Amplified radio emission from cosmic ray air showers in thunderstorms

Context. The detection of radio pulses from cosmic ray air showers is a potentially powerful new detection mechanism for studying spectrum and composition of ultra high energy cosmic rays that needs to be understood in greater detail. The radiation consists in large part of geosynchrotron radiation. The intensity of this radiation depends, among other factors, on the energy of the primary particle and the angle of the shower axis with respect to the geomagnetic field. Aims. Since the radiation mechanism is based on particle acceleration, the atmospheric electric field can play an important role. Especially inside thunderclouds large electric fields can be present. In this paper we examine the contribution of an electric field to the emission mechanism theoretically and experimentally. Methods. Two mechanisms of amplification of radio emission are considered: the acceleration radiation of the shower particles and the radiation from the current that is produced by ionization electrons moving in the electric field. For both mechanisms analytical estimates are made of their effects on the radio pulse height. We selected lopes data recorded during thunderstorms, periods of heavy cloudiness and periods of cloudless weather. We tested whether the correlations with geomagnetic angle and primary energy vary with atmospheric conditions. Results. We find that during thunderstorms the radio emission can be strongly enhanced. The present data suggests that the observed amplification is caused by acceleration of the shower electrons and positrons. In the near future, extensions of lopes and the construction of LOFAR will help to identify the mechanism in more detail. No amplified pulses were found during periods of cloudless sky or heavy cloudiness, suggesting that the electric field effect for radio air shower measurements can be safely ignored during non-thunderstorm conditions.