Monte Carlo simulations of radio pulses in atmospheric showers using   ZHAireS
  We present predictions for the radio pulses emitted by extensive air showers using ZHAireS, an AIRES-based Monte Carlo code that takes into account the full complexity of ultra-high energy cosmic-ray induced shower development in the atmosphere, and allows the calculation of the electric field in both the time and frequency domains. We do not presuppose any emission mechanism, but our results are compatible with a superposition of geomagnetic and charge excess radio emission effects. We investigate the polarization of the electric field as well as the effects of the refractive index n and shower geometry on the radio pulses. We show that geometry, coupled to the relativistic effects that appear when using a realistic n>1, play a prominent role on the radio emission of air showers. 
Jaime Alvarez-Mu\~niz, Washington R. Carvalho Jr., Enrique Zas
astro-ph.HE