Measurement of the circular polarization in radio emission from extensive air showers confirms emission mechanisms

TL;DR

This study analyzes the complete polarization of radio signals from extensive air showers, confirming the interplay of geomagnetic and charge-excess emission mechanisms, and demonstrating the potential of circular polarization as a tool for air shower analysis.

Contribution

It provides the first comprehensive measurement of all four Stokes parameters in air shower radio signals, confirming theoretical models and highlighting the diagnostic potential of circular polarization.

Findings

Circular polarization depends on azimuth angle and distance from shower axis.

Data matches first-principles calculations very well.

Circular polarization can be used to investigate air shower structures.

Abstract

We report here on a novel analysis of the complete set of four Stokes parameters that uniquely determine the linear and/or circular polarization of the radio signal for an extensive air shower. The observed dependency of the circular polarization on azimuth angle and distance to the shower axis is a clear signature of the interfering contributions from two different radiation mechanisms, a main contribution due to a geomagnetically-induced transverse current and a secondary component due to the build-up of excess charge at the shower front. The data, as measured at LOFAR, agree very well with a calculation from first principles. This opens the possibility to use circular polarization as an investigative tool in the analysis of air shower structure, such as for the determination of atmospheric electric fields.