The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >1020 eV remains a mystery1. They are likely to be of extragalactic origin, but should be absorbed within ∼50 Mpc through interactions with the cosmic microwave background. As there are no sufficiently powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics2. Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or γ-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques3 are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect4,5,6,7,8. Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing9,10.
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A.F.B. is on leave of absence from the National Institute of Physics and Nuclear Engineering, Bucharest; T.H. is now at the Institut für Kernphysik, Forschungszentrum Karlsruhe, Karlsruhe; S. Ostapchenko is on leave of absence from Moscow State University, Moscow. LOPES was supported by the German Federal Ministry of Education and Research (Verbundforschung Astroteilchenphysik). This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The KASCADE experiment is supported by the German Federal Ministry of Education and Research. The Polish group is supported by KBN; the Romanian group acknowledge support from the Romanian Ministry of Education and Research.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Layout of the LOPES and KASCADE array. (DOC 93 kb)
Radio pulse as a function of time. (DOC 264 kb)
Dependence of radio signal on geomagnetic angle. (DOC 53 kb)
Time lapsed video of the radio emission of an air shower. (MPG 314 kb)
Legend to accompany the above Supplementary Video. (DOC 23 kb)
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Falcke, H., Apel, W., Badea, A. et al. Detection and imaging of atmospheric radio flashes from cosmic ray air showers. Nature 435, 313–316 (2005). https://doi.org/10.1038/nature03614
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