Radio detection of cosmic rays with LOFAR

When high-energy cosmic rays (ionized atomic nuclei) impinge on the atmosphere of the Earth they interact with atomic nuclei and initiate cascades of secondary particles - the extensive air showers. Many of the secondary particles in the air showers are electrons and positrons. They cause radiation in the frequency range of tens of MHz. The LOFAR radio telescope detects this radiation in the frequency range 30 to 240 MHz. LOFAR has a high antenna density and good time resolution. In turn, the properties of the radio emission are measured in detail. The properties of the shower-inducing cosmic rays are derived from the air shower measurements, namely their direction, energy, and particle type (atomic mass). The uncertainties achieved are competative to established techniques. This demonstrates that the radio technique is now a standard tool to measure extensive air showers and to study the properties of the incoming cosmic rays. The mean logarithmic mass of cosmic rays as measured with LOFAR is derived as a function of energy. In an examplary study, these data are used to show that the radio measurements of air showers are now in a state to discriminate astrophysical models of the origin of cosmic rays.