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AIMS: We want to understand the emission mechanism of radio emission from airshowers to determine the origin of high-energy cosmic rays. Therefore, we studythe geometry of the air shower radio emission measured with LOPES and searchfor systematic effects between the direction determined on the radio signal andthe direction provided by the particle detector array KASCADE. METHODS: Weproduce 4D radio images on time-scales of nanoseconds using digitalbeam-forming. Each pixel of the image is calculated for three spatialdimensions and as a function of time. The third spatial dimension is obtainedby calculating the beam focus for a range of curvature radii fitted to thesignal wave front. We search this multi-dimensional parameter space for thedirection of maximum coherence of the air shower radio signal and compare it tothe direction provided by KASCADE. RESULTS: The maximum radio emission of airshowers is obtained for curvature radii being larger than 3 km. We find thatthe direction of the emission maximum can change when optimizing the curvatureradius. Furthermore, we find a tentative increase of the curvature radius tolower elevations, where the air showers pass through a larger atmosphericdepth. CONCLUSIONS: We conclude that the angular resolution of LOPES issufficient to determine the direction which maximizes the observed electricfield amplitude. However, the statistical uncertainty of the directions is notdetermined by the resolution of LOPES, but by the uncertainty of the curvatureradius. We do not find any systematic deviation between the directionsdetermined from the radio signal and from the detected particles. This resultplaces a strong supportive argument for the use of the radio technique to studythe origin of high-energy cosmic rays.

Direction identification in radio images of cosmic-ray air showers detected with LOPES and KASCADE