A Comparison of LOPES Lateral Distributions of the Air-shower Radio Signal with REAS 3.11 and CoREAS Simulations

We compare radio lateral distributions measured with LOPESto REAS 3.11 and CoREAS simulations of the radio emission. These simulation codes describe the measured radio signal significantly better than previous versions of REAS, which did not yet include the refractive index of air. The refractive index changes the coherence conditionsof the radio emission. This causes flatter lateral distributions at LOPESdistances (up to a few 100 m). In a few events the amplitude even falls towards the shower axis: a behavior which we observe both in simulations and measurements. Generally, REAS 3.11 and CoREAS can reproduce the measured slope of the lateral distributions within the uncertainties. With respect to the absolute amplitude of the radio signal, however, there is a difference between REAS 3.11 and CoREAS. The amplitude predicted by REAS 3.11 is approximately twice as large as the one predicted by CoREAS in frequency range (43 74 MHz) of LOPES, and REAS 3.11 is closer to the LOPESmeasurements. Overall, the comparison shows that the understanding of the radio emission has clearly advanced in the last years. It confirms that in addition to the dominant geomagnetic and the sub-dominant Askaryan effect(charge excess variation) the refractive index of the air plays an important role.