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Lightning is a dangerous yet poorly understood natural phenomenon. Lightning forms a network of plasma channels propagating away from the initiation point with both positively and negatively charged ends—called positive and negative leaders1. Negative leaders propagate in discrete steps, emitting copious radio pulses in the 30–300-megahertz frequency band2–8that can be remotely sensed and imaged with high spatial and temporal resolution9–11. Positive leaders propagate more continuously and thus emit very little high-frequency radiation12. Radio emission from positive leaders has nevertheless been mapped13–15, and exhibits a pattern that is different from that of negative leaders11–13,16,17. Furthermore, it has been inferred that positive leaders can become transiently disconnected from negative leaders9,12,16,18–20, which may lead to current pulses that both reconnect positive leaders to negative leaders11,16,17,20–22and cause multiple cloud-to-ground lightning events1. The disconnection process is thought to be due to negative differential resistance18, but this does not explain why the disconnections form primarily on positive leaders22, or why the current in cloud-to-ground lightning never goes to zero23. Indeed, it is still not understood how positive leaders emit radio-frequency radiation or why they behave differently from negative leaders. Here we report three-dimensional radio interferometric observations of lightning over the Netherlands with unprecedented spatiotemporal resolution. We find small plasma structures—which we call ‘needles’—that are the dominant source of radio emission from the positive leaders. These structures appear to drain charge from the leader, and are probably the reason why positive leaders disconnect from negative ones, and why cloud-to-ground lightning connects to the ground multiple times.

High-resolution radio measurements of air showers—cascades of secondary particles in the atmosphere initiated by cosmic rays—reveal that cosmic rays with energies of 1017–1017.5electronvolts have a mixed composition, with light elements (protons and helium nuclei) making up 80 per cent of their mass.

Nature 531, 70–73 (2016); doi:10.1038/nature16976 In this Letter, we omitted to cite preliminary results from the low-energy extension of the Pierre Auger Observatory, as presented at the International Cosmic Ray Conference 2015 (ref. 1). Figure 1 of this Corrigendum shows measurements of the average value of Xmax for the Low Frequency Array (LOFAR), and earlier experiments using different techniques, now including the data from the Pierre Auger Observatory1, specifically the contribution of A.

Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric “radio window”: 10–240 MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR.

Aims. Observations with the Very Large Array telescope at 1477 MHz revealed the presence of a radio mini-halo surrounding the faint central point-like radio source in the Ophiuchus cluster of galaxies. In this work we present a study of the radio emission from this cluster of galaxies at lower radio frequencies.Methods. We observed the Ophiuchus cluster at 153, 240, and 614 MHz with the Giant Metrewave Radio Telescope.Results. The mini-halo is clearly detected at 153 and 240 MHz, the frequencies at which we reached the best sensitivity to the low-surface brightness diffuse emission, while it is not detected at 610 MHz because of the too low signal-to-noise ratio at this frequency. The most prominent feature at low frequencies is a patch of diffuse steep spectrum emission located at about 5´ south-east from the cluster centre. By combining these images with that at 1477 MHz, we derived the spectral index of the mini-halo. Globally, the mini-halo has a low-frequency spectral index of $\alpha_{240}^{153}$$\simeq$1.4±0.3 and an high-frequency spectral index of $\alpha_{1477}^{240}$$\simeq$1.60±0.05. Moreover, we measure a systematic increase of the high-frequency spectral index with radius: the azimuthal radial average of $\alpha_{1477}^{240}$increases from about 1.3, at the cluster centre, up to about 2.0 in the mini-halo outskirts.Conclusions. The observed radio spectral index agrees with that obtained by modelling the non-thermal hard X-ray emission in this cluster of galaxies. We assume that the X-ray component arises from inverse-Compton scattering between the photons of the cosmic microwave background and a population of non-thermal electrons, which are isotropically distributed and whose energy spectrum is a power law with an index p. We derive that the electrons energy spectrum should extend from a minimum Lorentz factor of $\gamma_{\rm min}$≲700 up to a maximum Lorentz factor of $\gamma_{\rm max}$$\simeq$3.8 × 104with an index p= 3.8±0.4. The volume-averaged strength for a completely disordered intra-cluster magnetic field is BV$\simeq$0.3±0.1 μG.

Aims. Observations with the Very Large Array telescope at 1477 MHz revealed the presence of a radio mini-halo surrounding the faint central point-like radio source in the Ophiuchus cluster of galaxies. In this work we present a study of the radio emission from this cluster of galaxies at lower radio frequencies.