# Multi-wavelength observation of cosmic-ray air-showers with CODALEMA/EXTASIS

## TL;DR

This paper reports on multi-wavelength radio observations of cosmic-ray air-showers, especially exploring the underused 1-10 MHz band, to better understand the electric fields and improve cosmic ray reconstruction methods.

## Contribution

It introduces the EXTASIS experiment to study low-frequency emissions and demonstrates that the 20-80 MHz band contains sufficient information for accurate cosmic ray analysis.

## Key findings

- Confirmed strong electric fields in the 1-10 MHz band.
- Validated the effectiveness of the 20-80 MHz band for EAS reconstruction.
- Developed real-time comparison between simulations and data.

## Abstract

Since 2003, significant efforts have been devoted to the understanding of the radio emission of extensive air shower in the range [20-200] MHz. Despite some studies led until the early nineties, the [1-10] MHz band has remained unused for 20 years. However, it has been measured by some pioneering experiments that extensive air shower emit a strong electric field in this band and that there is evidence of a large increase in the amplitude of the radio pulse at lower frequencies. The EXTASIS experiment, located within the Nan\c{c}ay Radioastronomy Observatory and supported by the CODALEMA experiment, aims to reinvestigate the [1-10] MHz band, and especially to study the so-called "Sudden Death" contribution, the expected electric field emitted by shower front when hitting the ground level. Currently, EXTASIS has confirmed some results obtained by the pioneering experiments, and tends to bring explanations to the other ones, for instance the role of the underlying atmospheric electric field. Moreover, CODALEMA has demonstrated that in the most commonly used frequency band ([20-80] MHz) the electric field profile of EAS can be well sampled, and contains all the information needed for the reconstruction of EAS: an automatic comparison between the SELFAS3 simulations and data has been developed, allowing us to reconstruct in an almost real time the primary cosmic ray characteristics.