Enhanced detection of terrestrial gamma‐ray flashes by AGILE

Marisaldi, M.; Argan, A.; Ursi, A.; Gjesteland, Thomas; Fuschino, F.; Labanti, C.; Galli, M.; Tavani, M.; Pittori, C.; Verrecchia, F.; D’Amico, F.; Østgaard, Nikolai; Mereghetti, S.; Campana, R.; Cattaneo, P. W.; Bulgarelli, A.; Colafrancesco, S.; Dietrich, Stefano; Longo, F.; Gianotti, F.; Giommi, P.; Rappoldi, A.; Trifoglio, M.; Trois, A.

doi:10.1002/2015gl066100

Cited by 50 publications

(86 citation statements)

References 27 publications

(68 reference statements)

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“…After a change in their trigger logic in March 2009, this almost doubled to approximately eight events per month (Marisaldi et al, ). In 2015 the AGILE team removed the veto system for the anticoincidence shield and thereby increased their TGF detection rate by almost an order of magnitude (Marisaldi et al, ).…”

Section: Introductionmentioning

confidence: 99%

Observationally Weak TGFs in the RHESSI Data

et al. 2019

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Terrestrial gamma ray flashes (TGFs) are sub‐millisecond bursts of high energetic gamma radiation associated with intracloud flashes in thunderstorms. In this paper we use the simultaneity of lightning detections by World Wide Lightning Location Network to find TGFs in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data that are too faint to be identified by standard search algorithms. A similar approach has been used in an earlier paper, but here we expand the data set to include all years of RHESSI + World Wide Lightning Location Network data and show that there is a population of observationally weak TGFs all the way down to 0.22 of the RHESSI detection threshold (three counts in the detector). One should note that the majority of these are “normal” TGFs that are produced further away from the subsatellite point (and experience a 1/ r 2 effect) or produced at higher latitudes with a lower tropoause and thus experience increased atmospheric attenuation. This supports the idea that the TGF production rate is higher than currently reported. We also show that compared to lightning flashes, TGFs are more partial to ocean and coastal regions than over land.

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Section: Introductionmentioning

confidence: 99%

Observationally Weak TGFs in the RHESSI Data

et al. 2019

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“…Nevertheless, some common TGF properties have emerged from these data: the sources of TGFs occur within the altitude ranges of thunderclouds and are not associated with sprites as was initially proposed [ Dwyer and Smith , ; Cummer et al ., ]. TGFs have ( T 50 ) durations ranging from about 20 μs to about a millisecond [ Briggs et al ., ; Marisaldi et al ., ], with most having durations around 100 μs. TGFs can be composed of either single or multiple pulses [ Fishman et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Characterizing the source properties of terrestrial gamma ray flashes

et al. 2017

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Monte Carlo simulations are used to determine source properties of terrestrial gamma ray flashes (TGFs) as a function of atmospheric column depth and beaming geometry. The total mass per unit area traversed by all the runaway electrons (i.e., the total grammage) during a TGF, Ξ, is introduced, defined to be the total distance traveled by all the runaway electrons along the electric field lines multiplied by the local air mass density along their paths. It is shown that key properties of TGFs may be directly calculated from Ξ and its time derivative, including the gamma ray emission rate, the current moment, and the optical power of the TGF. For the calculations presented in this paper, a standard TGF gamma ray fluence, F0 = 0.1 cm−2 above 100 keV for a spacecraft altitude of 500 km, and a standard total grammage, Ξ0 = 1018 g/cm2, are introduced, and results are presented in terms of these values. In particular, the current moments caused by the runaway electrons and their accompanying ionization are found for a standard TGF fluence, as a function of source altitude and beaming geometry, allowing a direct comparison between the gamma rays measured in low‐Earth orbit and the VLF‐LF radio frequency emissions recorded on the ground. Such comparisons should help test and constrain TGF models and help identify the roles of lightning leaders and streamers in the production of TGFs.

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“…Only a small fraction of lightning (much less than 1%) is creating TGFs that can be observed from space by current instruments [ Fuschino et al , ; Østgaard et al , ; Briggs et al , ; Tierney et al , ]. But because TGFs are extremely brief (peaking from 100 to 500 μs in duration, with just a few lasting tens of microseconds or over 1 ms [ Briggs et al , , Marisaldi et al , ]), there are typically only 1 or 2 orders of magnitude of dynamic range between events that are barely detectable above background from satellites and those that begin to show saturation effects (high detector dead time). How many fainter events remain to be discovered is an open question and the main subject of this paper.…”

Section: Introductionmentioning

confidence: 99%

“…This method is also an ideal way to address the possibility that there is a large population of TGFs—or another high‐energy radiation mechanism—much shorter in duration than those currently known. The shortest values of t 50 , the time interval containing the middle 50% of counts in a TGF, are about 30 μs in Fermi data [ Briggs et al , ] and 20 μs in AGILE [ Marisaldi et al , ]. But even just a few μs is a plausible time scale for a single step of a stepped leader or for a lightning initiation event stimulated by a large cosmic ray shower in the atmosphere.…”

Section: Introductionmentioning

confidence: 99%

The rarity of terrestrial gamma‐ray flashes: 2. RHESSI stacking analysis

et al. 2016

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We searched for gamma‐ray emission from lightning using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite by identifying times when RHESSI was near over 2 million lightning discharges localized by the Worldwide Lightning Location Network (WWLLN). We then stacked together the gamma‐ray arrival times relative to the sferic times, correcting for light propagation time to the satellite. The resulting stacked gamma‐ray time profile is sensitive to an average level of gamma‐ray emission per lightning discharge far lower than what can be recognized above background for a single terrestrial gamma‐ray flash (TGF). The summed signal from presumed small, previously unknown TGFs simultaneous with WWLLN discharges is remarkably weak: for the region from 0 to 300 km beneath RHESSI's footprint, (6.2 ± 3.8) × 10−3 detector counts/discharge are measured, as opposed to a typical range of 12–50 detector counts for TGFs identified solely from the gamma‐ray signal. Under the assumption of a broken power law differential distribution of TGF intensities, we find that the index must harden dramatically or cut off just below the sensitivity limit of current satellites and that for most scenarios less than 1% of lightning can produce a TGF that belongs anywhere in the same distribution as those that are observable. For the minority of scenarios where more than a few percent of flashes produce a TGF, most of these “TGFs” are less than 10−4 of the luminosity of the faintest RHESSI TGFs and therefore closer to the luminosity of lightning stepped leaders. The rarity of TGFs holds not only for TGFs simultaneous with the sferic observed by WWLLN but also for any time within 10 ms of the sferic, allowing (for example) for the possibility that different events within the upward propagation of a negative leader in positive intracloud lightning triggered the TGF and WWLLN's detection.

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Enhanced detection of terrestrial gamma‐ray flashes by AGILE

Cited by 50 publications

References 27 publications

Observationally Weak TGFs in the RHESSI Data

Observationally Weak TGFs in the RHESSI Data

Characterizing the source properties of terrestrial gamma ray flashes

The rarity of terrestrial gamma‐ray flashes: 2. RHESSI stacking analysis

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