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AIMS: We wish to study the spectral dependence of the radio emission fromcosmic-ray air showers around 100 PeV (1017 eV). METHODS: We observe shortradio pulses in a broad frequency band with the dipole-interferometer LOPES(LOFAR Prototype Station), which is triggered by a particle detector arraynamed Karlsruhe Shower Core and Array Detector (KASCADE). LOFAR is the LowFrequency Array. For this analysis, 23 strong air shower events are selectedusing parameters from KASCADE. RESULTS: The resulting electric field spectrafall off to higher frequencies. An average electric field spectrum is fittedwith an exponential, or alternatively, with a power law. The spectral slopeobtained is not consistent within uncertainties and it is slightly steeper thanthe slope obtained from Monte Carlo simulations based on air showers simulatedwith CORSIKA (Cosmic Ray Simulations for KASCADE). One of the strongest eventswas measured during thunderstorm activity in the vicinity of LOPES and showsthe longest pulse length measured of 110 ns and a spectral slope of -3.6.CONCLUSIONS: We show with two different methods that frequency spectra from airshower radio emission can be reconstructed on event-by-event basis, with onlytwo dozen dipole antennae simultaneously over a broad range of frequencies.According to the obtained spectral slopes, the maximum power is emitted below40 MHz. Furthermore, the decrease in power to higher frequencies indicates aloss in coherence determined by the shower disc thickness. We conclude that abroader bandwidth, larger collecting area, and longer baselines, as will beprovided by LOFAR, are necessary to further investigate the relation of thecoherence, pulse length, and spectral slope of cosmic ray air showers.

Frequency spectra of cosmic ray air shower radio emission measured with LOPES