Needle-like structures discovered on positively charged lightning branches

\u3cp\u3e 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 leaders \u3csup\u3e1\u3c/sup\u3e . Negative leaders propagate in discrete steps, emitting copious radio pulses in the 30–300-megahertz frequency band \u3csup\u3e2–8\u3c/sup\u3e that can be remotely sensed and imaged with high spatial and temporal resolution \u3csup\u3e9–11\u3c/sup\u3e . Positive leaders propagate more continuously and thus emit very little high-frequency radiation \u3csup\u3e12\u3c/sup\u3e . Radio emission from positive leaders has nevertheless been mapped \u3csup\u3e13–15\u3c/sup\u3e , and exhibits a pattern that is different from that of negative leaders \u3csup\u3e11–13,16,17\u3c/sup\u3e . Furthermore, it has been inferred that positive leaders can become transiently disconnected from negative leaders \u3csup\u3e9,12,16,18–20\u3c/sup\u3e , which may lead to current pulses that both reconnect positive leaders to negative leaders \u3csup\u3e11,16,17,20–22\u3c/sup\u3e and cause multiple cloud-to-ground lightning events \u3csup\u3e1\u3c/sup\u3e . The disconnection process is thought to be due to negative differential resistance \u3csup\u3e18\u3c/sup\u3e , but this does not explain why the disconnections form primarily on positive leaders \u3csup\u3e22\u3c/sup\u3e , or why the current in cloud-to-ground lightning never goes to zero \u3csup\u3e23\u3c/sup\u3e . 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. \u3c/p\u3

Probing atmospheric electric fields in thunderstorms through radio emission from cosmic-ray-induced air showers

\u3cp\u3eWe present measurements of radio emission from cosmic ray air showers that took place during thunderstorms. The intensity and polarization patterns of these air showers are radically different from those measured during fair-weather conditions. With the use of a simple two-layer model for the atmospheric electric field, these patterns can be well reproduced by state-of-the-art simulation codes. This in turn provides a novel way to study atmospheric electric fields.\u3c/p\u3

The shape of the radio wavefront of extensive air showers as measured with LOFAR

Extensive air showers, induced by high energy cosmic rays impinging on the Earth's atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parameterization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle. © 2014 Elsevier B.V. All rights reserved

Corrigendum: A large light-mass component of cosmic rays at 1017-1017.5 electronvolts from radio observations.

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. Porcelli. Our values are in agreement with those of ref. 1 within systematic uncertainties

A large light-mass component of cosmic rays at 10(17)-10(17.5) electronvolts from radio observations

Cosmic rays are the highest-energy particles found in nature. Measurements of the mass composition of cosmic rays with energies of 10(17)-10(18) electronvolts are essential to understanding whether they have galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal comes from accelerators capable of producing cosmic rays of these energies. Cosmic rays initiate air showers--cascades of secondary particles in the atmosphere-and their masses can be inferred from measurements of the atmospheric depth of the shower maximum (Xmax; the depth of the air shower when it contains the most particles) or of the composition of shower particles reaching the ground. Current measurements have either high uncertainty, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays is a rapidly developing technique for determining Xmax (refs 10, 11) with a duty cycle of, in principle, nearly 100 per cent. The radiation is generated by the separation of relativistic electrons and positrons in the geomagnetic field and a negative charge excess in the shower front. Here we report radio measurements of Xmax with a mean uncertainty of 16 grams per square centimetre for air showers initiated by cosmic rays with energies of 10(17)-10(17.5) electronvolts. This high resolution in Xmax enables us to determine the mass spectrum of the cosmic rays: we find a mixed composition, with a light-mass fraction (protons and helium nuclei) of about 80 per cent. Unless, contrary to current expectations, the extragalactic component of cosmic rays contributes substantially to the total flux below 10(17.5) electronvolts, our measurements indicate the existence of an additional galactic component, to account for the light composition that we measured in the 10(17)-10(17.5) electronvolt range