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

Context. Recent Hi and Hα observations of NGC 4522 have revealed that this spiral galaxy represents one of the best cases of ongoing ram pressure stripping in the Virgo cluster. Aims. We determined the parameters of the interaction between the interstellar medium of NGC 4522 and the intracluster medium of the Virgo cluster. Methods. A dynamical model including ram pressure stripping is applied to the strongly Hi deficient Virgo spiral galaxy NGC 4522. A carefully chosen model snapshot is compared with existing VLA Hi observations. Results. The model successfully reproduces the large-scale gas distribution and the velocity field. However it fails to reproduce the large observed Hi linewidths in the extraplanar component, for which we give possible explanations. In a second step, we solve the induction equation on the velocity fields of the dynamical model and calculate the large scale magnetic field. Assuming a Gaussian distribution of relativistic electrons we obtain the distribution of polarized radio continuum emission which is also compared with our VLA observations at 6 cm. The observed maximum of the polarized radio continuum emission is successfully reproduced. Our model suggests that the ram pressure maximum occurred only ∼50 Myr ago. Conclusions. Since NGC 4522 is located far away from the cluster center (∼1 Mpc) where the intracluster medium density is too low to cause the observed stripping if the intracluster medium is static and smooth, two scenarios are envisaged: (i) the galaxy moves very rapidly within the intracluster medium and is not even bound to the cluster; in this case the galaxy has just passed the region of highest intracluster medium density; (ii) the intracluster medium is not static but moving due to the infall of the M 49 group of galaxies. In this case the galaxy has just passed the region of highest intracluster medium velocity. This study shows the strength of combining high resolution Hi and polarized radio continuum emission with detailed numerical modeling of the evolution of the gas and the large-scale magnetic field.

Abstract. We performed a high-sensitivity search for galaxy-scale magnetic fields by radio polarimetry at 10.45 GHz and 4.85 GHz with the Effelsberg 100 m radio telescope, accompanied by Hα imaging, for the two Local Group irregular galaxies IC 10 and NGC 6822. Their star-forming bodies are small and rotate slowly. IC 10 is known to have a very high star-forming activity, resembling blue compact dwarfs, while NGC 6822 has a low overall star-formation level. Despite very different current star formation rates, our Hα imaging revealed a large web of diffuse Hα filaments and shells in both IC 10 and NGC 6822. Some of them extend far away from the galaxy's main body. The total power emission of both objects shows bright peaks either at the positions of optically strong star-forming clumps (IC 10) or individual H  regions or supernova remnants (NGC 6822).However, in both cases we detect a smoothly distributed, extended component. In IC 10 we found clear evidence for the presence of a diffuse, mostly random magnetic field of 14 µG strength, probably generated by a fluctuation dynamo. One of the Hα-emitting filaments appears to be associated with enhanced magnetic fields. We also rediscuss the reddening of IC 10 and its implications for its distance. In the case of NGC 6822 we found only very weak evidence for nonthermal emission, except perhaps for some regions associated with local gas compression. We detect in both galaxies small spots of polarized emission, indicative of regular fields ( 3 µG), at least partly associated with local compressional phenomena.Key words. polarization -galaxies: irregular -galaxies: magnetic fields, galaxies: individual: IC10, NGC 6822 -radio continuum: galaxies IntroductionIrregular galaxies are low-mass objects exhibiting a variety of rotational properties with a subclass of them rotating very slowly (rotational speeds V rot ≤ 30 km s −1 ) and often chaotically (e.g. Lo et al. 1993). They constitute important laboratories for large-scale interactions of stars with the interstellar medium: the low gravitational potential and relatively small size increase the probability that superbubbles, forming close to star-forming regions, may break out of the galaxy (e.g. Mac Low & Ferrara 1998). Many irregular galaxies exhibit giant arcs or filaments of ionized gas (e.g. Sabbadin & Bianchini 1979;Hunter et al. 1993;Bomans et al. 1997). The role of magnetic fields in the origin and confinement of these ionized structures (e.g. Hunter & Gallagher 1990) is still a matter of debate.In spiral galaxies magnetic fields, which are sufficiently strong to trigger star formation via magnetic instabilities (Blitz & Shu 1980) or to influence the superbubble expansion (Ferriere et al. 1991), are probably generated by the mean-field dynamo (see . This requires strong Coriolis Send offprint requests to: K. Chyży, e-mail: chris@oa.uj.edu.pl forces (hence a rapid rotation) to give the turbulent motions a preferred sense of twisting. A sufficient size of the ionized gas envelope is also required. Large irregulars...

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