Radio detection of cosmic ray air showers is a technique that went
through a fast development in the last years. The low attenuation of
radio waves, high duty cycle of radio antennas and good angular
resolution make the technique an attractive option for stand-alone or
LOFAR (low frequency array) is a large network of radio antennas operating at frequencies below 250 MHz, currently being build in the Netherlands. It consists of 44 stations, containing 144 antennas each, which are connected by a wide area fiber network. It will be used for research on a variety of astrophysical topics, one of which is the detection of cosmic particle showers.
Particle showers can emit radio emission by different mechanisms. Inside dense media like water, ice or rock, the negative charge excess of the shower travels faster than the local speed of light and emits Cherenkov radiation. In the Earth's atmosphere the main driver of the radio emission is the geomagnetic field that separates the electrons and the positrons.
LOFAR covers a large range of cosmic rays energies. It is sensitive to the radio signal of air showers from 10^15 eV to 10^19 eV. Alternatively, the array can search for short radio flashes from the Moon, created by particle cascades below the lunar surface. In this mode, LOFAR is sensitive to cosmic rays and neutrinos above 1021 eV.
In this talk I will focus on two LOFAR pilot projects. With LOPES (lofar prototype station), an array of 30 LOFAR antennes co-located with the KASCADE air shower array, the key characteristics of the radio emission of air showers have been determined, providing proof that the technique can produce reliable measurements. In the NuMoon project, the Westerbork Synthesis Radio Telescope was used to search for short radio flashes from the Moon. This resulted in a limit on the neutrino flux at energies above 10^22 eV that is about an order of magnitude below existing limits.