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.

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.

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120–168 MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00′00″ to 57°00′00″) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325 694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ∼10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S144 MHz = 71 μJy beam−1 and the point-source completeness is 90% at an integrated flux density of 0.45 mJy. The resolution of the images is 6″ and the positional accuracy is within 0.2″. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ∼20 articles that highlight the scientific potential of LoTSS.

A new polarization survey of the northern sky at 1.41 GHz is presented. The observations were carried out using the 25.6 m telescope at the Dominion Radio Astrophysical Observatory in Canada, with an angular resolution of 36 . The data are corrected for ground radiation to obtain Stokes U and Q maps on a well-established intensity scale tied to absolute determinations of zero levels, containing emission structures of large angular extent, with an rms noise of 12 mK. Survey observations were carried out by drift scanning the sky between −29• and +90• declination. The fully sampled drift scans, observed in steps of 0.25• to ∼2.5• in declination, result in a northern sky coverage of 41.7% of full Nyquist sampling. The survey surpasses by a factor of 200 the coverage, and by a factor of 5 the sensitivity, of the Leiden/Dwingeloo polarization survey that was until now the most complete large-scale survey. The temperature scale is tied to the Effelsberg scale. Absolute zero-temperature levels are taken from the Leiden/Dwingeloo survey after rescaling those data by the factor of 0.94. The paper describes the observations, data processing, and calibration steps. The data are publicly available at

The lightning flashes used in this work were mapped using data from the LO-FAR (LOw Frequency ARray) radio telescope. Due to its effective lightning protection system, LOFAR is able to continue to operate during thunderstorm activity[1]. The Dutch LOFAR stations consist of 38 (24 core + 14 remote) stations spread over 3200 km 2 in the northern Netherlands. The largest baseline between core stations is about 3 km, the largest baseline between remote stations is about 100 km. From each station we use 6 dual-polarized low band dipole antennas (LBA), sampled at 200 MHz, to observe the 30-80 MHz band. The raw time series data were saved to the transient buffer boards, which continuously buffer the last 5 s of data from a maximum of 48 dual-polarized antennas per station. The resulting relative timing accuracy is better than 1 ns. See [2] for more details on LOFAR. When a lightning flash occurs within the area enclosed by the Dutch LOFAR stations, as observed by www.lightningmaps.org, the transient buffer boards are stopped and the data is read to disk. The method we used to map each lightning flash has three major steps. In the first step we fitted plane-waves to the time of pulses received by individual LOFAR stations. Note that the LOFAR stations are less than 100 m in diameter and the lighting is many kilometers from the closest LOFAR station, so that a plane-wave approximation is very good for individual LOFAR stations. These plane-waves were used to identify non-functional antennas, and the intersection of their arrival directions gave a rough first estimate of the flash location, accurate to a few kilometers. Since each station has its own clock and cable delays, in the second step we found the clock offsets between the different LOFAR stations by simultaneously fitting the locations of multiple events and station clock offsets to the measured times of radio pulses, with a Levenberg-Marquardt minimizer. In order to achieve the highest precision, we chose to fit locations of 5 events that created pulses that were strong but not saturating, had a simple structure, and did not change shape significantly across different stations. After fitting, the root-mean-square difference between the modeled and measured arrival times of the radio pulses was around 1 ns. The resulting station clock offsets are consistent with LOFAR station clock calibrations, which are known