A Terrestrial Gamma-ray Flash with an Asymmetric Footprint Observed at the Pierre Auger Observatory
Abstract
The Pierre Auger Observatory, a 3000km2 detector array that sits 1400m above sea level in Argentina, has the ability to map the entire footprint of Terrestrial Gamma-ray Flashes (TGFs), which are short, intense bursts of gamma rays associated with lightning. This is in contrast to the vast majority of TGF detections that only occur in a single detector in space. In this paper we leverage this capability to perform two important analyses on an event from 2007, chosen for its large footprint and high signal quality. First, we triangulate the source of the TGF to 2.3±0.5km height above ground level using the arrival time of the gamma rays themselves at the detector array, independently of any information regarding the lightning channel. With the source position established, we can analyze the spatial distribution of the TGF on the ground with respect to that source. Not only do we find that one side of the TGF footprint shows an order of magnitude higher flux than the other, but we also find an asymmetric azimuthal structure consisting of two clear peaks in intensity. These peaks are offset by about 120o from each other. This azimuthal structure is not possible under the traditional uniform electric-field model of a TGF. Using Geant4 simulations of collimated beams of the relativistic runaway electron-avalanche spectrum, we attempt to reconstruct the source geometry of the TGF and find that it likely had significant components in the θ> 75o range, where θ is the off-nadir angle.
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BibTeX
@misc{halim2026terrestrial,
title = {{A Terrestrial Gamma-ray Flash with an Asymmetric Footprint Observed at the Pierre Auger Observatory}},
author = {A. Abdul Halim and P Abreu and M Aglietta and M Ahmed and I Allekotte and K. Almeida Cheminant and R Aloisio and J Alvarez-Muniz and A Ambrosone and J. Ammerman Yebra and L Anchordoqui and B Andrada and L. Andrade Dourado and L Apollonio and C Aramo and E Arnone and J.C. Arteaga Velazquez and P Assis and G Avila and E Avocone and A Bakalova and Y Balibrea and A Baluta and F Barbato and A. Bartz Mocellin and J.P Behler and C Berat and M.E Bertaina and M Bianciotto and P.L Biermann and V Binet and K Bismark and T Bister and J Biteau and J Blazek and J Blümer and M Bohacova and D Boncioli and C Bonifazi and N Borodai and J Brack and P.G. Brichetto Orquera and A Bueno and S Buitink and A Bwembya and T.R. Caba Pineda and K.S Caballero-Mora and S Cabanafreire and L Caccianiga and J Caraca-Valente and R Caruso and A Castellina and F Catalani and G Cataldi and L Cazon and M Cerda and B Cermakova and A Cermenati and K Cerny and J.A Chinellato},
howpublished = {HAL},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.28885},
eprint = {2608.28885},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05743931v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Terrestrial Gamma-ray Flash with an Asymmetric Footprint Observed at the Pierre Auger Observatory
AU - A. Abdul Halim
AU - P Abreu
AU - M Aglietta
AU - M Ahmed
AU - I Allekotte
AU - K. Almeida Cheminant
AU - R Aloisio
AU - J Alvarez-Muniz
AU - A Ambrosone
AU - J. Ammerman Yebra
AU - L Anchordoqui
AU - B Andrada
AU - L. Andrade Dourado
AU - L Apollonio
AU - C Aramo
AU - E Arnone
AU - J.C. Arteaga Velazquez
AU - P Assis
AU - G Avila
AU - E Avocone
AU - A Bakalova
AU - Y Balibrea
AU - A Baluta
AU - F Barbato
AU - A. Bartz Mocellin
AU - J.P Behler
AU - C Berat
AU - M.E Bertaina
AU - M Bianciotto
AU - P.L Biermann
AU - V Binet
AU - K Bismark
AU - T Bister
AU - J Biteau
AU - J Blazek
AU - J Blümer
AU - M Bohacova
AU - D Boncioli
AU - C Bonifazi
AU - N Borodai
AU - J Brack
AU - P.G. Brichetto Orquera
AU - A Bueno
AU - S Buitink
AU - A Bwembya
AU - T.R. Caba Pineda
AU - K.S Caballero-Mora
AU - S Cabanafreire
AU - L Caccianiga
AU - J Caraca-Valente
AU - R Caruso
AU - A Castellina
AU - F Catalani
AU - G Cataldi
AU - L Cazon
AU - M Cerda
AU - B Cermakova
AU - A Cermenati
AU - K Cerny
AU - J.A Chinellato
PY - 2026
DA - 2026/08/28
PB - HAL
DO - 10.48550/arxiv.2608.28885
UR - https://hal.science/hal-05743931v1
AB - The Pierre Auger Observatory, a 3000km^2 detector array that sits 1400m above sea level in Argentina, has the ability to map the entire footprint of Terrestrial Gamma-ray Flashes (TGFs), which are short, intense bursts of gamma rays associated with lightning. This is in contrast to the vast majority of TGF detections that only occur in a single detector in space. In this paper we leverage this capability to perform two important analyses on an event from 2007, chosen for its large footprint and high signal quality. First, we triangulate the source of the TGF to 2.3±0.5km height above ground level using the arrival time of the gamma rays themselves at the detector array, independently of any information regarding the lightning channel. With the source position established, we can analyze the spatial distribution of the TGF on the ground with respect to that source. Not only do we find that one side of the TGF footprint shows an order of magnitude higher flux than the other, but we also find an asymmetric azimuthal structure consisting of two clear peaks in intensity. These peaks are offset by about 120^o from each other. This azimuthal structure is not possible under the traditional uniform electric-field model of a TGF. Using Geant4 simulations of collimated beams of the relativistic runaway electron-avalanche spectrum, we attempt to reconstruct the source geometry of the TGF and find that it likely had significant components in the θ> 75^o range, where θ is the off-nadir angle.
ER -