Context. Our position inside the Galaxy requires 3D-modelling to obtain the distribution of the Galactic magnetic field, cosmic-ray (CR) electrons and thermal electrons. Aims. Our intention is to find a Galactic 3D-model which agrees best with available radio observations. Methods. We constrain simulated all-sky maps in total intensity, linear polarization, and rotation measure (RM) by observations. For the simulated maps as a function of frequency we integrate in 15 wide cones the emission along the line of sight calculated from Galactic 3D-models. We test a number of large-scale magnetic field configurations and take the properties of the warm interstellar medium into account. Results. From a comparison of simulated and observed maps we are able to constrain the regular large-scale Galactic magnetic field in the disk and the halo of the Galaxy. The local regular field is 2 µG and the average random field is about 3 µG. The known local excess of synchrotron emission originating either from enhanced CR electrons or random magnetic fields is able to explain the observed high-latitude synchrotron emission. The thermal electron model (NE2001) in conjunction with a proper filling factor accounts for the observed optically thin thermal emission and low frequency absorption by optically thick emission. A coupling factor between thermal electrons and the random magnetic field component is proposed, which in addition to the small filling factor of thermal electrons increases small-scale RM fluctuations and thus accounts for the observed depolarization at 1.4 GHz. Conclusions. We conclude that an axisymmetric magnetic disk field configuration with reversals inside the solar circle fits available observations best. Out of the plane a strong toroidal magnetic field with different signs above and below the plane is needed to account for the observed high-latitude RMs. The large field strength is a consequence of the small thermal electron scale height of 1 kpc, which also limits the CR electron extent up to a height of 1 kpc not to contradict with the observed synchrotron emission out of the plane. Our preferred 3D-model fits the observed Galactic total intensity and polarized emission better than other models over a wide frequency range and also agrees with the observed RM from extragalactic sources.

Recently, Sun et al. (2008) published new Galactic 3D-models of magnetic fields in the disk and halo of the Milky Way and the distribution of cosmic-ray electron density by taking into account the thermal electron density model NE2001 by Cordes & Lazio (2002. The models successfully reproduce observed continuum and polarization all-sky maps and the distribution of rotation measures of extragalactic sources across the sky. However, the model parameters obtained for the Galactic halo, although reproducing the observations, seem physically unreasonable: the magnetic field needs to be significantly stronger in the Galactic halo than in the plane and the cosmicray distribution must be truncated at about 1 kpc to avoid excessive synchrotron emission from the halo. The reason for these unrealistic parameters was the low scale-height of the warm thermal gas of about 1 kpc adapted in the NE2001 model. However, this scaleheight seemed well settled by numerous investigations. Recently, the scale-height of the warm gas in the Galaxy was revised by Gaensler et al. (2008) to about 1.8 kpc, by showing that the 1 kpc scale-height results from a systematic bias in the analysis of pulsar data. This implies a higher thermal electron density in the Galactic halo, which in turn reduces the halo magnetic field strength to account for the observed rotation measures of extragalactic sources. We slightly modified the NE2001 model for the new scale-height and revised the Sun et al. (2008) model parameters accordingly: the strength of the regular halo magnetic field is now 2 µG or lower, and the physically unrealistic cutoff in z for the cosmic-ray electron density is removed. The simulations based on the revised 3D-models reproduce all-sky observations as before.

Aims. Polarization measurements of the Galactic plane at λ6 cm probe the interstellar medium (ISM) to larger distances compared to measurements at longer wavelengths, enabling us to investigate properties of the Galactic magnetic fields and electron density. Methods. We are conducting a new λ6 cm continuum and polarization survey of the Galactic plane covering 10• ≤ l ≤ 230• and |b| ≤ 5• . Missing large-scale structures in the U and Q maps are restored based on extrapolated polarization K-band maps from the WMAP satellite. The λ6 cm data are analyzed together with maps in other bands. Results. We discuss some results for the first survey region, 7• × 10 • in size, centered at (l, b) = (125.• 5, 0 • ). Two new passive Faraday screens, G125.6−1.8 and G124.9+0.1, were detected. They cause significant rotation of background polarization angles but little depolarization. G124.9+0.1 was identified as a new faint HII region at a distance of 2.8 kpc. G125.6−1.8, with a size of about 46 pc, has neither a counterpart in enhanced Hα emission nor in total intensity. A model combining foreground and background polarization modulated by the Faraday screen was developed. Using this model, we estimated the strength of the ordered magnetic field along the line of sight to be 3.9 µG for G124.9+0.1, and exceeding 6.4 µG for G125.6−1.8. We obtained an estimate of 2.5 and 6.3 mK kpcfor the average polarized and total synchrotron emissivity towards G124.9+0.1. The synchrotron emission beyond the Perseus arm is quite weak. A spectral curvature previously reported for SNR G126.2+1.6 is ruled out by our new data, which prove a straight spectrum. Conclusions. The new λ6 cm survey will play an important role in improving the understanding of the properties of the magnetoionic ISM. The magnetic fields in HII regions can be measured. Faraday screens with very low electron densities but large rotation measures were detected indicating strong and regular magnetic fields in the ISM. Information about the local synchrotron emissivity can be obtained.Key words. surveys -polarization -radio continuum: general -methods: observational -ISM: magnetic fields -ISM: supernova remnants IntroductionThe first detection of diffuse polarized emission from the Milky Way Galaxy (Westerhout et al. 1962;Wielebinski et al. 1962) confirmed that its non-thermal radiation originates from synchrotron emission. The two major sources of polarized emission from our Galaxy are diffuse radio emission associated with the Galactic disk produced by relativistic electrons spiraling in interstellar magnetic fields, and discrete sources such as supernova remnants (SNRs) with compressed interstellar magnetic fields, where relativistic electrons are accelerated by shocks.To understand the properties of the ISM in our Galaxy a number of whole sky surveys (e.g. as reviewed by Reich 2003) have been made. Also large-scale radio surveys of the Galactic plane with higher angular resolution have been performed. The large-scale surveys clearly show the concentration of emission ...