Polarized radio emission from extensive air showers measured with LOFAR

TL;DR

This study uses LOFAR to measure highly polarized radio emissions from extensive air showers, revealing the superposition of charge-excess and geomagnetic mechanisms and how their contributions vary with shower geometry and observer position.

Contribution

First detailed measurement of polarization and charge-excess fraction in air shower radio emissions, confirming theoretical models and revealing dependencies on shower inclination and observer distance.

Findings

Radio emission is nearly 99% polarized.

Charge-excess fraction varies with shower zenith angle and observer distance.

Measured charge-excess fraction ranges from 3.3% to 20.3%.

Abstract

We present LOFAR measurements of radio emission from extensive air showers. We find that this emission is strongly polarized, with a median degree of polarization of nearly $99\%$, and that the angle between the polarization direction of the electric field and the Lorentz force acting on the particles, depends on the observer location in the shower plane. This can be understood as a superposition of the radially polarized charge-excess emission mechanism, first proposed by Askaryan and the geomagnetic emission mechanism proposed by Kahn and Lerche. We calculate the relative strengths of both contributions, as quantified by the charge-excess fraction, for $163$ individual air showers. We find that the measured charge-excess fraction is higher for air showers arriving from closer to the zenith. Furthermore, the measured charge-excess fraction also increases with increasing observer distance from the air shower symmetry axis. The measured values range from $(3.3\pm 1.0)\%$ for very inclined air showers at $25\, \mathrm{m}$ to $(20.3\pm 1.3)\%$ for almost vertical showers at $225\, \mathrm{m}$. Both dependencies are in qualitative agreement with theoretical predictions.