Experimental evidence for the sensitivity of the air-shower radio signal   to the longitudinal shower development
  We observe a correlation between the slope of radio lateral distributions, and the mean muon pseudorapidity of 59 individual cosmic-ray-air-shower events. The radio lateral distributions are measured with LOPES, a digital radio interferometer co-located with the multi-detector-air-shower array KASCADE-Grande, which includes a muon-tracking detector. The result proves experimentally that radio measurements are sensitive to the longitudinal development of cosmic-ray air-showers. This is one of the main prerequisites for using radio arrays for ultra-high-energy particle physics and astrophysics. 
W.D. Apel, J.C. Arteaga, L. B\"ahren, K. Bekk, M. Bertaina, P.L.   Biermann, J. Bl\"umer, H. Bozdog, I.M. Brancus, P. Buchho...
astro-ph.IM astro-ph.HE