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Study of downward Terrestrial Gamma-ray Flashes with the surface detector of the Pierre Auger Observatory
Roberta Colalillo, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2023, objavljeni znanstveni prispevek na konferenci

Opis: The surface detector (SD) of the Pierre Auger Observatory, consisting of 1660 water-Cherenkov detectors (WCDs), covers 3000 km2 in the Argentinian pampa. Thanks to the high efficiency of WCDs in detecting gammarays, it represents a unique instrument for studying downward Terrestrial Gamma-ray Flashes (TGFs) over a large area. Peculiar events, likely related to downward TGFs, were detected at the Auger Observatory. Their experimental signature and time evolution are very different from those of a shower produced by an ultrahigh-energy cosmic ray. They happen in coincidence with low thunderclouds and lightning, and their large deposited energy at the ground is compatible with that of a standard downward TGF with the source a few kilometers above the ground. A new trigger algorithm to increase the TGF-like event statistics was installed in the whole array. The study of the performance of the new trigger system during the lightning season is ongoing and will provide a handle to develop improved algorithms to implement in the Auger upgraded electronic boards. The available data sample, even if small, can give important clues about the TGF production models, in particular, the shape of WCD signals. Moreover, the SD allows us to observe more than one point in the TGF beam, providing information on the emission angle.
Ključne besede: Pierre Auger Observatory, ultra-high energy cosmic rays, surface detectors, extensive air shower, terrestrial gamma-ray flashes
Objavljeno v RUNG: 23.01.2024; Ogledov: 729; Prenosov: 6
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Insight Into Lightning Initiation via Downward Terrestrial Gamma-ray Flash Observations at Telescope Array
J. Remington, Jon Paul Lundquist, 2022, objavljeni znanstveni prispevek na konferenci

Opis: Due to the difficulty of direct measurement of the thunderstorm environment, in particular the electric field strengths, the initial stages of lightning breakdown remain mysterious. The 1994 discovery of Terrestrial Gamma-ray Flashes (TGFs) and their implications for megaVolt potentials within thunderclouds has proved to be a valuable source of information about the breakdown process. The Telescope Array Surface Detector (TASD) --- a 700 km^2 scintillator array in Western Utah, U.S.A --- coupled with a lightning mapping array, fast sferic (field change) sensor and broadband interferometer, has provided unique insight into the properties of this energetic radiation and of lightning initiation in general. In particular, microsecond-scale timing comparisons have clearly established that downward TGFs occur during strong initial breakdown pulses (IBPs) of downward negative cloud-to-ground and intracloud flashes. In turn, the IBPs are produced by streamer-based fast negative breakdown. Investigations into downward TGFs with the TASD have significantly evolved with recent upgrades to lightning instrumentation. A second state-of-the-art broadband interferometer allows high-resolution stereo observation of lightning development. A high-speed optical video camera, set to be deployed in Spring 2021, will allow simultaneous observation of the visual component of lightning responsible for TGF production. Finally, a suite of ground based static electric field mills will provide new information on the large-scale properties of the thunderstorms in which downward TGFs arise. In this talk, we present the most recent TGF observations from the Telescope Array.
Ključne besede: Telescope Array, ground array, ultra-high energy, cosmic rays, photons, terrestrial gamma-ray flashes, gamma-rays, lightning
Objavljeno v RUNG: 02.10.2023; Ogledov: 920; Prenosov: 6
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Observations of the origin of downward terrestrial gamma-ray flashes
J. W. Belz, P. R. Krehbiel, J. Remington, M. A. Stanley, R. U. Abbasi, R. LeVon, W. Rison, D. Rodeheffer, T. Abu-Zayyad, Jon Paul Lundquist, 2020, izvirni znanstveni članek

Opis: In this paper we report the first close, high‐resolution observations of downward‐directed terrestrial gamma‐ray flashes (TGFs) detected by the large‐area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud‐to‐ground and low‐altitude intracloud flashes and that the IBPs are produced by a newly identified streamer‐based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark‐like transient conducting events (TCEs) within the fast streamer system and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub‐pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub‐pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely, as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.
Ključne besede: terrestrial gamma-ray flashes, lightning, fast breakdown, initial breakdown pulse, atmospheric electricity, transient conducting events
Objavljeno v RUNG: 04.02.2021; Ogledov: 2739; Prenosov: 22
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