[Paper Review] Radio Detection of Horizontal Extensive Air Showers with AERA

O. Kambeitz|arXiv (Cornell University)|Sep 28, 2015

Astrophysics and Cosmic Phenomena4 references3 citations

TL;DR

This paper presents the first experimental analysis of horizontal extensive air showers using the AERA radio array at the Pierre Auger Observatory, demonstrating a clear correlation between shower energy and radio signal amplitude. By optimizing time-window selection and validating detection efficiency for inclined showers (62°–80° zenith angle), AERA confirms the feasibility of radio detection for high-inclination cosmic rays, enhancing exposure and enabling new insights into shower development and radio emission physics.

ABSTRACT

AERA, the Auger Engineering Radio Array, located at the Pierre Auger Observatory in Malargüe, Argentina measures the radio emission of extensive air showers in the 30-80 MHz frequency range and is optimized for the detection of air showers up to 60$^{\circ}$ zenith angle. In this contribution the motivation, the status, and first results of the analysis of horizontal air showers with AERA will be presented.

Motivation & Objective

• To extend AERA's detection capability to horizontal air showers (zenith angles 62°–80°) to increase observational exposure.
• To validate the radio detection of inclined showers, which are less attenuated in the atmosphere and offer unique insights into shower development.
• To assess the significance of the vertical electric field component in radio emission for inclined showers using prototype antenna stations.
• To improve event selection efficiency and purity for horizontal showers by refining time-window and amplitude-based criteria.
• To evaluate the performance of new antenna types (whisk, tripole) in detecting the vertical polarization component of radio signals.

Proposed method

• AERA uses 124 antenna stations (24 LPDAs and 100 butterfly antennas) to measure radio emission in the 30–80 MHz range.
• Events are selected using external triggers from the surface detector, with timing and distance cuts applied to identify candidate horizontal showers.
• A time-window of ±7 µs around the expected signal arrival time is used to distinguish real cosmic ray signals from noise.
• The amplitude of the electric field from the three closest AERA stations is averaged and correlated with shower energy to validate signal detection.
• Prototype stations with tripole and whisk-type antennas are deployed to measure the vertical electric field component and improve sensitivity to inclined showers.
• Simulations (CoREAS) are used to model footprint size and wavefront shape, informing the interpretation of radio signal distribution.

Experimental results

Research questions

• RQ1 Can AERA detect horizontal extensive air showers with zenith angles up to 80° using radio emission?
• RQ2 What is the correlation between the measured radio amplitude and the primary cosmic ray energy in inclined showers?
• RQ3 How does the footprint size and asymmetry of the radio signal change with increasing zenith angle?
• RQ4 What is the contribution of the vertical electric field component to the radio signal in inclined showers, and how can it be measured?
• RQ5 How do new antenna types (whisk, tripole) improve detection sensitivity and noise suppression for horizontal showers?

Key findings

• A clear correlation is observed between the primary shower energy and the electric field amplitude measured by AERA, confirming the feasibility of radio energy estimation for inclined showers.
• The selected events show a consistent signal-to-noise ratio, with no correlation observed outside the ±7 µs time window, validating the event selection method.
• Footprint sizes for horizontal showers (e.g., 75° zenith angle) reach up to 6 km in length, with significant asymmetry, as predicted by CoREAS simulations.
• The deployment of five whisk-type and four tripole antenna stations enables the measurement of the vertical electric field component, crucial for accurate reconstruction of inclined showers.
• The analysis of data from January 2012 to March 2014 identified candidate events in the 62°–80° zenith angle range, demonstrating the potential for increased exposure.
• The current event selection efficiency is limited by core uncertainty and time calibration; future improvements with better timing will enhance purity and detection rate.