9 research outputs found
On the HighâEnergy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes
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On the HighâEnergy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes
Terrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highestâenergy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and highâenergy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a highâenergy component in TGF spectra extended up to â100 MeV, which is difficult to reconcile with the predictions from the Relativistic Runaway Electron Avalanche (RREA) mechanism at the basis of many TGF production models. Here we present a new set of TGFs detected by the AGILE satellite and associated to lightning measurements capable to solve this controversy. Detailed endâtoâend Monte Carlo simulations and an improved understanding of the instrument performance under highâflux conditions show that it is possible to explain the observed highâenergy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single highâenergy counts may be the signature of a fineâpulsed time structure of TGFs on time scales â4 ÎŒs, but we find no clear evidence for this. The presented data set and modeling results allow also for explaining the observed TGF distribution in the (Fluence Ă duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled
The 3rd AGILE Terrestrial Gamma-ray Flashes Catalog. Part II: Optimized Selection Criteria and Characteristics of the New Sample
We present in this work the third catalog of terrestrial gamma-ray flashes (TGFs) by the AGILE mission and the new search algorithm that was developed to produce it. We firstly introduce the new selection criteria, designed from the characteristics of WWLLN-identified TGFs, and then applied on all data from March 2015 to September 2018. Association with sferics was performed by an independent search, described in a companion paper by Lindanger et al. (2020, https://doi.org/10.1029/2019JD031985). This search showed that many TGFs were not recognized by the existing selection algorithm, hence the need for this work. Several new selection criteria were tested and are compared in this paper. We then present the chosen selection criteria and the obtained sample, which includes 2,780 events and represents the most extensive TGF catalog available for the equatorial regions. Finally, we discuss the characteristics of this sample, including geographic distribution, intensity and duration, and seasonal variations
The ListafjordenâDrangedal Fault Complex of the AgderâTelemark Lineament Zone, southern Norway. A structural analysis based on remote sensing and potential field data
Very-high-frequency oscillations in the main peak of a magnetar giant flare
Magnetars are strongly magnetized, isolated neutron stars1â3 with magnetic fields up to around 1015 gauss, luminosities of approximately 1031â1036 ergs per second and rotation periods of about 0.3â12.0 s. Very energetic giant flares from galactic magnetars (peak luminosities of 1044â1047 ergs per second, lasting approximately 0.1 s) have been detected in hard X-rays and soft Îł-rays4, and only one has been detected from outside our galaxy5. During such giant flares, quasi-periodic oscillations (QPOs) with low (less than 150 hertz) and high (greater than 500 hertz) frequencies have been observed6â9, but their statistical significance has been questioned10. High-frequency QPOs have been seen only during the tail phase of the flare9. Here we report the observation of two broad QPOs at approximately 2,132 hertz and 4,250 hertz in the main peak of a giant Îł-ray flare11 in the direction of the NGC 253 galaxy12â17, disappearing after 3.5 milliseconds. The flare was detected on 15 April 2020 by the AtmosphereâSpace Interactions Monitor instrument18,19 aboard the International Space Station, which was the only instrument that recorded the main burst phase (0.8â3.2 milliseconds) in the full energy range (50 Ă 103 to 40 Ă 106 electronvolts) without suffering from saturation effects such as deadtime and pile-up. Along with sudden spectral variations, these extremely high-frequency oscillations in the burst peak are a crucial component that will aid our understanding of magnetar giant flares