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On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes.

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Title: On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes.
Authors: Marisaldi, M.1,2 martino.marisaldi@uib.no, Galli, M.3, Labanti, C.2, Østgaard, N.1, Sarria, D.1, Cummer, S. A.4, Lyu, F.4, Lindanger, A.1, Campana, R.2, Ursi, A.5, Tavani, M.5, Fuschino, F.2, Argan, A.6, Trois, A.7, Pittori, C.8, Verrecchia, F.8
Source: Journal of Geophysical Research. Atmospheres. 7/27/2019, Vol. 124 Issue 14, p7484-7497. 14p.
Subject Terms: Gamma rays, Atmospheric electricity, Lightning, Atmospheric physics, Monte Carlo method
Abstract: Terrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest‐energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and high‐energy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a high‐energy component in TGF spectra extended up to ≈100 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 end‐to‐end Monte Carlo simulations and an improved understanding of the instrument performance under high‐flux conditions show that it is possible to explain the observed high‐energy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single high‐energy counts may be the signature of a fine‐pulsed time structure of TGFs on time scales ≈4 μs, 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 × duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled. Key Points: TGFs detected by AGILE with counts energy larger than 40 MeV are compatible with RREAData show no evidence of a fine time structure of TGFs on microsecond time scale.TGFs simultaneous to lightning suggest that the AGILE TGF sample can be significantly increased [ABSTRACT FROM AUTHOR]
Copyright of Journal of Geophysical Research. Atmospheres is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes.
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  Data: <searchLink fieldCode="AR" term="%22Marisaldi%2C+M%2E%22">Marisaldi, M.</searchLink><relatesTo>1,2</relatesTo><i> martino.marisaldi@uib.no</i><br /><searchLink fieldCode="AR" term="%22Galli%2C+M%2E%22">Galli, M.</searchLink><relatesTo>3</relatesTo><br /><searchLink fieldCode="AR" term="%22Labanti%2C+C%2E%22">Labanti, C.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Østgaard%2C+N%2E%22">Østgaard, N.</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Sarria%2C+D%2E%22">Sarria, D.</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Cummer%2C+S%2E+A%2E%22">Cummer, S. A.</searchLink><relatesTo>4</relatesTo><br /><searchLink fieldCode="AR" term="%22Lyu%2C+F%2E%22">Lyu, F.</searchLink><relatesTo>4</relatesTo><br /><searchLink fieldCode="AR" term="%22Lindanger%2C+A%2E%22">Lindanger, A.</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Campana%2C+R%2E%22">Campana, R.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Ursi%2C+A%2E%22">Ursi, A.</searchLink><relatesTo>5</relatesTo><br /><searchLink fieldCode="AR" term="%22Tavani%2C+M%2E%22">Tavani, M.</searchLink><relatesTo>5</relatesTo><br /><searchLink fieldCode="AR" term="%22Fuschino%2C+F%2E%22">Fuschino, F.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Argan%2C+A%2E%22">Argan, A.</searchLink><relatesTo>6</relatesTo><br /><searchLink fieldCode="AR" term="%22Trois%2C+A%2E%22">Trois, A.</searchLink><relatesTo>7</relatesTo><br /><searchLink fieldCode="AR" term="%22Pittori%2C+C%2E%22">Pittori, C.</searchLink><relatesTo>8</relatesTo><br /><searchLink fieldCode="AR" term="%22Verrecchia%2C+F%2E%22">Verrecchia, F.</searchLink><relatesTo>8</relatesTo>
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+Geophysical+Research%2E+Atmospheres%22">Journal of Geophysical Research. Atmospheres</searchLink>. 7/27/2019, Vol. 124 Issue 14, p7484-7497. 14p.
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  Data: <searchLink fieldCode="DE" term="%22Gamma+rays%22">Gamma rays</searchLink><br /><searchLink fieldCode="DE" term="%22Atmospheric+electricity%22">Atmospheric electricity</searchLink><br /><searchLink fieldCode="DE" term="%22Lightning%22">Lightning</searchLink><br /><searchLink fieldCode="DE" term="%22Atmospheric+physics%22">Atmospheric physics</searchLink><br /><searchLink fieldCode="DE" term="%22Monte+Carlo+method%22">Monte Carlo method</searchLink>
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  Data: Terrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest‐energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and high‐energy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a high‐energy component in TGF spectra extended up to ≈100 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 end‐to‐end Monte Carlo simulations and an improved understanding of the instrument performance under high‐flux conditions show that it is possible to explain the observed high‐energy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single high‐energy counts may be the signature of a fine‐pulsed time structure of TGFs on time scales ≈4 μs, 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 × duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled. Key Points: TGFs detected by AGILE with counts energy larger than 40 MeV are compatible with RREAData show no evidence of a fine time structure of TGFs on microsecond time scale.TGFs simultaneous to lightning suggest that the AGILE TGF sample can be significantly increased [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
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  Data: <i>Copyright of Journal of Geophysical Research. Atmospheres is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1029/2019JD030554
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        Text: English
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      – SubjectFull: Lightning
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