LOPES — Recent Results and Open Questions on the Radio Detection of Air Showers

LOPESwas a digital antenna arrayoperating for approximately 10 years until spring 2013 at the Karlsruhe Institute of Technology (KIT). Triggered by the co-locatedKASCADE-Grande air-showerexperiment, it measured the radio signal of around 1000 cosmic-ray air showerswith energies E 1017 eV in an effective band of 43 – 74 MHz. Using the interferometric technique of cross-correlation beamforming, LOPEScould reconstruct the showerdirection with an accuracy < 0.7°, the showerenergy with a precision < 20%, and the atmospheric depth of the showermaximum, Xmax, with a precision < 95g/cm2. In particular the reconstruction of the showermaximum suffers from significant measurement uncertaintiesdue to the radio-loud environment of the site. This article summarizes our latest results on the reconstruction of the showermaximum, using two independent methods: the steepness of the hyperbolic radio wavefront and the slope of the lateral distribution of the radio amplitude. Moreover, we show vectorial measurements of the electric field with the tripole antennas of the latest LOPESsetup. Finally, we discuss open questions as well as the potential impact of the lessons learned at LOPESfor future antenna arrays.