RM) in these arms are consistent with the superposition of two low azimuthal dynamo modes. Three more magnetic arms are discovered in the outer galaxy, located between H I arms. The RM structure function confirms large-scale coherent fields. The observed anti-correlation between the field's pitch angles and the RM values is a possible signature of helical fields. -Due to strong Faraday depolarization the galaxy is not transparent to polarized waves at λ18 cm and λ20 cm. The large-scale asymmetry in depolarization with respect to the major axis may be another indication of large-scale helical fields. Three depolarization rings of almost zero polarization seen at λ20 cm are probably generated by differential Faraday rotation in H II complexes in NGC 6946 of 300−500 pc size. -In the gas/optical spiral arms, the total (mostly turbulent) magnetic field is amplified to 20 µG. Its energy density is 10 times larger than that of the ionized gas and is similar to that of the turbulent gas motions in the inner galaxy. The magnetic energy exceeds that of the turbulent energy in the outer galaxy. All energy densities in NGC 6946 are about one order of magnitude larger than those in the Milky Way. Conclusions. Density waves in the inner gaseous spiral arms mildly compress the field. Dynamo action probably generates the magnetic spiral arms. The magnetic field is dynamically important, interacts with the gas flow and possibly determines the properties of the gas spiral arms.

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We report here infrared spectra of protonated and lithiated valine with varying degrees of hydration in the gas phase and interpret them with the help of DFT calculations at the B3LYP/6-31++G** level. In both the protonated and lithiated species our results clearly indicate that the solvation process is driven first by solvation of the charge site and subsequently by formation of a second solvation shell. The infrared spectra of Val x Li+ (H2O)4 and Val x H+ (H2O)4 are strikingly similar in the region of the spectrum corresponding to hydrogen-bonded stretches of donor water molecules, suggesting that in both cases similar extended water structures are formed once the charge site is solvated. In the case of the lithiated species, our spectra are consistent with a conformation change of the amino acid backbone from syn to anti accompanied by a change in the lithium binding from a NO coordination to OO coordination configuration upon addition of the third water molecule. This change in the mode of metal ion binding was also observed previously by Williams and Lemoff [J. Am. Soc. Mass Spectrom. 2004, 15, 1014-1024] using blackbody infrared radiative dissociation (BIRD). In contrast to the zwitterion formation inferred from results of the BIRD experiments upon addition of a third water molecule, our spectra, which are a more direct probe of structure, show no evidence for zwitterion formation with the addition of up to four water molecules.