  The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10^20 eV
remains a mystery. 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 sufficient powerful accelerators within this
distance from the Galaxy, explanations for UHECRs range from unusual
astrophysical sources to exotic string physics. Also unclear is whether UHECRs
consist of protons, heavy nuclei, neutrinos or gamma-rays. To resolve these
questions, larger detectors with higher duty cycles and which combine multiple
detection techniques 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 effect. 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
mixing.
