arXiv subjects

Joseph Dwyer

Publications and source records attributed to Joseph Dwyer.

6 recordsLinked to original sources

arXiv2026

A Terrestrial Gamma-ray Flash with an Asymmetric Footprint Observed at the Pierre Auger Observatory

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 $2.3\pm0.5$km 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 $\theta > 75^o$ range, where $\theta$ is the off-nadir angle.

astro-ph.HE↗

arXiv2026

Unveiling the Mysteries of Lightning: Exploring its fundamental Physical Processes with SKA-LOW

Lightning is a surprisingly poorly understood phenomena. It consists of a wide variety of complex processes such as initiation, propagation, connection to ground, even emission of high-energy radiation. However, due to the extreme challenges in observing lightning at fast time scales, small spatial scales, and behind obscuring clouds, these processes are not well understood. In the past, interferometers such as the LOFAR radio telescope have provided unique insight and discoveries into the physics of lightning. The new SKA-LOW being built in western Australia will provide unrivaled spectral bandwidth and sensitivity, which will be combined with high resolution resulting from large antenna baselines. We will use SKA-LOW to observe lightning in order to explore its fundamental plasma physics, such as how it initiates and propagates. SKA's high bandwidth will allow us to test how lightning emits VHF radiation, giving tremendous insight into precisely how the plasma behaves. SKA's sensitivity will allow us to explore extremely faint lightning processes, such as the very first radio emission from a lightning flash. Here, we detail the lightning physics that can be explored with SKA, as well as the observation strategy needed explore such physics.

astro-ph.EP↗

arXiv2025

Characterization of downward Terrestrial Gamma-ray Flashes detected at the Pierre Auger Observatory

Downward Terrestrial Gamma-ray Flashes (TGFs) are sub-millisecond bursts of MeV gamma rays produced in thunderclouds. According to the Relativistic Runaway Electron Avalanche model, gamma rays are produced, via bremsstrahlung, from electron cascades activated by a relativistic "seed" electron. It is not clear what mechanism is responsible for the acceleration of electrons to relativistic energies in electric discharges. To better understand the acceleration sites and the TGF production mechanisms, it is critically important to identify the TGF source position and geometry in the atmosphere and to study the gamma emission characteristics. The Surface Detector of the Pierre Auger Observatory, with its 1600 water-Cherenkov detectors very sensitive to high-energy photons and with a very fine time-sampling, is a valuable instrument to study downward TGFs. The possibility to analyze the radiation emission in detail led to the observation of the first TGFs with an asymmetric azimuthal structure, suggesting a complex source different from the initially hypothesized downward beam. We report on these observations and the new perspectives which may open with the incorporation of new instruments at the Auger site to study lightning development alongside gamma emission, and the increasingly detailed data provided by satellites and global lightning networks.

astro-ph.HE↗

arXiv2015

Pulse properties of terrestrial gamma-ray flashes detected by the Fermi Gamma-Ray Burst Monitor

The Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope has triggered on over 300 terrestrial gamma-ray flashes (TGFs) since its launch in June 2008. With 14 detectors, GBM collects on average ~100 counts per triggered TGF, enabling unprecedented studies of the time profiles of TGFs. Here we present the first rigorous analysis of the temporal properties of a large sample of TGFs (278), including the distributions of the rise and fall times of the individual pulses and their durations. A variety of time profiles are observed with 19 of TGFs having multiple pulses separated in time and 31 clear cases of partially overlapping pulses. The effect of instrumental dead time and pulse pileup on the temporal properties are also presented. As the observed gamma ray pulse structure is representative of the electron flux at the source, TGF pulse parameters are critical to distinguish between relativistic feedback discharge and lightning leader models. We show that at least 67% of TGFs at satellite altitudes are significantly asymmetric. For the asymmetric pulses, the rise times are almost always shorter than the fall times. Those which are not are consistent with statistical fluctuations. The median rise time for asymmetric pulses is ~3 times shorter than for symmetric pulses while their fall times are comparable. The asymmetric shapes observed are consistent with the relativistic feedback discharge model when Compton scattering of photons between the source and Fermi is included, and instrumental effects are taken into account.

astro-ph.HE↗

arXiv2015

Compton scattering in terrestrial gamma-ray flashes detected with the Fermi gamma-ray burst monitor

Terrestrial gamma-ray flashes (TGFs) are short intense flashes of gamma rays associated with lightning activity in thunderstorms. Using Monte Carlo simulations of the relativistic runaway electron avalanche (RREA) process, theoretical predictions for the temporal and spectral evolution of TGFs are compared to observations made with the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope. Assuming a single source altitude of 15 km, a comparison of simulations to data is performed for a range of empirically chosen source electron variation time scales. The data exhibit a clear softening with increased source distance, in qualitative agreement with theoretical predictions. The simulated spectra follow this trend in the data, but tend to underestimate the observed hardness. Such a discrepancy may imply that the basic RREA model is not sufficient. Alternatively, a TGF beam that is tilted with respect to the zenith could produce an evolution with source distance that is compatible with the data. Based on these results, we propose that the source electron distributions of TGFs observed by GBM vary on time scales of at least tens of microseconds, with an upper limit of approx. 100 microseconds.

astro-ph.HE↗

arXiv1995

The Inverse-Compton And Extragalactic Components Of The Diffuse Gamma-Ray Emission

We present spectra of the inverse-Compton and extragalactic components of the high-energy gamma-radiation based on an analysis of the emission at high galactic latitudes ($\vert$b$\vert$ $\geq$ $30°$). We correlate the gamma-ray intensity with a model consisting of an isotropic component, a component proportional to the 408 MHz synchrotron radiation, and a component proportional to atomic hydrogen (H I) column density with different emissivities in eight galactic octants. The spectrum of gamma-radiation that is correlated with the H I column density indicates that this component originates in cosmic-ray/matter interactions. The cosmic-ray electrons which produce the 408 MHz radio continuum emission also produce gamma-radiation through inverse-Compton interactions with interstellar photons. By correlating the gamma-radiation with the 408 MHz continuum, we measure the spectrum and absolute intensity of the IC emission. The isotropic component gives us the spectrum and intensity of the extragalactic gamma-radiation. We discuss interpretation of the extragalactic spectrum.

astro-ph↗