Radio pulses emitted in the Atmosphere during the air shower development of high-energy primary cosmic rays were measured during the late 1960ies in the frequency range from 2 MHz to 520 MHz. Mainly due to difficulties with radio interference these measurements ceased in the late 1970ies.
LOFAR (Low Frequency Array) is a new digital radio interferometer under development. Using high bandwidth ADCs and fast data processing it it will be able to filter out most of the interference. By storing the whole waveform information in digital form one can analyze transient events like air showers even after they have been recorded.
To test this new technology and to demonstrate its ability to measure air showers a "LOFAR Prototype Station" (LOPES) is set up to operate in conjunction with an existing air shower array (KASCADE-Grande).
The first phase consisting of 10 antennas is already running. It operates in the frequency range of 40 to 80 MHz, using simple short dipole antennas and direct 2nd Nyquist sampling of the incoming wave.
It has proven to be able to do simple astronomical measurements, like imaging of a solar burst. It has also demonstrated how digital interference suppression and beamforming can overcome the problem of
radio interference and pick out air shower events.
The main aim of the KASCADE extensive air shower (EAS) experiment is the determination of the chemical composition of cosmic rays in the energy range around and above the knee at Ek ≈ 3 PeV. A large number of observables are measured simultaneously for each individual event, by the combination of various detection techniques for the electromagnetic, the muonic, and the hadronic component of the extensive air showers. Detailed investigations have been performed with the data measured by the KASCADE experiment since the start of data taking at the end of 1995. The results allow to evaluate hadronic interaction models, used in simulations to interpret air shower data. The all-particle spectrum of cosmic rays and their mass composition, as well as individual spectra for groups of elements have been reconstructed in the energy range between 1015 and 1017 eV . The results suggest, the knee in the all-particle cosmic-ray energy spectrum is caused by a rigidity-dependent cut-off of individual element groups. To improve the statistics around 1017 eV, where the “iron knee” in the cosmic ray spectrum is indicated in our data, the KASCADE experiment has recently been extended to KASCADE-Grande by a large collecting area (0.5km2) electromagnetic array, contributed from the former EAS-TOP experiment. The Grande part will cover the primary energy range 1016 eV<E0<1018 eV, overlapping with KASCADE around 1016 eV, thus providing a continuous information from 3•1014 eV to 1018 eV. In addition, new technologies in detecting radio emission from cosmic ray air showers will be tested by an array of antennas (LOPES project) at the site of KASCADE.