Radio detection of cosmic rays in [1.7–3.7] MHz: The EXTASIS experiment

Abstract

Since 2003, significant efforts have been devoted to the understanding of the radio emission of extensive air showers above 20 MHz. Despite some studies led until the early nineties, the band available below 20 MHz has remained unused for 20 years. However, it has been claimed by some pioneering experiments that extensive air showers emit a strong electric field in this band and that there is evidence of a large increase of the radio pulse amplitude with decreasing frequencies. The EXTASIS experiment, located within the Nançay Radioastronomy Observatory and supported by the scintillator array and the autonomous radio stations of the CODALEMA experiment, aims to re-investigate the low-frequency band, and especially to study the so-called "sudden death" contribution, the expected electric field radiated by the shower front when hitting ground level. In this work, we present the instrumental setup of the EXTASIS experiment composed of 7 low-frequency antennas operating in [1.7 − 3.7] MHz and covering approximately 1 km 2 . We report the observation of 18 air showers detected in coincidence in the three instruments, and estimate a detection threshold of 23 ± 4 µV · m −1 from comparisons with detailed SELFAS3 simulations. We also report a strong correlation of the low-frequency signal observation with the atmospheric electric field.

From the Observation of UHECR Radio Signal in [1-200] MHz to the Composition: CODALEMA and EXTASIS Status Report

“…In its current version, CODALEMA consists of a square array (0.4 × 0.4 km 2 ) of 13 scintillators (surface detector) ; a set of 57 so-called "autonomous" crossed dipoles (hereafter noted SA for standalone antennas), synchronized by GPS dating and operating in the EMF band, distributed over 1.1 km 2 ; a so-called "Compact Array" (here after noted CA) of 10 cross-polarized antennas arranged in a star shape of 150 m extension and whose signal acquisition (in the MF band) is triggered by the particle detector. CODALEMA is today the support infrastructure of EXTASIS [7], an array of 7 low-frequency antennas also triggered by the particle detector. The LF antenna locations have been chosen to cover the overall Nançay Radioastronomy Observatory area and to have a nearby MF autonomous station (see figure 1)).…”

“…Thanks to its design, CODALEMA operates in the [20 − 200] MHz band, referred as EMF in the following for Extended Medium Frequencies. Moreover, the EXTASIS experiment [7] hosted by CODALEMA, dedicated to resume the low frequency (LF) studies made in the 70's and up to the 90's, permits to widen the frequency range, and to demonstrate for the first time that the study of atmospheric showers is possible continuously from 1 to 200 MHz in a continuous way, making it possible to scan different zones and processes of their development. The goal of the study of cosmic rays is the reconstruction of their parameters, especially their mass and energy, primordial to understand the mechanisms of acceleration of cosmic rays, their propagation and their sources.…”

“…These characteristics can be estimated by comparing the electric field detected at ground level to a large catalogue predictions obtained from detailed simulations. Figure 1: Area experiment at the Nançay Radioastronomy Observatory [7]. Red crosses represent the 57 standalone antennas of CO-DALEMA, black squares the 13 scintillators, the blue points represent the 7 LF antennas.…”

“…Over the years, significant efforts have been devoted to the understanding of the radio emission of extensive air shower (EAS) in the range [20-80] MHz but, despite some studies led until the nineties, the [1][2][3][4][5][6][7][8][9][10] MHz band has remained unused for nearly 30 years. The EXTASIS experiment, located within the radio astronomy observatory of Nançay and supported by the CO-DALEMA instrument, aims to reinvestigate the [1][2][3][4][5][6][7][8][9][10] MHz band, and to study the so-called "Sudden Death" contribution, the expected radiation electric field created by the particles that are stopped upon arrival to the ground. We present the instrumental setup and the results obtained with EXTASIS.…”

Coherent transition radiation from the geomagnetic air shower current

“…In this work we discuss that for observers located far from the air-Earth boundary a different emission has to be considered, coherent transition radiation from the air shower traversing from air into Earth or more interestingly ice. This emission was already discussed in [8,9,10] where the focus was put on transition radiation from the net excess charge in the cascade front, where in [11,12,13], the sudden death signal due to the absorption of the air shower by Earth was considered. In this work, we show that transition radiation is a geometrical effect and as such is not limited to a net charge traversing a boundary layer, but applies equally well to the geomagnetically induced current in the air shower front, to which we will refer to as CTR-GM.…”