Radio Emission from Cosmic Ray Air Showers

Abstract

Charged particles with energies ranging from a few times 10⁶ eV up to ≥ 10²⁰ eV continuously impinge on the earth as so-called “cosmic rays”. Their study constitutes an important field of research in both astronomy and particle physics, and some cosmic ray related effects even have significance for the public’s every-day life.<br /> Radio emission from cosmic ray air showers offers the opportunity to use radio observations as an additional powerful observing technique in cosmic ray research, thereby building a bridge between astroparticle physics and radio astronomy. As a necessary prerequisite, a detailed theoretical analysis of the processes responsible for the radio emission has to be performed. In this thesis, we analyse the emission in the scheme of “coherent geosynchrotron radiation” emitted by electron-positron pairs created in the air shower cascade as they are deflected in the earth’s magnetic field.<br /> We first perform an analytic calculation of the emission based on realistic parametrisations of the particle distributions in the air shower. The analytic approach allows us to gain a solid understanding of general emission features and the coherence effects arising from the different physical scales present in the air shower. We compare our predictions with the available historical data and find that geosynchrotron radiation can indeed explain the emission.<br /> Afterwards, we conceive and implement a sophisticated Monte Carlo code, performing the calculation with increased precision and taking into account a more realistic air shower model. We describe and test the concepts envisaged to allow a high-precision modelling of realistic air showers on standard computer hardware and compare the Monte Carlo results with the analytic calculations to ensure the correctness of our modelling efforts.<br /> In a last step, we use our Monte Carlo code to simulate a large number of air showers in order to study the effect of important air shower parameters and geometries on the associated radio emission. Our main result is a parametrisation formula relating the radio emission characteristics directly to important air shower and observer parameters such as the air shower geometry, the primary particle energy, the depth of the shower maximum, the observer distance from the shower centre, and the observing frequency.<br /> With this analysis, we build the foundation for the interpretation of experimental measurements of radio emission from extensive air showers and thus make a great step forward in the establishment of radio observations as an additional observing technique in cosmic ray research.