Coherent Radiation from Extensive Air Showers in the Ultra-High Frequency Band

TL;DR

This study uses detailed simulations to analyze the radio emission of inclined air showers in the GHz frequency range, revealing the coherent emission characteristics and their implications for cosmic ray detection and background noise understanding.

Contribution

The paper provides a comprehensive simulation-based characterization of ultra-high frequency radio emission from inclined air showers, highlighting the coherence effects and spatial emission patterns.

Findings

Radio emission extends into GHz frequencies with significant intensity.

Maximum emission forms an elliptical ring around the Cherenkov intersection.

Simulation results align with ANITA experiment observations.

Abstract

Using detailed Monte Carlo simulations we have characterized the features of the radio emission of inclined air showers in the Ultra-High Frequency band (300 MHz - 3 GHz). The Fourier-spectrum of the radiation is shown to have a sizable intensity well into the GHz frequency range. The emission is mainly due to transverse currents induced by the geomagnetic field and to the excess charge produced by the Askaryan effect. At these frequencies only a significantly reduced volume of the shower around the axis contributes coherently to the signal observed on the ground. The size of the coherently emitting volume depends on frequency, shower geometry and observer position, and is interpreted in terms of the relative time delays. At ground level, the maximum emission at high frequencies is concentrated in an elliptical ring-like region around the intersection of a Cherenkov cone with its vertex at shower maximum and the ground. The frequency spectrum of inclined showers when observed at positions that view shower maximum in the Cherenkov direction, is shown to be in broad agreement with the pulses detected by the Antarctic Impulsive Transient Antenna (ANITA) experiment, making the interpretation that they are due to Ultra-High Energy Cosmic Ray atmospheric showers consistent with our simulations. These results are also of great importance for experiments aiming to detect molecular bremsstrahlung radiation in the GHz range as they present an important background for its detection.