We observe a large fraction of circular polarization in radio emission from extensive air showers recorded during thunderstorms, much higher than in the emission from air showers measured during fair-weather circumstances. We show that the circular polarization of the air showers measured during thunderstorms can be explained by the change in the direction of the transverse current as a function of altitude induced by atmospheric electric fields. Thus by using the full set of Stokes parameters for these events, we obtain a good characterization of the electric fields in thunderclouds. We also measure a large horizontal component of the electric fields in the two events that we have analysed.
@article{arxiv.1703.06008,
title = {Thunderstorm electric fields probed by extensive air showers through their polarized radio emission},
author = {T. N. G. Trinh and O. Scholten and A. Bonardi and S. Buitink and A. Corstanje and U. Ebert and J. E. Enriquez and H. Falcke and J. R. Horandel and B. M. Hare and P. Mitra and K. Mulrey and A. Nelles and J. P. Rachen and L. Rossetto and C. Rutjes and P. Schellart and S. Thoudam and S. ter Veen and T. Winchen},
journal= {arXiv preprint arXiv:1703.06008},
year = {2017}
}