| The existence of cosmic rays (UHECR) observed at energies of 1 joule and beyond
(>Â 1019Â eV) lies among the more challenging questions of contemporary particle physics
and astrophysics.
But their observation is difficult and only indirect. High-energy cosmic rays passing
through the Earth’s atmosphere produce extensive showers (EAS) currently being studied by
using giant ground-based particle detectors, whereas additional information can
also be provided by observing fluorescence and Cerenkov radiation in the optical
range.
Another observable feature of EAS, not accessed by previous methods, is the electric field
created by charged secondary particles of the shower. This field is detectable in the decametre
radio wave range with dedicated antennas as very fast pulses (up to tens of nanoseconds)
while secondaries propagate down to the ground.
Based on the renewal of this 1960’s idea (then abandoned for technical reasons), new
researches (since 2002) have permitted to develop a new operational method of detecting and
studying UHECR air showers. As an illustration, we describe the CODALEMA experiment,
operated at Nançay (France) around the Nançay Decameter Array, and discuss the most
significant results obtained so far. The radio technique appears to be an useful complement of
the particle counting at the ground and, by giving a way to follow the shower developments
both in spatial and temporal domains, to potentially provide unique information on the
primary particle.
Among specific methods developed for UHECR study, several are of general interest for
Radio Astronomy, namely: development of wideband active antennas, mitigation of ultra fast
RFI, methodology for studying transient phenomena (both of astrophysical or geophysical
origins). |