Search

Radio antennas have become a standard tool for the detection of cosmic-ray air showers in the energy range above $10^{16}\,$eV. The radio signal of these air showers is generated mostly due to the deflection of electrons and positrons in the geomagnetic field, and contains information about the energy and the depth of the maximum of the air showers. Unlike the traditional air-Cherenkov and air-fluorescence techniques for the electromagnetic shower component, radio detection is not restricted to clear nights, and recent experiments have demonstrated that the measurement accuracy can compete with these traditional techniques. Numerous particle detector arrays for air showers have thus been or will be complemented by radio antennas. In particular when combined with muon detectors, the complementary information provided by the radio antennas can enhance the total accuracy for the arrival direction, energy and mass of the primary cosmic rays. Digitization and computational techniques have been crucial for this recent progress, and radio detection will play an important role in next-generation experiments for ultra-high-energy cosmic rays. Moreover, stand-alone radio experiments are under development and will search for ultra-high-energy photons and neutrinos in addition to cosmic rays. This article provides a brief introduction to the physics of the radio emission of air showers, an overview of air-shower observatories using radio antennas, and highlights some of their recent results., Comment: accepted for EPJ ST

IceCube-Gen2, the next generation extension of the IceCube Neutrino Observatory at the South Pole, offers a unique scientific potential for cosmic-ray physics at PeV to EeV energies complementing the main science case of neutrino astronomy. The cosmic-ray science case will be enabled by a surface array on top of an extended optical array deep in the polar ice. The optical array measures TeV muons of air showers, and the surface array primarily measures the electromagnetic shower component and low-energy muons. The design of the surface array foresees scintillation panels providing a full-efficiency threshold for near-vertical proton showers of 0.5 PeV and radio antennas increasing the measurement accuracy for the electromagnetic shower component in the energy range of the Galactic-to-extragalactic transition. Compared to IceCube, the aperture for air showers measured in coincidence with the surface and optical arrays will increase by a factor of 30, due to the larger area and angular acceptance in zenith angle. The science potential includes both, the particle physics of air showers, such as prompt muons, and the astrophysics of the highest energy Galactic cosmic-rays, enabled by the higher sensitivity for the mass composition and anisotropy of cosmic rays, and by the search for PeV photons. This proceeding summarizes the science case and design of the surface array as presented in the recently released IceCube-Gen2 Technical Design Report: https://icecube-gen2.wisc.edu/science/publications/tdr/, Comment: Proceedings of UHECR 2024

Machine learning is a useful tool for identifying radio pulses from cosmic-ray air showers and for cleaning such pulses from radio background. This can lower the detection threshold and increase the accuracy for the pulse time and amplitude. We have trained Convolutional Neural Networks (CNNs) using CoREAS simulations and background recorded by a prototype station at the IceTop surface array at the South Pole and have applied them to air-shower measurements by this station. The station consists of 3 SKALA antennas and 8 scintillators, which are used to trigger the readout of the antennas upon a sixfold coincidence. Afterwards, the radio signal is filtered to the band of 70-350 MHz. By applying neural networks to search for radio signals in about four months of data, we find about five times more events than by a traditional method based on a signal-to-noise ratio cut after filtering for radio frequency interferences. Despite the lower threshold, the purity of the selected events seems to improve, and the angular resolution of the radio measurements does not deteriorate, which we have confirmed by a comparison of the reconstructed shower direction with IceTop. This analysis thus provides experimental confirmation that neural networks can indeed be used to clean air-shower radio signals from background and to lower the radio detection threshold of hybrid arrays combining particle and radio detectors., Comment: Proceedings of ARENA 2024

This is an index of the contributions by the Giant Radio Array for Neutrino Detection (GRAND) Collaboration to the 10th International Workshop on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2024, University of Chicago, June 11-14, 2024). The contributions include an overview of GRAND in its present and future incarnations, methods of radio-detection that are being developed for them, and ongoing joint work between the GRAND and BEACON experiments., Comment: Note: To access the list of contributions, please follow the "HTML" link that can be found on the arXiv page

Neutrinos at ultrahigh energies can originate both from interactions of cosmic rays at their acceleration sites and through cosmic-ray interactions as they propagate through the universe. These neutrinos are expected to have a low flux which drives the need for instruments with large effective areas. Radio observations of the inclined air showers induced by tau neutrino interactions in rock can achieve this, because radio waves can propagate essentially unattenuated through the hundreds of kilometers of atmosphere. Proposed arrays for radio detection of tau neutrinos focus on either arrays of inexpensive receivers distributed over a large area, the GRAND concept, or compact phased arrays on elevated mountains, the BEACON concept, to build up a large detector area with a low trigger threshold. We present a concept that combines the advantages of these two approaches with a trigger driven by phased arrays at a moderate altitude (1 km) and sparse, high-gain outrigger receivers for reconstruction and background rejection. We show that this design has enhanced sensitivity at 100 PeV over the two prior designs with fewer required antennas and discuss the need for optimized antenna designs., Comment: ARENA2024 Conference Proceeding PoS(ARENA2024)058

This special issue of the Astroparticle Physics journal is dedicated to the memory of Thomas Korff Gaisser, the Martin A. Pomerantz Professor Emeritus of Physics at the University of Delaware. Tom was one of the most prominent scientists in cosmic-ray and astroparticle physics, and also one of the founding editors of this very journal. A theoretical particle physicist by training, he dedicated his career to cosmic-ray physics. He worked on the phenomenology of high-energy particle interactions in the atmosphere, modeling extensive air showers, including the famous 'Gaisser-Hillas' function describing their longitudinal profile. A focus of his work were muon and neutrino fluxes resulting from cosmic-ray showers in the atmosphere, and their measurement with specialized experiments at the South Pole., Comment: Editorial of Special Issue in Astroparticle Physics journal in memory of Tom Gaisser. Link to Special Issue: https://www.sciencedirect.com/special-issue/10Q7TR3833C

The surface array of IceCube, IceTop, operates primarily as a cosmic-ray detector, as well as a veto for astrophysical neutrino searches for the IceCube in-ice instrumentation. However, the snow accumulation on top of the IceTop detectors increases the detection threshold and attenuates the measured IceTop signals. Enhancing IceTop by a hybrid array of scintillation detectors and radio antennas will lower the energy threshold for air-shower measurements, provide more efficient veto capabilities, enable more accurate cosmic-ray measurements, and improve the detector calibration by compensating for snow accumulation. After the initial commissioning period, a prototype station at the South Pole has been recording air-shower data and has successfully observed coincident events of both the scintillation detectors and the radio antennas with the IceTop array. The production and calibration of the detectors for the full planned array has been ongoing. Additionally, one station each has been installed at the Pierre Auger Observatory and the Telescope Array for further R\&D of these detectors in different environmental conditions. This contribution will present the status and future plans of the hybrid detector stations for the IceCube Surface Array Enhancement., Comment: Presented at the 38th International Cosmic Ray Conference (ICRC2023). See arXiv:2307.13047 for all IceCube contributions

Using CORSIKA simulations, we investigate the mass sensitivity of cosmic-ray air-shower observables for sites at the South Pole and Malarg\"ue, Argentina, the respective locations of the IceCube Neutrino Observatory and the Pierre Auger Observatory. Exact knowledge of observables from air-shower simulations was used to study the event-by-event mass separation between proton, helium, oxygen, and iron primary cosmic rays with a Fisher linear discriminant analysis. Dependencies on the observation site as well as the energy and zenith angle of the primary particle were studied in the ranges from $10^{16.0}-10^{18.5}\,$eV and $0^\circ$ to $60^\circ$: they are mostly weak and do not change the qualitative results. Promising proton-iron mass separation is achieved using combined knowledge of all studied observables, also when typical reconstruction uncertainties are accounted for. However, even with exact measurements, event-by-event separation of intermediate-mass nuclei is challenging and better methods than the Fisher discriminant and/or the inclusion of additional observables will be needed. As an individual observable, high-energy muons ($> 500\,$GeV) provide the best event-by-event mass discrimination, but the combination of muons of any energy and $X_{\text{max}}$ provides already a high event-by-event separation between proton-iron primaries at both sites. We also confirm that the asymmetry and width parameters of the air-shower longitudinal profile, $R$ and $L$, are mass sensitive. Only $R$ seems to be suitable for event-by-event mass separation, but $L$ can potentially be used to statistically determine the proton-helium ratio. Overall, our results motivate the coincident measurement of several air-shower observables, including at least $X_{\text{max}}$ and the sizes of the muonic and electromagnetic shower components, for the next generation of air-shower experiments.

Signal-to-noise ratios are a widely used concept for astroparticle radio detectors, such as air-shower radio arrays for cosmic-ray measurements or detectors searching for radio signals induced by neutrino interactions in ice. Nonetheless, no common standards or methods are established for the determination of the signal-to-noise ratio: values cannot be compared between experiments, and for the same signal and noise, various methods differ by large factors on the signal-to-noise ratio. This was the motivation to discuss a community-specific standardization at the ARENA conference 2022. No agreement on a common method to calculate signal-to-noise ratios was reached, however, awareness was raised that signal-to-noise ratios need to be well defined in any publications. This includes providing sufficient information on the procedure used to determine the signal-to-noise ratio, in addition to simply stating the formula. Even when using the same definition of the signal-to-noise ratio, there is still a significant dependence on the procedure of calculation, e.g., the signal-to-noise ratio of waveforms containing only background can vary significantly depending on the size of the time interval used as signal search window. To facilitate the interpretation of any signal-to-noise ratios in a specific study, the recommendation is to also state the mean value of the signal-to-noise ratio that the used method yields when applied to noise used in the study, e.g., the radio background measured by the corresponding experiment., Comment: Proceedings of ARENA 2022

IceCube-Gen2, the next generation of the IceCube Neutrino Observatory at the South Pole, will consist of three co-located arrays: a deep Optical Array and a more shallow and larger Radio Array for neutrino detection in the ice, and a Surface Array above the footprint of the Optical Array. The Surface Array will be comprised of hybrid stations featuring elevated radio antennas and scintillation detectors, following the design of a prototype station successfully operating at the South Pole since 2020. Besides providing a veto for neutrino detection, the Surface Array will make IceCube-Gen2 a unique laboratory for cosmic-ray air showers. Compared to the current IceCube detector with its IceTop surface array, the aperture for coincident air-shower measurements detected by both, the deep optical and surface arrays, will increase by about a factor of 30. In addition to particle physics questions, such as the production of PeV muons and neutrinos in prompt decays, these surface-deep coincidences will be used to target astrophysical questions of the most energetic Galactic cosmic rays. The combination of particle and radio measurements at the surface and high-energy muons measured in the ice promises unprecedented accuracy for the mass composition in the energy range of the presumed Galactic-to-extragalactic transition - complementing the multimessenger science case of IceCube-Gen2. This proceeding provides an overview of the IceCube-Gen2 Surface Array and, in particular, its radio component., Comment: Proceeding of ARENA 2022

Digital radio detection of cosmic-ray air showers has emerged as an alternative technique in high-energy astroparticle physics. Estimation of the detection efficiency of cosmic-ray radio arrays is one of the few remaining challenges regarding this technique. To address this problem, we developed a model based on the explicit probabilistic treatment of key elements of the radio technique for air showers: the footprint of the radio signal on ground, the detection of the signal in an individual antenna, and the detection criterion on the level of the entire array. The model allows for estimation of sky regions of full efficiency and can be used to compute the aperture of the array, which is essential to measure the absolute flux of cosmic rays. We also present a semi-analytical method that we apply to the generic model, to calculate the efficiency and aperture with high accuracy and reasonable calculation time. The model in this paper is applied to the Tunka-Rex array as example instrument and validated against Monte Carlo simulations. The validation shows that the model performs well, in particular, in the prediction of regions with full efficiency. It can thus be applied to other antenna arrays to facilitate the measurement of absolute cosmic-ray fluxes and to minimize a selection bias in cosmic-ray studies.

Digital radio arrays have become an effective tool to measure air showers at energies around and above 100 PeV. Compared to optical techniques, the radio technique is not restricted to clear nights. Thanks to recent progress on computational analysis techniques, radio arrays can provide an equally accurate measurement of the energy and the depth of the shower maximum. Stand-alone radio arrays offer an economic way towards huge apertures, e.g., for the search for ultra-high-energy neutrinos, but still require technical demonstration on large scales. Hybrid arrays combining radio antennas and particle detectors have already started to contribute to cosmic-ray physics in the energy range of the presumed Galactic-to-extragalactic transition. In particular, the combination of radio and muon detectors can pave a path to unprecedented accuracy for the mass composition of cosmic rays. This proceeding reviews recent developments regarding the radio technique and highlights selected running and planned antennas arrays, such as GCOS, GRAND, the SKA, the AugerPrime Upgrade of the Pierre Auger Observatory, and IceCube-Gen2., Comment: Proceedings of the 28th Epiphany Conference 2022

We use CoREAS simulations to study the ratio of geomagnetic and Askaryan radio emission from cosmic-ray air showers at the location of the South Pole. The fraction of Askaryan emission relative to the total emission is determined by the polarization of the radio signal at the time of its peak amplitude. We find that the relative Askaryan fraction has a radial dependence increasing with the distance from the shower axis -- with a plateau around the Cherenkov ring. We further find that the Askaryan fraction depends on shower parameters like zenith angle and the distance to the shower maximum. While these dependencies are in agreement with earlier studies, they have not yet been utilized to determine the depth of the shower maximum, $X_\mathrm{max}$, based on the Askaryan fraction. Fitting these dependencies with a polynomial model, we arrive at an alternative method to reconstruct $X_\mathrm{max}$ using a measurement of the Askaryan fraction and shower geometry as input. Depending on the measurement uncertainties of the Askaryan fraction, this method provides a measurement of $X_{\mathrm max}$ with a resolution similar to other methods of reconstructing $X_{\mathrm max}$ from radio observables. Although existing template methods give superior resolution, they are more computationally intensive. Consequently, the use of polarization to extract the Askaryan fraction of the radio signal should be considered as an additional input observable in future generations of template-fitting reconstruction and other multivariate approaches for measuring $X_{\mathrm max}$.

Cosmic rays are messengers from highly energetic events in the Universe. These rare ultra-high-energy particles can be detected efficiently and in an affordable way using large arrays of radio antennas. Linearly polarized geomagnetic emission is the dominant emission mechanism produced when charged particles in air showers get deflected in the Earth's magnetic field. The sub-dominant Askaryan emission is radially polarized and produced due to the time-varying negative-charge excess in the shower front. The relative amplitude of these two emission components depends on various air shower parameters, such as the arrival direction and the depth of the shower maximum. We studied these dependencies using CoREAS simulations of the radio emission from air showers at the South Pole using a star-shaped antenna layout. On the one hand, the parametrization of the Askaryan-to-geomagnetic ratio can be used as input for a more accurate reconstruction of the shower energy. On the other hand, if measured precisely enough, this ratio may provide a new method to reconstruct the atmospheric depth of the shower maximum., Comment: 37th International Cosmic Ray Conference (ICRC2021)

IceCube-Gen2, the extension of the IceCube Neutrino Observatory, will feature three main components: an optical array in the deep ice, a large-scale radio array in the shallow ice and firn, and a surface detector above the optical array. Thus, IceCube-Gen2 will not only be an excellent detector for PeV neutrinos, but also constitutes a unique setup for the measurement of cosmic-ray air showers, where the electromagnetic component and low-energy muons are measured at the surface and high-energy muons are measured in the ice. As for ongoing enhancement of IceCube's current surface array, IceTop, we foresee a combination of elevated scintillation and radio detectors for the Gen2 surface array, aiming at high measurement accuracy for air showers. The science goals are manifold: The in-situ measurement of the cosmic-ray flux and mass composition, as well as more thorough tests of hadronic interaction models, will improve the understanding of muons and atmospheric neutrinos detected in the ice, in particular, regarding prompt muons. Moreover, the surface array provides a cosmic-ray veto for the in-ice detector and contributes to the calibration of the optical and radio arrays. Last but not least, the surface array will make major contributions to cosmic-ray science in the energy range of the transition from Galactic to extragalactic sources. The increased sensitivities for photons and for cosmic-ray anisotropies at multi-PeV energies provide a chance to solve the puzzle of the origin of the most energetic Galactic cosmic rays and will serve IceCube's multimessenger mission.

This rapport summarizes the cosmic-ray indirect (CRI) session of the 36th ICRC conference. Updated measurements from several air-shower arrays with higher precision lead to the discovery of new features in the energy spectrum: HAWC measures a softening of the light component (p+He) around $10^{13.5}\,$eV; measurements of the Pierre Auger Observatory show that the second knee is a smooth feature extending at least over the range of $100-200\,$PeV and that the energy spectrum between the ankle and the cut-off cannot be described by a simple broken power law. Measurements of the mass composition confirm that the composition is a varying mixture of protons and nuclei at least up to several $10\,$EeV. A joint effort of several collaborations helps to better assess deficiencies of hadronic interaction models, e.g., by quantifying the muon deficit in the models over the shower energy. Anisotropy measurements with higher precision generally confirm earlier results. A change of the amplitude and phase of the equatorial dipole indicates that in the energy range between the second knee and the ankle there likely is a transition from Galactic to extragalactic sources. However, neither the most energetic Galatic nor the extragalactic sources have been discovered, which remains a primary science goal. Next to more exposure, an increase of measurement accuracy and decrease of systematic uncertainties will provide future progress. It is particularly exciting that new experiments are built and existing experiments upgraded to increase the measurement accuracy of the energy and mass composition, e.g., by combining radio antennas with particle detectors. Last but not least, there is a trend that experiments are designed such that they can target cosmic rays, photons, and neutrinos at the same time, which will facilitate multi-messenger astrophysics at the highest energies., Comment: CRI Rapporteur proceeding of the 36th ICRC in Madison, WI, USA

The upgrade of IceTop, the surface array of IceCube, by a hybrid array of scintillation and radio detectors is motivated by a rich science case. The scintillators will lower the threshold for the measurement of air showers to about 100 TeV, provide a more efficient veto of air showers for neutrino measurements, and improve the separation of the electromagnetic and muonic shower components due to the different responses of scintillators and ice-Cherenkov tanks. Furthermore, the scintillators will enable the calibration and compensation of the effect of snow accumulation above IceTop. The radio antennas will provide a calorimetric measurement of the electromagnetic shower component and a direct sensitivity to the shower maximum. Consequently, the combination of the existing ice-Cherenkov detectors in the ice and at the surface with the new scintillation and radio detectors at the surface will enable unprecedented accuracy for event-by-event mass classification in the PeV to EeV range. This will transform IceCube into the most accurate instrument for high-energy Galactic cosmic rays in the Southern Hemisphere. Hence, the hybrid array will make an important contribution to the main science case of IceCube of understanding the origin of cosmic rays. In addition to its cosmic-ray science goals, the hybrid array provides essential R&D for IceCube-Gen2 which will feature a larger surface array, sophisticated timing and communication technology, and elevated surface structures. Moreover, the hybrid array will improve the understanding of the atmospheric background to the neutrino measurements and opens new scientific opportunities, such as the searches for PeV gamma rays from the Galactic Center and for mass-dependent anisotropies., Comment: Presented at the 36th International Cosmic Ray Conference (ICRC 2019). See arXiv:1907.11699 for all IceCube contributions

Radio detection of air showers in the current era has progressed immensely to effectively extract the properties of these air showers. Primary cosmic rays with energies of hundreds of PeV have been successfully measured with the method of radio detection. There are also attempts to observe high-energy neutrinos with this technique. Current radio experiments measuring cosmic-ray air showers mostly operate in the frequency range of 30-80 MHz. An optimization of the frequency band of operation can be done for maximizing the signal-to-noise ratio that can be achieved by an array of radio antennas at the South Pole, operated along with IceTop. Such an array can improve the reconstruction of air showers performed with IceTop. The prospect of using such an optimized radio array for measuring gamma rays of PeV energies from the Galactic Center is discussed., Comment: To be published in the proceedings of the ARENA2018 conference

The muonic and electromagnetic components of air showers are sensitive to the mass of the primary cosmic particle. The sizes of the components can be measured with particle detectors on ground, and the electromagnetic component in addition indirectly via its radio emission in the atmosphere. The electromagnetic particles do not reach the ground for very inclined showers. On the contrary, the atmosphere is transparent for the radio emission and its footprint on ground increases with the zenith angle. Therefore, the radio technique offers a reliable detection over the full range of zenith angles, and in particular for inclined showers. In this work, the mass sensitivity of a combination of the radio emission with the muons is investigated in a case study for the site of the Pierre Auger Observatory using CORSIKA Monte Carlo simulations of showers in the EeV energy range. It is shown, that the radio-muon combination features superior mass separation power in particular for inclined showers, when compared to established mass observables such as a combination of muons and electrons or the shower maximum Xmax. Accurate measurements of the energy-dependent mass composition of ultra-high energy cosmic rays are essential to understand their still unknown origin. Thus, the combination of muon and radio detectors can enhance the scientific performance of future air-shower arrays and offers a promising upgrade option for existing arrays.

The origin of the highest energy Galactic cosmic rays is still not understood, nor is the transition to EeV extragalactic particles. Scientific progress requires enhancements of existing air-shower arrays, such as: IceCube with its surface detector IceTop, and the low-energy extensions of both the Telescope Array and the Pierre Auger Observatory., Comment: Science White Paper for the Astronomy and Astrophysics Decadal Survey (Astro2020)

Antenna arrays are beginning to make important contributions to high energy astroparticle physics supported by recent progress in the radio technique for air showers. This article provides an update to my more extensive review published in Prog. Part. Nucl. Phys. 93 (2017) 1 [arXiv: 1607.08781]. It focuses on current and planned radio arrays for atmospheric particle cascades, and briefly references to a number of evolving prototype experiments in other media, such as ice. While becoming a standard technique for cosmic-ray nuclei today, in future radio detection may drive the field for all type of primary messengers at PeV and EeV energies, including photons and neutrinos. In cosmic-ray physics accuracy becomes increasingly important in addition to high statistics. Various antenna arrays have demonstrated that they can compete in accuracy for the arrival direction, energy and position of the shower maximum with traditional techniques. The combination of antennas and particles detectors in one array is a straight forward way to push the total accuracy for high-energy cosmic rays for low additional cost. In particular the combination of radio and muon detectors will not only enhance the accuracy for the cosmic-ray mass composition, but also increase the gamma-hadron separation and facilitate the search for PeV and EeV photons. Finally, the radio technique can be scaled to large areas providing the huge apertures needed for ultra-high-energy neutrino astronomy., Comment: Proceedings of the 20th ISVHECRI 2018, Nagoya, Japan (to appear in EPJ WoC)

A surface array of radio antennas will enhance the performance of the IceTop array and enable new, complementary science goals. First, the accuracy for cosmic-ray air showers will be increased since the radio array provides a calorimetric measurement of the electromagnetic component and is sensitive to the position of the shower maximum. This enhanced accuracy can be used to better measure the mass composition, to search for possible mass-dependent anisotropies in the arrival directions of cosmic rays, and for more thorough tests of hadronic interaction models. Second, the sensitivity of the radio array to inclined showers will increase the sky coverage for cosmic-ray measurements. Third, the radio array can be used to search for PeV photons from the Galactic Center. Since IceTop is planned to be enhanced by a scintillator array in the near future, a radio extension sharing the same infrastructure can be installed with minimal additional effort and excellent scientific prospects. The combination of ice-Cherenkov, scintillation, and radio detectors at IceCube will provide unprecedented accuracy for the study of highenergy Galactic cosmic rays., Comment: Proceedings of 8th ARENA 2018, to appear in EPJ WoC

The Giant Radio Array for Neutrino Detection (GRAND) is a planned large-scale observatory of ultra-high-energy (UHE) cosmic particles, with energies exceeding 10^8 GeV. Its goal is to solve the long-standing mystery of the origin of UHE cosmic rays. To do this, GRAND will detect an unprecedented number of UHE cosmic rays and search for the undiscovered UHE neutrinos and gamma rays associated to them with unmatched sensitivity. GRAND will use large arrays of antennas to detect the radio emission coming from extensive air showers initiated by UHE particles in the atmosphere. Its design is modular: 20 separate, independent sub-arrays, each of 10 000 radio antennas deployed over 10 000 km^2. A staged construction plan will validate key detection techniques while achieving important science goals early. Here we present the science goals, detection strategy, preliminary design, performance goals, and construction plans for GRAND., Comment: 45 pages, 34 figures. Matches published version

Tunka-Rex is a $3\,$km$^2$ large antenna array for cosmic-ray air showers at the TAIGA observatory in Siberia close to Lake Baikal. In autumn 2016 Tunka-Rex has been extended to a total of 63 stations, each equipped with SALLA antennas for two polarization directions operating in the band of $30-80\,$MHz. All antenna stations are triggered by the other cosmic-ray detectors at the site, i.e., Tunka-Rex runs in coincidence with the Tunka-Grande array of scintillation detectors and during clear nights additionally in coincidence with the Tunka-133 non-imaging air-Cherenkov array. Compared to previous measurements, the day-time and bad-weather trigger increases the annual exposure by an order of magnitude, and the higher density of antennas is expected to increase the accuracy - in particular for the cosmic-ray mass composition. In addition to the status and potential of Tunka-Rex an overview of recently published results will be presented: by comparing the radio and the air-Cherenkov signals of the same air showers, the Tunka-Rex precision for the most important shower parameters has been determined. The direction precision is about $1^\circ$, the precision for the depth of the shower maximum, $X_\mathrm{max}$, is better than $40\,$g/cm$^2$ for high-quality events, and the energy precision is better than $15\,\%$. A rough estimation of the shower energy and of $X_\mathrm{max}$ is even possible with only a single antenna station when using the shower geometry measured by Tunka-133. Moreover, by using exactly the same external calibration source for the antennas as at the LOPES experiment we have shown that the absolute energy scale of different air-shower arrays can be compared with each other at $10\,\%$ precision. Thus, Tunka-Rex will improve the total accuracy of TAIGA for the energy as well as for the mass composition of cosmic rays of energy above $0.1\,$EeV., Comment: Proceedings of the 35th ICRC 2017, Busan, Korea

LOPES was the radio extension of the KASCADE-Grande particle-detector array consisting of up to 30 antennas measuring the radio emission of cosmic-ray air showers between 40 and 80 MHz with an energy threshold of around 100 PeV. Even with the external trigger by the particle detectors, the separation of the air-shower signal from the radio background was difficult in the noisy environment of the Karlsruhe Institute of Technology. For the typical event this was only possible because of the digital, interferometric beamforming technique pioneered by LOPES for cosmic-ray detection. Using this technique LOPES made important discoveries with respect to the radio emission of air showers and their relation to the shower properties, such as its energy and its longitudinal development. By now, practically all results have been confirmed by subsequent antenna arrays, but regarding digital interferometry the LOPES results are still unique. Lately we completed an end-to-end pipeline for CoREAS simulations of the radio emission including measured background and all known detector responses as well as the interferometric analysis technique. As result we present an update on the reconstruction of the most important shower parameters: arrival direction, energy, and $X_\mathrm{max}$., Comment: Proceedings of the 35th ICRC 2017, Busan, Korea

Extensive air showers still are our only access to the highest-energy particles in the universe, namely cosmic-ray nuclei with energies up to several 100 EeV. Studying open questions in cosmic-ray physics, like their yet unknown origin requires the reconstruction of the energy and mass of the primary particles from the air-shower measurements. Great progress has been achieved lately in the development of the radio detection technique for this purpose. There now is a consistent picture of the mechanisms behind the radio emission, which is in agreement with measurements. Several second-generation, digital antenna arrays are operating in different parts of the world not only aiming at the further development of the technique, but also contributing to cosmic-ray physics at energies above 100 PeV. Recently it has been demonstrated experimentally that radio detection can compete in precision with established techniques for air showers, like the measurement of secondary particles on ground, or fluorescence and Cherenkov light emitted by air showers. Consequently, cosmic-ray observatories can benefit from radio extensions to maximize their total measurement accuracy., Comment: Proceedings of VULCANO Workshop 2016, ISBN 978-88-86409-65-0

Detection of the mostly geomagnetically generated radio emission of cosmic-ray air showers provides an alternative to air-Cherenkov and air-fluorescence detection, since it is not limited to clear nights. Like these established methods, the radio signal is sensitive to the calorimetric energy and the position of the maximum of the electromagnetic shower component. This makes antenna arrays an ideal extension for particle-detector arrays above a threshold energy of about 100 PeV of the primary cosmic-ray particles. In the last few years the digital radio technique for cosmic-ray air showers again made significant progress, and there now is a consistent picture of the emission mechanisms confirmed by several measurements. Recent results by the antenna arrays AERA and Tunka-Rex confirm that the absolute accuracy for the shower energy is as good as the other detection techniques. Moreover, the sensitivity to the shower maximum of the radio signal has been confirmed in direct comparison to air-Cherenkov measurements by Tunka-Rex. The dense antenna array LOFAR can already compete with the established techniques in accuracy for cosmic-ray mass-composition. In the future, a new generation of radio experiments might drive the field: either by providing extremely large exposure for inclined cosmic-ray or neutrino showers or, like the SKA core in Australia with its several 10,000 antennas, by providing extremely detailed measurements., Comment: XXV ECRS 2016 Proceedings - eConf C16-09-04.3

This proceeding gives a summary of the current status and open questions of the radio technique for cosmic-ray air showers, assuming that the reader is already familiar with the principles. It includes recent results of selected experiments not present at this conference, e.g., LOPES and TREND. Current radio arrays like AERA or Tunka-Rex have demonstrated that areas of several km^2 can be instrumented for reasonable costs with antenna spacings of the order of 200 m. For the energy of the primary particle such sparse antenna arrays can already compete in absolute accuracy with other precise techniques, like the detection of air-fluorescence or air-Cherenkov light. With further improvements in the antenna calibration, the radio detection might become even more accurate. For the atmospheric depth of the shower maximum, Xmax, currently only the dense array LOFAR features a precision similar to the fluorescence technique, but analysis methods for the radio measurement of Xmax are still under development. Moreover, the combination of radio and muon measurements is expected to increase the accuracy of the mass composition, and this around-the-clock recording is not limited to clear nights as are the light-detection methods. Consequently, radio antennas will be a valuable add-on for any air shower array targeting the energy range above 100 PeV., Comment: Proceedings of ARENA 2016

As of 2023, the low-frequency part of the Square Kilometre Array will go online in Australia. It will constitute the largest and most powerful low-frequency radio-astronomical observatory to date, and will facilitate a rich science programme in astronomy and astrophysics. With modest engineering changes, it will also be able to measure cosmic rays via the radio emission from extensive air showers. The extreme antenna density and the homogeneous coverage provided by more than 60,000 antennas within an area of one km$^2$ will push radio detection of cosmic rays in the energy range around 10$^{17}$ eV to ultimate precision, with superior capabilities in the reconstruction of arrival direction, energy, and an expected depth-of-shower-maximum resolution of 6~g/cm${^2}$., Comment: To be published in the Proceedings of the ARENA2016 conference, Groningen, The Netherlands; replaced with final published version in v3

This review provides an introduction to the radio emission by particle cascades, an overview on the various experiments, and explains methods for the radio measurement of air-shower properties. Furthermore, potential applications of the radio technique in high-energy astroparticle physics are discussed. Due to the successful operation of digital radio experiments and due to the improved quantitative understanding of the emission, radio detection is back on the list of promising techniques for extensive air showers. With a threshold of about 100 PeV radio detectors are particularly useful to study the highest-energy galactic cosmic rays and ultra-high-energy extragalactic particles of all types. Various antenna arrays like LOPES, CODALEMA, AERA, LOFAR, and Tunka-Rex have shown that radio measurements can compete in precision with other techniques, in particular for the arrival direction, the energy, and the position of the shower maximum. The scientific potential of the radio technique seems to be maximum in combination with particle detectors, which increases the total accuracy for air-shower measurements. This is crucial for a better separation of different primary particles, like gamma-ray photons, neutrinos, or different types of nuclei. In addition to air-showers the radio technique can be used for particle cascades in dense media. Several pioneering experiments like ARA, ARIANNA, and ANITA are currently searching for cascades induced by ultra-high-energy neutrinos in ice. Moreover, several future projects aim at both, high-energy cosmic-rays and neutrinos. SKA will search for neutrino and cosmic-ray initiated cascades in the lunar regolith and provide unprecedented detail for air-shower measurements. Finally, radio detectors with huge exposure like GRAND, SWORD, or EVA are being considered to study the highest energy cosmic rays and neutrinos., Comment: review article invited and published by "Progress in Particle and Nuclear Physics (ELSEVIER)"; 97 pages

We measure the energy emitted by extensive air showers in the form of radio emission in the frequency range from 30 to 80 MHz. Exploiting the accurate energy scale of the Pierre Auger Observatory, we obtain a radiation energy of 15.8 \pm 0.7 (stat) \pm 6.7 (sys) MeV for cosmic rays with an energy of 1 EeV arriving perpendicularly to a geomagnetic field of 0.24 G, scaling quadratically with the cosmic-ray energy. A comparison with predictions from state-of-the-art first-principle calculations shows agreement with our measurement. The radiation energy provides direct access to the calorimetric energy in the electromagnetic cascade of extensive air showers. Comparison with our result thus allows the direct calibration of any cosmic-ray radio detector against the well-established energy scale of the Pierre Auger Observatory., Comment: Replaced with published version. Added journal reference and DOI. Supplemental material in the ancillary files

The Auger Engineering Radio Array (AERA) is an enhancement of the Pierre Auger Observatory in Argentina. Covering about View the $17\,$km$^2$, AERA is the world-largest antenna array for cosmic-ray observation. It consists of more than 150 antenna stations detecting the radio signal emitted by air showers, i.e., cascades of secondary particles caused by primary cosmic rays hitting the atmosphere. At the beginning, technical goals had been in focus: first of all, the successful demonstration that a large-scale antenna array consisting of autonomous stations is feasible. Moreover, techniques for calibration of the antennas and time calibration of the array have been developed, as well as special software for the data analysis. Meanwhile physics goals come into focus. At the Pierre Auger Observatory air showers are simultaneously detected by several detector systems, in particular water-Cherenkov detectors at the surface, underground muon detectors, and fluorescence telescopes, which enables cross-calibration of different detection techniques. For the direction and energy of air showers, the precision achieved by AERA is already competitive; for the type of primary particle, several methods are tested and optimized. By combining AERA with the particle detectors we aim for a better understanding of cosmic rays in the energy range from approximately $0.3$ to $10\,$EeV, i.e., significantly higher energies than preceding radio arrays., Comment: Proceedings of PISA 2015 (published in Nucl. Instr. Meth A)

The Auger Engineering Radio Array (AERA) is part of the Pierre Auger Observatory and is used to detect the radio emission of cosmic-ray air showers. These observations are compared to the data of the surface detector stations of the Observatory, which provide well-calibrated information on the cosmic-ray energies and arrival directions. The response of the radio stations in the 30 to 80 MHz regime has been thoroughly calibrated to enable the reconstruction of the incoming electric field. For the latter, the energy deposit per area is determined from the radio pulses at each observer position and is interpolated using a two-dimensional function that takes into account signal asymmetries due to interference between the geomagnetic and charge-excess emission components. The spatial integral over the signal distribution gives a direct measurement of the energy transferred from the primary cosmic ray into radio emission in the AERA frequency range. We measure 15.8 MeV of radiation energy for a 1 EeV air shower arriving perpendicularly to the geomagnetic field. This radiation energy -- corrected for geometrical effects -- is used as a cosmic-ray energy estimator. Performing an absolute energy calibration against the surface-detector information, we observe that this radio-energy estimator scales quadratically with the cosmic-ray energy as expected for coherent emission. We find an energy resolution of the radio reconstruction of 22% for the data set and 17% for a high-quality subset containing only events with at least five radio stations with signal., Comment: Replaced with published version. Added journal reference and DOI

Neutrinos in the cosmic ray flux with energies near 1 EeV and above are detectable with the Surface Detector array of the Pierre Auger Observatory. We report here on searches through Auger data from 1 January 2004 until 20 June 2013. No neutrino candidates were found, yielding a limit to the diffuse flux of ultra-high energy neutrinos that challenges the Waxman-Bahcall bound predictions. Neutrino identification is attempted using the broad time-structure of the signals expected in the SD stations, and is efficiently done for neutrinos of all flavors interacting in the atmosphere at large zenith angles, as well as for "Earth-skimming" neutrino interactions in the case of tau neutrinos. In this paper the searches for downward-going neutrinos in the zenith angle bins $60^\circ-75^\circ$ and $75^\circ-90^\circ$ as well as for upward-going neutrinos, are combined to give a single limit. The $90\%$ C.L. single-flavor limit to the diffuse flux of ultra-high energy neutrinos with an $E^{-2}$ spectrum in the energy range $1.0 \times 10^{17}$ eV - $2.5 \times 10^{19}$ eV is $E_\nu^2 dN_\nu/dE_\nu < 6.4 \times 10^{-9}~ {\rm GeV~ cm^{-2}~ s^{-1}~ sr^{-1}}$., Comment: Replaced with published version. Added journal reference and DOI

A measurement of the cosmic-ray spectrum for energies exceeding $4{\times}10^{18}$ eV is presented, which is based on the analysis of showers with zenith angles greater than $60^{\circ}$ detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above $5.3{\times}10^{18}$ eV, the "ankle", the flux can be described by a power law $E^{-\gamma}$ with index $\gamma=2.70 \pm 0.02 \,\text{(stat)} \pm 0.1\,\text{(sys)}$ followed by a smooth suppression region. For the energy ($E_\text{s}$) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find $E_\text{s}=(5.12\pm0.25\,\text{(stat)}^{+1.0}_{-1.2}\,\text{(sys)}){\times}10^{19}$ eV., Comment: Replaced with published version. Added journal reference and DOI

Contributions of the Pierre Auger Collaboration to the 33rd International Cosmic Ray Conference, Rio de Janeiro, Brazil, July 2013, Comment: Contributions to the 33rd International Cosmic Ray Conference, Rio de Janeiro, Brazil, July 2013

LOPES is a digitally read out antenna array consisting of 30 calibrated dipole antennas. It is located at the site of the KASCADE-Grande experiment at Forschungszentrum Karlsruhe and measures the radio emission of cosmic ray air showers in the frequency band from 40 to 80 MHz. LOPES is triggered by KASCADE and uses the KASCADE reconstruction of the shower axis as an input for the analysis of the radio pulses. Thereby LOPES works as an interferometer when the signal of all antennas is digitally merged to form a beam into the shower direction. To be sensitive to the coherence of the radio signal, a precise time calibration with an accuracy in the order of 1 ns is required. Thus, it is necessary to know the delay of each antenna which is time and frequency dependent. Several calibration measurements are performed to correct for this delay in the analysis: The group delay of every antenna is measured regularly (roughly once per year) by recording a test pulse which is emitted at a known time. Furthermore, the delay is monitored continuously by the so called phase calibration method: A beacon (a dipole antenna) emits continuously two sine waves at 63.5 MHz and 68.1 MHz. By that a variation of the delay can be detected in a subsequent analysis of the radio events as a change of the phase at these frequencies. Finally, the dispersion of the analog electronics has been measured to account for the frequency dependence of the delay., Comment: 4 pages, Proceedings of the 31st ICRC, Lodz 2009