Ultra high frequency geomagnetic radiation from extensive air showers

Using the ZHAireS Monte Carlo code, we show that the Fourier-spectrum of the radio emission of inclined air showers can have a sizable intensity up to the GHz frequency range. At these frequencies only a significantly reduced volume of the shower around the axis contributes coherently to the signal observed on the ground, which is mainly due to the geomagnetic and charge excess mechanisms. At ground level, the maximum emission at high frequencies is concentrated in a ring-like elliptical region defined by the intersection with the ground of a Cherenkov cone with its vertex at shower maximum. The frequency-spectrum of inclined showers, when observed at positions close to the ring-like maximum emission region, is in broad agreement with the pulses detected by the ANITA experiment, making the interpretation that they are due to ultrahigh energy cosmic ray atmospheric showers consistent with our simulations. These results are also relevant for ground-based radio experiments aiming at detecting molecular bremsstrahlung radiation in the GHz range - an entirelly different emission mechanism which is not included in ZHAireS simulations - since they present an important background for such experiments.