Polarization measurements of radio emission of cosmic ray air showers with LOPES

Abstract

It has been recently definitively established that the development of Extensive Air Showers (EAS) which are induced in the Earth's atmosphere by impinging cosmic particles from the outermost space is accompanied by the emission of radio waves. This phenomenon is experimentally investigated by the LOPES experiment, co-located at Karlsruhe Institute of Technology (KIT) with the EAS detector array KASCADE-Grande using traditional detection techniques. The LOPES experiment is an absolutely amplitude calibrated array of radio antennae for observing radio waves from EAS in the frequency range of 40-80~MHz. The KASCADE-Grande array provides the trigger information and experimentally determined parameters of the associated EAS observed in the energy range of $10^{16}-10^{18}$~eV. The studies are focussed to understand and clarify the phenomena of EAS radio emission, in particular in view of an eventual large scale application of a corresponding detection technique, like in LOFAR (Low Frequency Array for which LOPES is a prototype station) and for the Pierre Auger Observatory. Until summer 2006, all 30 antennas were equipped in the east-west polarization direction only, measuring a single polarization of the radio emission. The data of this LOPES set-up provided the possibility for a detailed investigation of correlations of the radio signal with basic EAS parameters like arrival direction, particle energy and mass of the primary. The results enable studies of the radio signal on a single EAS event basis, in particular of its lateral extension. Nevertheless, the north-south polarization component is required for an improved understanding of the radio emission signal and for a verification of the geo-synchrotron effect as the dominant mechanism of radio emission in air showers. The focus of the present work are measurements and analyses of the polarization of the radio signals from EAS. Investigations of the radio pulse height in correlation with EAS parameters, including an adequate parametrization of the pulse height per single polarization in east-west and north-south direction, respectively, have been performed. The studies of the lateral distribution per single event and per single polarization of the electric field are helpful for an improved understanding of the shower development. The polarization vector was finally reconstructed by observations with dual polarization antennae which are configured for both polarizations of signals recorded at the same place and simultaneously for the east-west and north-south direction. Comparisons with theoretical studies complete the investigations. The geo-synchrotron emission process is confirmed as the radio emission mechanism in cosmic ray air showers. In addition, it could be found that a not negligib le contribution to the total signal stems from another emission process. It is most probably induced by the negative charge excess during the shower development. This contribution mainly modifies the North-South polarization component of the dominant geo-synchrotron induced signal.