We present a microscopic computation of the radio emission from air showers initiated by ultra-high energy cosmic rays in the atmosphere.

Figure: the principle of SELFAS calculations. The length of an individual particle track of the particle trajectory is divided in short tracks allowing us to compute energy losses and deviations due to geomagnetic field and to multiple scattering. At the starting point and at each end of short track, sti(x,t), ci(x,t) and cui(x,t) are calculated independently and added to three independent histograms corresponding to St(x,t), C(x,t) and Cu(x,t). Once all particles have been considered, the time derivatives for C(x,t) and Cu(x,t) are performed and the complete electric field at this position is obtained summing up the three total contributions. Figure: the principle of SELFAS calculations. The length of an individual particle track of the particle trajectory is divided in short tracks allowing us to compute energy losses and deviations due to geomagnetic field and to multiple scattering. At the starting point and at each end of short track, sti(x,t), ci(x,t) and cui(x,t) are calculated independently and added to three independent histograms corresponding to St(x,t), C(x,t) and Cu(x,t). Once all particles have been considered, the time derivatives for C(x,t) and Cu(x,t) are performed and the complete electric field at this position is obtained summing up the three total contributions.
The strategy adopted is to compute each secondary particle contribution of the electromagnetic component and to construct the total signal at any location. SELFAS2 is a code which does not rely on air shower generators like AIRES or CORSIKA and it is based on the concept of air shower universality which makes it completely autonomous. Each positrons and electrons of the air shower are generated randomly following relevant distributions and tracking them along their travel in the atmosphere. We confirm in this paper earlier results that the radio emission is mainly due to the time derivative of the transverse current and the time derivative of the charge excess. The time derivative of the transverse current created by systematic deviations of charges in the geomagnetic field is usually dominant compared to the charge excess contribution except for the case of an air shower parallel to the geomagnetic field.

Published by V. Marin, B. Revenu, in Astroparticle Physics 35, March 2012, P. 733–741 http://dx.doi.org/10.1016/j.astropartphys.2012.03.007