Measuring a Cherenkov ring in the radio emission from air showers at 110-190 MHz with LOFAR

TL;DR

This paper reports the first detailed measurements of Cherenkov ring structures in radio emissions from air showers at 110-190 MHz using LOFAR, confirming theoretical predictions and enabling new methods to determine cosmic ray properties.

Contribution

It provides the first detailed observational evidence of Cherenkov rings in radio air shower emissions and validates CoREAS simulations including air's refractive index effects.

Findings

Measured Cherenkov ring structures in radio emissions at 110-190 MHz.

Data agree with CoREAS simulations incorporating air's refractive index.

Cherenkov ring enables geometrical measurement of shower maximum depth.

Abstract

Measuring radio emission from air showers offers a novel way to determine properties of the primary cosmic rays such as their mass and energy. Theory predicts that relativistic time compression effects lead to a ring of amplified emission which starts to dominate the emission pattern for frequencies above ~100 MHz. In this article we present the first detailed measurements of this structure. Ring structures in the radio emission of air showers are measured with the LOFAR radio telescope in the frequency range of 110 - 190 MHz. These data are well described by CoREAS simulations. They clearly confirm the importance of including the index of refraction of air as a function of height. Furthermore, the presence of the Cherenkov ring offers the possibility for a geometrical measurement of the depth of shower maximum, which in turn depends on the mass of the primary particle.