Terrestrial Gamma-Ray Flashes Observed up to 20 MeV (original) (raw)
Thunderstorm characteristics associated with RHESSI identified terrestrial gamma ray flashes
Journal of Geophysical Research, 2010
1] The characteristics of thunderstorms that produce terrestrial gamma-ray flashes (TGFs) observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are determined using climatological and meteorological data. RHESSI observed TGFs follow diurnal, seasonal, and geographic patterns that are very similar to those of thunderstorms confirming, in part, that these events are directly connected to thunderstorm activity. The TGF producing thunderstorms are shown to be closely associated with tall (ranging from 13.6 km to 17.3 km) tropical thunderstorm systems, a finding that is consistent with theoretical expectations from models of relativistic breakdown that relate the source region to the spectral signatures observed by RHESSI. Unlike sprites, there appears to be no predilection for TGFs to occur with large thunderstorm complexes. Rather, TGF producing thunderstorms are shown to range in areal extent by several orders of magnitude. Analysis of a single TGF event within the Mozambique Channel indicates an elevated mixed phase (both liquid water and ice present) level of approximately 6 km which is consistent with the climatological findings.
Gamma-ray burst detection with the AGILE mini-calorimeter
Astronomy & Astrophysics, 2008
The Mini-Calorimeter (MCAL) instrument on-board the AGILE satellite is a non-imaging gamma-ray scintillation detector sensitive in the 300keV-100MeV energy range with a total on-axis geometrical area of 1400cm^2. Gamma-Ray Bursts (GRBs) are one of the main scientific targets of the AGILE mission and the MCAL design as an independent self-triggering detector makes it a valuable all-sky monitor for GRBs. Furthermore MCAL is one of the very few operative instruments with microsecond timing capabilities in the MeV range. In this paper the results of GRB detections with MCAL after one year of operation in space are presented and discussed. A flexible trigger logic implemented in the AGILE payload data-handling unit allows the on-board detection of GRBs. For triggered events, energy and timing information are sent to telemetry on a photon-by-photon basis, so that energy and time binning are limited by counting statistics only. When the trigger logic is not active, GRBs can be detected offline in ratemeter data, although with worse energy and time resolution. Between the end of June 2007 and June 2008 MCAL detected 51 GRBs, with a detection rate of about 1 GRB/week, plus several other events at a few milliseconds timescales. Since February 2008 the on-board trigger logic has been fully active. Comparison of MCAL detected events and data provided by other space instruments confirms the sensitivity and effective area estimations. MCAL also joined the 3rd Inter-Planetary Network, to contribute to GRB localization by means of triangulation.
Discharges in the Stratosphere and Mesosphere
Space Science Reviews, 2012
In the present paper salient features of discharges in the stratosphere and mesosphere (namely sprites, halos, blue starters, blue jets, gigantic jets and elves), are discussed. The electrostatic field due to charge imbalance during lightning processes may lead to stratospheric/mesospheric discharges either through the conventional breakdown based on streamers and leaders or relativistic runaway mechanism. Most (not all) of the observed features of sprites, halos and jets are explained by this processes. Development and evolution of streamers are based on the local transient electrostatic field and available ambient electron density which dictate better probability in favor of positive cloud-to-ground discharges, and thus explains the polarity asymmetry in triggering sprites and streamers. Elves are generated by electromagnetic pulse radiated by return stroke currents of cloud-to-ground/inter-cloud discharges. Generation of the both donut and pancake shape elves are explained. Electrodynamic features of thunderstorms associated with stratospheric/mesospheric discharges are summarized including current and charge moment associated with relevant cloud-to-ground discharges. The hypothesis relating tropospheric generated gravity waves and mesospheric discharges are also discussed. Finally some interesting problems are listed.
Solar Activity, Lightning and Climate
Surveys in Geophysics, 2011
The physics of solar forcing of the climate and long term climate change is summarized, and the role of energetic charged particles (including cosmic rays) on cloud formation and their effect on climate is examined. It is considered that the cosmic raycloud cover hypothesis is not supported by presently available data and further investigations (during Forbush decreases and at other times) should be analyzed to further examine the hypothesis. Another player in climate is lightning through the production of NO x ; this greenhouse gas, water vapour in the troposphere (and stratosphere) and carbon dioxide influence the global temperature through different processes. The enhancement of aerosol concentrations and their distribution in the troposphere also affect the climate and may result in enhanced lightning activity. Finally, the roles of atmospheric conductivity on the electrical activity of thunderstorms and lightning discharges in relation to climate are discussed.
Polarity asymmetry of sprite-producing lightning: A paradox?
Radio Science, 2007
1] Sprites and halos in the mesosphere are produced electrostatically by lightning ground flashes whose polarity is positive, by a margin of at least 1000 to 1 in collected observations. The initiation of these events is controlled by the vertical charge moment change of the flash. Schumann resonance ELF methods have been used to measure the charge moments of millions of flashes worldwide. The bipolar distributions of these events show stronger positive than negative tails, consistent with the predominance of ''positive'' sprites, but the negative tail of supercritical events is still of the order of 10% of the total supercritical population, more than 1 order of magnitude larger than the observed fraction of ''negative'' sprites. This juxtaposition constitutes a paradox. The suggested resolution of the paradox is that the more impulsive population of supercritical negative flashes is producing dim halos that are not readily detected in conventional video imagery. Additional sensitive, high-resolution, and high-speed imager (<1 ms) studies of halos and their lightning parents are needed to verify this hypothesis.
Geophysical Research Letters, 2011
1] The AGILE satellite detects Terrestrial Gamma-ray Flashes (TGFs) in the 0.35-100 MeV energy range using its Mini-Calorimeter (MCAL) instrument with an average detection rate of 10 TGFs/month. Thanks to its Low Earth Orbit with only 2.5 degree of inclination, AGILE guarantees an unprecedented exposure above the equator, where both lightning activity and TGF detection peak. Here we report the comparison between the AGILE TGFs detected between March 2009 and February 2010 and full climatology lightning worldwide distribution based on satellite optical observations from LIS (Lightning Imaging Sensor) and OTD (Optical Transient Detector) instruments. This approach is complementary to the one-to-one TGF/lightning correlations by ground-based sferics measurements. Based on mono and bi-dimensional Kolmogorov-Smirnov tests, we show that the AGILE TGFs and time-averaged global lightning in the equatorial area are not drawn from the same distribution. However, we find significant regional differences in the degree of correlation as well as in the TGF/lightning ratio. In the case of south east Asia we find a 87% probability for the TGF and lightning being samples of the same distribution. This result supports the idea that the physical conditions at play in TGF generation can have strong geographical and climatological modulation. Based on the assumption that the observed range of TGF/flash ratio holds at all latitudes we can estimate a global rate of _ N ' 220 ÷ 570 TGFs per day. The observed TGF/flash geographical modulation as well as the TGF global rate estimate are in agreement with previous observations. Citation : Fuschino, F., et al. (2011), High spatial resolution correlation of AGILE TGFs and global lightning activity above the equatorial belt, Geophys.
Detection of terrestrial gamma ray flashes up to 40 MeV by the AGILE satellite
Journal of Geophysical Research, 2010
We report the detection by the AGILE satellite of Terrestrial Gamma-Ray Flashes (TGFs) obtained with the Mini-Calorimeter (MCAL) detector operating in the energy range 0.3-100 MeV. We select events typically lasting a few milliseconds with spectral and directional selections consistent with the TGF characteristics previously reported by other space missions. During the period 2008 1 June -2009 March 31 we detect 34 high-confidence events showing millisecond durations and a geographical distribution peaked over continental Africa and South-East Asia. For the first time, AGILE-MCAL detects photons associated with TGF events up to 40 MeV. We determine the cumulative spectral properties of the spectrum in the range 0.5-40 MeV,
Terrestrial gamma ray flashes and lightning discharges
Geophysical Research Letters, 2006
1] Analysis of ELF/VLF broadband data from Palmer Station, Antarctica indicates that 76% Terrestrial Gammaray Flashes (TGFs) detected on the RHESSI spacecraft occur in association with lightning-generated radio atmospherics arriving from near the footprint of RHESSI and within a few ms of the TGF. The remaining TGFs are not associated with any radio atmospheric, thus by implication CG lightning. The peak currents of TGFassociated lightning discharges are often among the most intense from a given storm, with the degree of this association apparently varying between oceanic and land regions. The time-integrated ELF energy of the associated sferics (and thus the lightning charge moment) exhibit much less tendency to be large. Statistical analysis of the spread in arrival time suggests a $2 ms variance due to factors other than geometry and measurement error.
Properties of terrestrial gamma ray flashes detected by AGILE MCAL below 30 MeV
Journal of Geophysical Research: Space Physics, 2014
Marisaldi, M., et al. (2014), Properties of terrestrial gamma ray flashes detected by AGILE MCAL below 30 MeV Abstract We present the characteristics of 308 terrestrial gamma ray flashes (TGFs) detected by the Minicalorimeter (MCAL) instrument on board the AGILE satellite during the period March 2009-July 2012 in the ±2.5 • latitude band and selected to have the maximum photon energy up to 30 MeV. The characteristics of the AGILE events are analyzed and compared to the observational framework established by the two other currently active missions capable of detecting TGFs from space, RHESSI and Fermi. A detailed model of the MCAL dead time is presented, which is fundamental to properly interpret our observations. The most significant contribution to dead time is due to the anticoincidence shield in its current configuration and not to the MCAL detector itself. Longitude and local time distributions are compatible with previous observations, while the duration distribution is biased toward longer values because of dead time. The intensity distribution is compatible with previous observations, when dead time is taken into account. The TGFs cumulative spectrum supports a low production altitude, in agreement with previous measurements. We also compare our sample to lightning sferics detected by the World Wide Lightning Location Network and suggest a new method to assess quantitatively the consistency of two TGF populations based on the comparison of the associated lightning activity. According to this method, AGILE and RHESSI samples are compatible with the same parent population. The AGILE TGF catalog below 30 MeV is accessible online at the website of the ASI Science Data Center http://www.asdc.asi.it/mcaltgfcat/.
Relativistic runaway breakdown in low-frequency radio
Journal of Geophysical Research, 2010
1] The electromagnetic radiation emitted by an electron avalanche beam resulting from relativistic runaway breakdown within the Earth's atmosphere is investigated. It is found from theoretical modeling with a computer simulation that the electron beam emits electromagnetic radiation which is characterized by consecutive broadband pulses in the low-frequency radio range from 10to300kHzatadistanceof10 to 300 kHz at a distance of 10to300kHzatadistanceof800 km. Experimental evidence for the existence of consecutive broadband pulses is provided by low-frequency radio observations of sprite-producing lightning discharges at a distance of 550km.Themeasuredbroadbandpulsesoccur550 km. The measured broadband pulses occur 550km.Themeasuredbroadbandpulsesoccur4-9 ms after the sprite-producing lightning discharge, they exhibit electromagnetic radiation which mainly spans the frequency range from 50to350kHz,andtheyexhibitcomplexwaveformswithoutthetypicalionosphericreflectionofthefirsthopskywave.Twoconsecutivepulsesoccur50 to 350 kHz, and they exhibit complex waveforms without the typical ionospheric reflection of the first hop sky wave. Two consecutive pulses occur 50to350kHz,andtheyexhibitcomplexwaveformswithoutthetypicalionosphericreflectionofthefirsthopskywave.Twoconsecutivepulsesoccur4.5 ms and $3 ms after the causative lightning discharge and coincide with the sprite luminosity. It is concluded that relativistic runaway breakdown within the Earth's atmosphere can emit broadband electromagnetic pulses and possibly generates sprites. The source location of the broadband pulses can be determined with an interferometric network of wideband low-frequency radio receivers to lend further experimental support to the relativistic runaway breakdown theory. Citation: Füllekrug, M., R. Roussel-Dupré, E. M. D. Symbalisty, O. Chanrion, A. Odzimek, O. van der Velde, and T. Neubert (2010), Relativistic runaway breakdown in low-frequency radio,
Production of runaway electrons by negative streamer discharges
Journal of Geophysical Research, 2010
1] In this paper we estimate the probability that cold electrons can be accelerated by an ambient electric field into the runaway regime, and discuss the implications for negative streamer formation. The study is motivated by the discovery of ms duration bursts of g-rays from the atmosphere above thunderstorms, the so-called Terrestrial Gamma-Ray Flashes. The radiation is thought to be bremsstrahlung from energetic (MeV) electrons accelerated in a thunderstorm discharge. The observation goes against conventional wisdom that discharges in air are carried by electrons with energies below a few tens of eV. Instead the relativistic runaway electron discharge has been proposed which requires a lower threshold electric field; however, seed electrons must be born with energies in the runaway regime. In this work we study the fundamental problem of electron acceleration in a conventional discharge and the conditions on the electric field for the acceleration of electrons into the runaway regime. We use particle codes to describe the process of stochastic acceleration and introduce a novel technique that improves the statistics of the relatively few electrons that reach high energies. The calculation of probabilities for electrons to reach energies in the runaway regime shows that even with modest fields, electrons can be energized in negative streamer tips into the runaway regime, creating a beamed distribution in front of the streamer that affects its propagation. The results reported here suggest that theories of negative streamers and spark propagation should be reexamined with an improved characterization of the kinetic effects of electrons.
Electron acceleration above thunderclouds
Environmental Research Letters, 2013
The acceleration of electrons results in observable electromagnetic waves which can be used for remote sensing. Here, we make use of ∼4 Hz-66 MHz radio waves emitted by two consecutive intense positive lightning discharges to investigate their impact on the atmosphere above a thundercloud. It is found that the first positive lightning discharge initiates a sprite where electrons are accelerated during the exponential growth and branching of the sprite streamers. This preconditioned plasma above the thundercloud is subsequently exposed to a second positive lightning discharge associated with a bouncing-wave discharge. This discharge process causes a re-brightening of the existing sprite streamers above the thundercloud and initiates a subsequent relativistic electron beam.
Terrestrial Gamma-Ray Flashes as Powerful Particle Accelerators
Physical Review Letters, 2011
3 Tropical thunderstorms can produce terrestrial gamma-ray flashes 1 (TGFs), i.e., intense bursts of X-rays and gamma-rays associated with very powerful lightning. TGFs typically last a few milliseconds and produce impulsive radiation that can be detected by space satellites 1,2,3,4 from tens of kiloelectronvolt up to tens of megaelectronvolt. TGFs are now established 5 to occur deep in the atmosphere (~10-25 km above ground) near the upper regions of tropical thunderclouds. Here we present new TGF timing and spectral data based on the observations of the Italian Space Agency's AGILE satellite. We determine that the TGF photon energies may reach 50 MeV and above, implying accelerating potential differences of order of hundreds of megavolts in the cloud-to-ground or intercloud discharges.
Gamma-Ray Localization of Terrestrial Gamma-Ray Flashes
Physical Review Letters, 2010
Terrestrial Gamma-Ray Flashes (TGFs) are very short bursts of high energy photons and electrons originating in Earth's atmosphere. We present here a localization study of TGFs carried out at gamma-ray energies above 20 MeV based on an innovative event selection method. We use the AGILE satellite Silicon Tracker data that for the first time have been correlated with TGFs detected by the AGILE Mini-Calorimeter. We detect 8 TGFs with gamma-ray photons of energies above 20 MeV localized by the AGILE gamma-ray imager with an accuracy of ∼ 5 − 10 • at 50 MeV. Remarkably, all TGF-associated gamma rays are compatible with a terrestrial production site closer to the sub-satellite point than 400 km. Considering that our gamma rays reach the AG-ILE satellite at 540 km altitude with limited scattering or attenuation, our measurements provide the first precise direct localization of TGFs from space.
Confining the angular distribution of terrestrial gamma ray flash emission
Journal of Geophysical Research, 2011
1] Terrestrial gamma ray flashes (TGFs) are bremsstrahlung emissions from relativistic electrons accelerated in electric fields associated with thunder storms, with photon energies up to at least 40 MeV, which sets the lowest estimate of the total potential of 40 MV. The electric field that produces TGFs will be reflected by the initial angular distribution of the TGF emission. Here we present the first constraints on the TGF emission cone based on accurately geolocated TGFs. The source lightning discharges associated with TGFs detected by RHESSI are determined from the Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education (AWESOME) network and the World Wide Lightning Location Network (WWLLN). The distribution of the observation angles for 106 TGFs are compared to Monte Carlo simulations. We find that TGF emissions within a half angle >30°are consistent with the distributions of observation angle derived from the networks. In addition, 36 events occurring before 2006 are used for spectral analysis. The energy spectra are binned according to observation angle. The result is a significant softening of the TGF energy spectrum for large (>40°) observation angles, which is consistent with a TGF emission half angle (<40°). The softening is due to Compton scattering which reduces the photon energies.
Connecting the terrestrial gamma-ray flash source strength and observed fluence distributions
Journal of Geophysical Research, 2012
1] Terrestrial gamma-ray flashes (TGFs) as observed by satellites have a broad fluence distribution. This fluence distribution is not trivially related to the source strength distribution, since even a very strong TGF may still be observed at low fluence if the source is far from the satellite. In this paper we connect the source strength distribution with the observed fluence distribution by calculating the effective size and probability of detection of TGFs as a function of their source strength. For sources at a single altitude, power law distributions of source strength give softer power law distributions in observed fluence with especially pronounced softening for very hard source power law indices. This result holds even with broad source altitude distributions in regions of the fluence distribution away from the peak fluence since such regions tend to be dominated by TGFs produced in a relatively narrow range of higher altitudes.
Journal of Geophysical Research, 2008
1] On the basis of the RHESSI results it has been suggested that terrestrial gamma flashes (TGFs) are produced at very low altitudes. On the other hand some of the Burst and Transient Source Experiment (BATSE) spectra show unabsorbed fluxes of X rays in the 25-50 keV energy range, indicating a higher production altitude. To investigate this, we have developed a Monte Carlo code for X-ray propagation through the atmosphere. The most important features seen in the modeled spectra are (1) a low-energy cutoff which moves to lower energies as TGFs are produced at higher altitudes, (2) a high-energy cutoff which moves to lower energies as TGFs are observed at larger zenith angles, and (3) time delays are observed for TGFs produced at 20 km (and some at 30 km) altitude when observed at larger zenith angle than the half-angle defining the initial isotropic X-ray beam. This is a pure Compton effect. The model results and an optimization procedure are used to estimate production altitudes of the BATSE TGFs. The main findings are (1) half or more of the BATSE TGFs are produced at low altitudes, 20 km, (2) a significant portion of the BATSE TGFs are produced at higher altitudes, 30 km to 40 km, (3) for the TGFs produced at 20 km (and some at 30 km) altitudes the dispersion signatures can be explained as a pure Compton effect, and (4) the softening of the BATSE spectra for increasing zenith angles and the time dispersions both indicate that the initial TGF distribution is beamed. Citation: Østgaard, N., T. Gjesteland, J. Stadsnes, P. H. Connell, and B. Carlson (2008), Production altitude and time delays of the terrestrial gamma flashes: Revisiting the Burst and Transient Source Experiment spectra,
Terrestrial gamma-ray flash electron beam geometry, fluence, and detection frequency
Journal of Geophysical Research, 2011
1] Terrestrial gamma-ray flashes (TGFs) are associated with emission of detectable beams of electrons into space. In this paper we use simulations of TGF and electron beam emission and escape from the atmosphere to determine how the geometry and fluence of such events depend on the angular distribution of the source photons. Given a photon source, the geometry of the electron beam depends on the geomagnetic latitude of the source but can be well-predicted by tracing a disk at 57 km altitude along the geomagnetic field to satellite orbit. The fluence and geometry are then used to infer the relative detection probabilities of TGF and electron beam in the context of a variety of photon sources and intensities. Analysis of detection probabilities and the relative frequency of TGF and electron beam detections suggests the existence of a population of electron beams emitted by TGFs too faint to be detected as photons.
Journal of Geophysical Research, 2010
1] Measurements from the Burst and Transient Source Experiment (BATSE) instrument on the Compton Gamma Ray Observatory (CGRO) are the only ones where characteristics of single terrestrial gamma ray flashes (TGFs) have been obtained thus far. However, it has been reported that the measurements suffer from significant dead time losses which complicates the analysis and raises question about earlier BATSE studies. These losses are due to the high-intensity flux combined with limitations of the time resolution of the instrument. Since these losses will affect both the spectrum and the temporal distribution of the individual TGFs, results based on BATSE data need to be revisited, including our own. We have therefore developed a Monte Carlo method to study the effects of these dead time losses. We show that the energy spectrum of TGFs becomes softer as the dead time losses increase. We also show that the time delay between the light curves of hard (E > 300 keV) and soft (E < 300 keV) photons increases significantly as the dead time losses increase. The Monte Carlo approach also enables us to identify the BATSE TGFs where the dead time effects can be corrected. These are the short-duration single-peaked TGFs. Without correcting for dead time losses we find that these short single-peak TGFs have a softer energy spectrum and larger time delay than the multipeaked TGFs. After correcting for dead time losses we perform a new analysis of production altitudes and find that the production altitude is reduced compared to analysis without dead time losses. The new production altitudes combined with dead time losses are also consistent with the apparent large time delays. Our method gives consistent results regarding production altitude and time delays and indicates that the corrected TGF intensities measured by BATSE are 3 to 4 times brighter than the uncorrected measurements would indicate. We also show that the production mechanism of these TGFs has a typical duration of 250 ms. Citation: Gjesteland, T., N. Østgaard, P. H. Connell, J. Stadsnes, and G. J. Fishman (2010), Effects of dead time losses on terrestrial gamma ray flash measurements with the Burst and Transient Source Experiment,
Assessing the power law distribution of TGFs
Journal of Geophysical Research, 2011
A seed population of relativistic electrons is required to induce a RREA. One possible source 60 is cosmic rays, although Carlson et al. [2008] conclude that these are typically insufficient. 61 Alternatively, thermal electrons can be accelerated to the required energies by the acute electric 62 fields in streamer tips [Moss et al., 2006]. Dwyer [2007] proposed a feedback mechanism which 63 can regenerate seed electrons from back-scattered energetic photons and positrons, leading to a 64 RREA able to rapidly discharge the ambient electric field so that lightning does not occur.
Journal of Geophysical Research: Space Physics, 2013
Up to a few years ago, Terrestrial Gamma-Ray Flashes (TGF) were only observed 4 by space borne instruments. The aircraft campaign ADELE was able to observe one 5 TGF and more attempts on aircraft observations are planned. There is also a planned 6 campaign with stratospheric balloons, COBRAT. In this context an important question 7 that arises is what count rates we can expect and how these estimates are affected by 8 the initial properties of the TGFs. Based on simulations of photon propagation in air we 9 find the photon fluence at different observation points at aircraft and balloon altitudes.
Physical Review D, 2010
The Aragats Space Environmental Center facilities continuously measure fluxes of neutral and charged secondary cosmic ray incidents on the Earth's surface. Since 2003 in the 1-minute time series we have detected more than 100 enhancements in the electron, gamma ray, and neutron fluxes correlated with thunderstorm activities. During the periods of the count rate enhancements, lasting tens of minutes, millions of additional particles were detected. Based on the largest particle event of September 19, 2009, we show that our measurements support the existence of long-lasting particle multiplication and acceleration mechanisms in the thunderstorm atmosphere. For the first time we present the energy spectra of electrons and gamma rays from the particle avalanches produced in the thunderstorm atmosphere, reaching the Earth's surface.
Journal of Geophysical Research, 2010
1] In Russia several space missions are now planned to study transient luminous events in the atmosphere and high-energy charged particles at satellite altitudes. The experimental goal is to investigate the origin of the high-energy electrons and gamma ray quanta for specific transient luminous events (TLEs) and their role in the ionosphere-magnetosphere system. Simultaneous measurements of electrons at the orbit of the satellite and TLE atmospheric radiation in many wavelength bands will be performed in two missions, Tatiana-2 and RELEC. In the TUS mission UV transient event detection will be accompanied by measurements of the weak UV emission from the "seed" electrons of extensive air showers of extremely high-primary energies. Citation: Panasyuk, M. I., et al. (2010), Transient luminous event phenomena and energetic particles impacting the upper atmosphere: Russian space experiment programs,
Energetic Charged Particles Above Thunderclouds
Surveys in Geophysics, 2013
The French government has committed to launch the satellite TARANIS to study transient coupling processes between the Earth's atmosphere and near-Earth space. The prime objective of TARANIS is to detect energetic charged particles and hard radiation emanating from thunderclouds. The British Nobel prize winner C.T.R. Wilson predicted lightning discharges from the top of thunderclouds into space almost a century ago. However, new experiments have only recently confirmed energetic discharge processes which
Compton scattering in terrestrial gamma-ray flashes detected with the Fermi gamma-ray burst monitor
Physical Review D, 2014
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 ∼100 μs.
Runaway breakdown and electrical discharges in thunderstorms
Journal of Geophysical Research, 2010
This review considers the precise role played by runaway breakdown (RB) in 5 the initiation and development of lightning discharges. RB remains a fundamental research 6 topic under intense investigation. The question of how lightning is initiated and 7 subsequently evolves in the thunderstorm environment rests in part on a fundamental 8 understanding of RB and cosmic rays and the potential coupling to thermal runaway 9 (as a seed to RB) and conventional breakdown (as a source of thermal runaways). In 10 this paper, we describe the basic mechanism of RB and the conditions required to initiate 11 an observable avalanche. Feedback processes that fundamentally enhance RB are 12 discussed, as are both conventional breakdown and thermal runaway. Observations 13 that provide clear evidence for the presence of energetic particles in thunderstorms/ 14 lightning include g-ray and X-ray flux intensifications over thunderstorms, g-ray and 15 X-ray bursts in conjunction with stepped leaders, terrestrial g-ray flashes, and neutron 16 production by lightning. Intense radio impulses termed narrow bipolar pulses 17 (or NBPs) provide indirect evidence for RB particularly when measured in association 18 with cosmic ray showers. Our present understanding of these phenomena and their 19 enduring enigmatic character are touched upon briefly. .
Origin of neutron flux increases observed in correlation with lightning
Journal of Geophysical Research, 2007
1] The past decade of research into the phenomenon of lightning has seen an accumulation of evidence for the existence of penetrating radiation (X-and g-rays) in direct association with many forms of discharges. As a result, our basic understanding of the mechanisms that produce lightning has shifted from the present paradigm based on conventional breakdown to a picture that incorporates the acceleration and avalanche of energetic particles. Experiments conducted at high mountainous facilities in Gulmarg, India, have further confirmed the need for a paradigm shift. These measurements have shown an enhancement in neutron flux in the atmosphere in correlation with lightning electromagnetic pulses. We demonstrate here that the prevailing neutron generation theory based on synthesis of deuterium nuclei in the lightning channel is not feasible. Instead, this phenomenon is most likely connected with photonuclear reactions produced as part of the recently elaborated theory of relativistic runaway breakdown.
Journal of Geophysical Research, 2010
1] Detailed Boltzmann kinetic calculations of the electron distribution functions resulting from thermal runaway in a constant electric field are presented. Thermal runaway is considered to occur when an initially thermal electron is accelerated above the 150 eV peak in the dynamical friction force in air and becomes a runaway electron. We investigate the role of runaway breakdown in situations where thermal runaway, as well as conventional breakdown, is occurring. The electric field strengths studied span the range from the threshold for runaway breakdown in air (∼0.3 MV/m at sea level) through conventional breakdown (2.4-3.2 MV/m at sea level) and exceeding the Dreicer field (25 MV/m at sea level), above which all electrons are runaways. We initiate our simulations with a population of pseudothermal electrons or with a combination of thermal and runaway (∼1 MeV) electrons. We find that when thermal runaway occurs the selfsimilar electron distribution function is identical in the presence or absence of a seed runaway population. We show that attempts to obtain the electric field from remote measurements of optical line ratios are ambiguous both in the context of the absolute field and in the underlying kinetics. By considering the runaway electrons as a separate population we conclude that the avalanche rate of low-energy electrons is equivalent to that of runaway electrons at a reduced field of 140 Td (3.8 MV/m at standard temperature and pressure). Above that field the conventional avalanche rate will control the avalanche rate of the entire population. Below that field the runaway avalanche rate will control the avalanche rate of the entire population.
Journal of Geophysical Research, 2007
We study local time variation in high peak current lightning over land versus over ocean by using lightning locations from the World Wide Lightning Location Network (WWLLN). Optical lightning data from the photodiode detector on the Fast On-Orbit Recording of Transient Events (FORTE) satellite are used to determine the relative detection efficiency of the WWLLN for lightning events by region, as well as over land versus over ocean. We find that the peak lightning flash density varies for the different continents by up to 5 hours in local time. Because the WWLLN measures lightning strokes with large peak currents, the variation in local time of WWLLN-detected strokes suggests a similar variation in local time of transient luminous events (e.g., elves) and their effects on the lower ionosphere.
Remote Sensing
In this article, we report the first investigation over time of the atmospheric conditions around terrestrial gamma-ray flash (TGF) occurrences, using GPS sensors in combination with geostationary satellite observations and ERA5 reanalysis data. The goal is to understand which characteristics are favorable to the development of these events and to investigate if any precursor signals can be expected. A total of 9 TGFs, occurring at a distance lower than 45 km from a GPS sensor, were analyzed and two of them are shown here as an example analysis. Moreover, the lightning activity, collected by the World Wide Lightning Location Network (WWLLN), was used in order to identify any links and correlations with TGF occurrence and precipitable water vapor (PWV) trends. The combined use of GPS and the stroke rate trends identified, for all cases, a recurring pattern in which an increase in PWV is observed on a timescale of about two hours before the TGF occurrence that can be placed within the...
Thunderstorm ground enhancements: Gamma ray differential energy spectra
Physical Review D, 2013
The shape and evolution of the energy spectra of the thunderstorm ground enhancement (TGE) electrons and gamma rays shed light on the origin of TGEs, on the relationship of modification of the energy spectra (MOS) and relativistic runaway electron avalanche processes, on the nature of the seed particles, and on the strength and elongation of an atmospheric electric field. However, till now the measurements of energy spectra of TGE electrons and gamma rays have been rather scarce. For the first time, we present differential energy spectra of gamma rays in the wide energy range 4-100 MeV for five TGE events detected in 2012-2013 at Aragats. We use the special technique of electron/gamma ray fraction determination to select TGE events with very small contamination of electrons. The network of large NaI spectrometers located 3200 m above sea level measured energy spectra of gamma rays. The power law indices of ''small'' TGEs are rather close to the background cosmic gamma ray spectrum ($ À2); thus, we may deduce that these small events are due to MOS of cosmic ray electrons in the electric field of a thundercloud. Larger TGEs measured by the NaI network and the two largest TGE events earlier recovered from energy releases in a 60-cm-thick scintillator have much steeper energy spectra typical for the avalanche process in atmosphere. The classification of TGEs according to intensity and gamma ray spectral index pointed toward two main mechanisms of the TGE gamma ray origin: the runaway process and modification of electron energy spectra in the thunderstorm atmospheres.
Journal of Physics: Conference Series
A recent theoretical explanation for observed anomalous low energy nuclear phenomena which have puzzled physicists for many years is expanded on. Based on covariant relativistic quantum mechanics and historical time wave equations, it explains a large number of observed anomalous effects by supposing that nuclear masses can vary in "nuclear active environments" in condensed matter settings. The modified quantum wave equation originally introduced by Fock and Stueckelberg in the 1930s with significant enhancements up to the present by Horwitz and others prove that variable masses are compatible with the principles of both quantum mechanics and relativity. They can explain all of these effects by modifying the kinematic constraints of the reaction, enhancing electron screening and quantum tunneling rates, and allowing for resonant tunneling. Some previous results are recounted, and experimental evidence based on variable radioactive decay rates and other evidence for variable masses is presented which adds some new potential support for this theory.
Implications of GNSS-Inferred Tropopause Altitude Associated with Terrestrial Gamma-ray Flashes
Remote Sensing
The thermal structure of the environmental atmosphere associated with Terrestrial Gamma-ray Flashes (TGFs) is investigated with the combined observations from several detectors (FERMI, RHESSI, and Insight-HXMT) and GNSS-RO (SAC-C, COSMIC, GRACE, TerraSAR-X, and MetOp-A). The geographic distributions of TGF-related tropopause altitude and climatology are similar. The regional TGF-related tropopause altitude in Africa and the Caribbean Sea is 0.1–0.4 km lower than the climatology, whereas that in Asia is 0.1–0.2 km higher. Most of the TGF-related tropopause altitudes are slightly higher than the climatology, while some of them have a slightly negative bias. The subtropical TGF-producing thunderstorms are warmer in the troposphere and have a colder and higher tropopause over land than the ocean. There is no significant land–ocean difference in the thermal structure for the tropical TGF-producing thunderstorms. The TGF-producing thunderstorms have a cold anomaly in the middle and upper ...
Journal of Geophysical Research: Space Physics
On 25 October 2012 the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) and the Tropical Rainfall Measuring Mission (TRMM) satellites passed over a thunderstorm on the coast of Sri Lanka. RHESSI observed a terrestrial gamma ray flash (TGF) originating from this thunderstorm. Optical measurements of the causative lightning stroke were made by the lightning imaging sensor (LIS) on board TRMM. The World Wide Lightning Location Network (WWLLN) detected the very low frequency (VLF) radio emissions from the lightning stroke. The geolocation from WWLLN, which we also assume is the TGF source location, was in the convective core of the cloud. By using new information about both RHESSI and LIS timing accuracy, we find that the peak in the TGF light curve occurs 230 μs before the WWLLN time. Analysis of the optical signal from LIS shows that within the uncertainties, we cannot conclude which comes first: the gamma emission or the optical emission. We have also applied the new information about the LIS timing on a previously published event by Østgaard et al. (2012). Also for this event we are not able to conclude which signal comes first. More accurate instruments are needed in order to get the exact timing between the TGF and the optical signal. Plain Language Summary In this paper we present two terrestrial gamma ray flash (TGF) events with observations from two different spacecraft. The Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) and the Tropical Rainfall Measuring Mission (TRMM) satellites passed over the same thunderstorm when TGF and lightning were produced. RHESSI measured gamma ray and the lightning imaging sensor on board TRMM satellite measured optical emission from the lightning stroke. We found that the TGF (gamma rays) and the optical part of the lightning stroke were produced simultaneous to within 1.6 ms. This indicates the TGF occurs very close to the lightning stroke.
A Distinct Class of High Peak‐Current Lightning Pulses Over Mountainous Terrain in Thunderstorms
Geophysical Research Letters
Analysis of the radio emissions from lightning discharges continues to provide new insight into lightning and atmospheric electricity processes, especially classes of energetic discharges that have particularly strong radio emissions. Cloud-to-ground (CG) return strokes with large peak-current or large charge moment changes (Lyons et al., 1998) produce transient luminous events between cloud tops and the ionosphere. Narrow bipolar events (NBEs) (Le Vine, 1980; Smith et al., 1999; Willett et al., 1989) demonstrate the existence of powerful very high frequency (VHF) radio frequency emissions inside thunderclouds and also serve as the initiation process for some lightning flashes (
A Terrestrial Gamma-Ray Flash inside the Eyewall of Hurricane Patricia
Journal of Geophysical Research: Atmospheres
On 23 October 2015 at~1732 UTC, the Airborne Detector for Energetic Lightning Emissions (ADELE) flew through the eyewall of Hurricane Patricia aboard National Oceanic and Atmospheric Administration's Hurricane Hunter WP-3D Orion, observing the first terrestrial gamma-ray flash (TGF) ever seen in that context, and the first ever viewed from behind the forward direction of the main TGF gamma-ray burst. ADELE measured 184 counts of ionizing radiation within 150 μs, coincident with the detection of a nearby lightning flash. Lightning characteristics inferred from the associated radio signal and comparison of the gamma-ray energy spectrum to simulations suggests that this is the first observation of a reverse beam of positrons predicted by the leading TGF production model, relativistic runaway electron avalanches. This paper presents the first experimental evidence of a previously predicted second component of gamma-ray emission from TGFs. The brightest emission, commonly observed from orbit, is from the relativistic runaway electron avalanche bremsstrahlung; the second, fainter component reported here is from the bremsstrahlung of positrons propagating in the reverse direction. This reverse gamma-ray beam penetrates to low enough altitudes to allow ground-based detection of typical upward TGFs from mountain observatories. Plain Language Summary We report the first observation of gamma-ray emission from lightning within a hurricane eyewall, consistent with production by a downward beam of positrons.
Characteristics of Radio Emissions Associated With Terrestrial Gamma-Ray Flashes
Journal of Geophysical Research: Space Physics
In this study, we analyze 44 terrestrial gamma-ray flashes (TGFs) detected by the Fermi Gamma-ray Burst Monitor (GBM) occurring in 2014-2016 in conjunction with data from the U.S. National Lightning Detection Network (NLDN). We examine the characteristics of magnetic field waveforms measured by NLDN sensors for 61 pulses that occurred within 5 ms of the start-time of the TGF photon flux. For 21 (out of 44) TGFs, the associated NLDN pulse occurred almost simultaneously with (that is, within 200 μs of) the TGF. One TGF had two NLDN pulses within 200 μs. The median absolute time interval between the beginning of these near-simultaneous pulses and the TGF flux start-time is 50 μs. We speculate that these RF pulses are signatures of either TGF-associated relativistic electron avalanches or currents traveling in conducting paths "preconditioned" by TGF-associated electron beams. Compared to pulses that were not simultaneous with TGFs (but within 5 ms of one), simultaneous pulses had higher median absolute peak current (26 kA versus 11 kA), longer median threshold-to-peak rise time (14 μs versus 2.8 μs), and longer median peak-to-zero time (15 μs versus 5.5 μs). A majority (77%) of our simultaneous RF pulses had NLDN-estimated peak currents less than 50 kA indicating that TGF emissions can be associated with moderate-peak-amplitude processes. The lightning flash associated with one of the TGFs in our data set was observed by a Lightning Mapping Array, which reported a relatively high-power source at an altitude of 25 km occurring 101 μs after the GBM-reported TGF discovery-bin start-time.
Surveys in Geophysics
In 1999, the first sprites were observed above European thunderstorms using sensitive cameras. Since then, Eurosprite campaigns have been conducted to observe sprites and other transient luminous events (TLEs), expanding into a network covering large parts of Europe and coastal areas. In 2009 through 2013, the number of optical observations of TLEs reached a peak of 2000 per year. Because of this unprecedented number of European observations, it was possible to construct 5 a climatology of 8394 TLEs observed above 1018 thunderstorm systems, and study for the first time their distribution and seasonal cycle above Europe and parts of the Mediterranean Sea. The number of TLEs per thunderstorm was found to follow a power law, with less than 10 TLEs for 801 thunderstorms and up to 195 TLEs above the most prolific one. The majority of TLEs were classified as sprites, 641 elves, 280 halos, 70 upward lightning, 2 blue jets and 1 gigantic jet. The climatology 10 shows intense TLE activity during summer over continental areas, and in late autumn over coastal 1 vised manuscript with changes highlighted Click here to access/download;attachment to manuscript;Arnone2019_SG_revised_manuscript_RM_changes_ lick here to view linked References areas and sea. The two seasons peak respectively in August and November, separated by March and April with almost no TLEs, and a relative minimum around September. The observed TLE activity, i.e. mostly sprites, is shown to be largely consistent with lightning activity, with a 1/1000 of observed TLE to lightning ratio in regions with most observations. The overall behavior is consistent among individual years, making the observed seasonal cycle a robust general feature of TLE activity above Europe. Article Highlights • Detailed climatological study of TLEs and lightning above Europe produced for the first time • Seasonal cycle of TLEs and lightning with two active seasons in summer over land and autumn over sea and coastal areas • Power law distribution of number of TLEs per thunderstorm Keywords Thunderstorms, lightning, transient luminous events, ground-based observations, Europe, climatology. 1 Introduction 1.1 Transient luminous events Three decades ago, a test low-light camera recorded a sprite (Franz et al., 1990), a spectacular discharge extending for tens of km above a thunderstorm. It was the first discovery of a whole family of upper atmosphere electrical processes, collectively known as transient luminous events (TLEs-see
TETRA observation of gamma-rays at ground level associated with nearby thunderstorms
Journal of Geophysical Research - Space Physics, 2013
Terrestrial gamma-ray flashes (TGFs)-very short, intense bursts of electrons, positrons, and energetic photons originating from terrestrial thunderstorms-have been detected with satellite instruments. TGF and Energetic Thunderstorm Rooftop Array (TETRA), an array of NaI(Tl) scintillators at Louisiana State University, has now been used to detect similar bursts of 50 keV to over 2 MeV gamma-rays at ground level. After 2.6 years of observation, 24 events with durations 0.02-4.2 ms have been detected associated with nearby lightning, three of them coincident events observed by detectors separated by~1000 m. Nine of the events occurred within 6 ms and 5 km of negative polarity cloud-to-ground lightning strokes with measured currents in excess of 20 kA. The events reported here constitute the first catalog of TGFs observed at ground level in close proximity to the acceleration site.
Journal of Geophysical Research: Space Physics, 2013
Terrestrial gamma-ray flashes (TGFs) have been correlated with an early development stage of high altitude positive intracloud (+IC) flashes in which the negative leader propagates up toward the upper positive charge region, while the positive leader propagates down toward the lower negative charge region. The resultant bidirectional leaders develop electrical potential differences in the vicinity of their heads with respect to the ambient potential distribution created by the thundercloud charges. These potential differences are believed to be of essential importance for the generation of TGFs. Using electrostatic calculations and a three-dimensional Cartesian fractal model, we quantify these potential differences produced in a developing +IC lightning discharge for given thunderstorm electric configurations. We present a case of a +IC lightning discharge in a realistic thunderstorm configuration that leads to a very high ($300 MV) potential difference and show how a delay in the development of the negative leader with respect to the positive one in a bidirectional leader system can facilitate a high potential difference in the negative leader head region.
Observation of Neutron Bursts Produced by Laboratory High-Voltage Atmospheric Discharge
Physical Review Letters, 2013
For the first time the emission of neutron bursts in the process of high-voltage discharge in air was observed. Experiments were carried out at an average electric field strength of 1MVAˊmAˋ1anddischargecurrentof1 MV Á m À1 and discharge current of 1MVAˊmAˋ1anddischargecurrentof10 kA. Two independent methods (CR-39 track detectors and plastic scintillation detectors) registered neutrons within the range from thermal energies up to energies above 10 MeV and with an average flux density of *10 6 cm À2 per shot inside the discharge zone. Neutron generation occurs at the initial phase of the discharge and correlates with x-ray generation. The data obtained allow us to assume that during the discharge fast neutrons are mainly produced.
Constraints to do realistic modeling of the electric field ahead of the tip of a lightning leader
Journal of Geophysical Research: Atmospheres, 2017
Several computer models exist to explain the observation of terrestrial gamma-ray flashes (TGFs). Some of these models estimate the electric field ahead of lightning leaders and its effects on electron acceleration and multiplication. In this paper, we derive a new set of constraints to do more realistic modeling. We determine initial conditions based on in situ measurements of electric field and vertical separation between the main charge layers of thunderclouds. A maximum electric field strength of 50 kV/cm at sea level is introduced as the upper constraint for the leader electric field. The threshold for electron avalanches to develop of 2.86 kV/cm at sea level is introduced as the lower value. With these constraints, we determine a region where acceleration and multiplication of electrons occur. The maximum potential difference in this region is found to be ∼52 MV, and the corresponding number of avalanche multiplication lengths is ∼3.5. We then quantify the effect of the ambient electric field compared to the leader field at the upper altitude of the negative tip. Finally, we argue that only leaders with the highest potential difference between its tips (∼600 MV) can be candidates for the production of TGFs. However, with the assumptions we have used, these cannot explain the observed maximum energies of at least 40 MeV. Open questions with regard to the temporal development of the streamer zone and its effect on the shape of the electric field remain.
Universat-SOCRAT multi-satellite project to study TLEs and TGFs
Progress in Earth and Planetary Science, 2019
We present concept of a new multi-satellite Universat-SOCRAT project aimed to study transient phenomena in the upper atmosphere such as transient luminous events (TLEs) and terrestrial gamma-ray flashes (TGFs). It is a new space project of Lomonosov Moscow State University based on the use of a few satellites in the near-Earth orbit for real-time monitoring of radiation environment, natural (asteroids, meteoroids) and artificial (space debris) potentially dangerous objects, electromagnetic transients including cosmic gamma-ray bursts, terrestrial gamma-ray flashes, and optical and ultraviolet bursts in the Earth's atmosphere. Study of TLEs and TGFs remains an important and demanding task despite of a multitude of recently acquired data for these phenomena. This might be explained by the absence of comprehensive theoretical understanding of physical nature of high-energy processes in the Earth's atmosphere. Multi-wavelength synchronous observations with moderate accuracy of localization of TGF and TLE events are necessary to gain an insight of physics governing these high-energy processes in the Earth's atmosphere. In the article, we present results of TLE observations in space experiments of Moscow State University and discuss advanced instruments for optical observations of TLEs, as well as gamma-ray burst monitor and tracking gamma spectrometer for TGFs observations.
Observation of an Energetic Radiation Burst from Mountain-Top Thunderclouds
Physical Review Letters, 2009
During thunderstorms on 2008 September 20, a simultaneous detection of γ rays and electrons was made at a mountain observatory in Japan located 2770 m above sea level. Both emissions, lasting 90 seconds, were associated with thunderclouds rather than lightning. The photon spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung γ rays arriving from a source which is 60 − 130 m in distance at 90% confidence level. The observed electrons are likely to be dominated by a primary population escaping from an acceleration region in the clouds.
Insights into high peak current in‐cloud lightning events during thunderstorms
Geophysical Research Letters, 2015
We investigated National Lightning Detection Network reports and lightning radio waveforms in a 44 day observation period to analyze the in-cloud (IC) events producing currents above 200 kA. The results show that there are two distinct classes of IC lightning events with very high peak currents: the well-known narrow bipolar events, and a previously unreported type that we call energetic in-cloud pulses (EIPs). Their temporal and spatial context shows that EIPs are generated from existing negative polarity leaders that are propagating usually upward but sometimes downward. The nearly identical characteristics of EIPs and some previously reported terrestrial gamma ray flashes (TGFs) indicate a likely connection between the two, which further suggests the possibility of downward directed TGFs. These very high peak current IC events also suggest the association of EIPs with ionospheric perturbations and optical emissions known as elves.
Journal of geophysical research. Atmospheres : JGR, 2017
It has been discussed that lightning flashes emit high-energy electrons, positrons, photons, and neutrons with single energies of several tens of MeV. In the first part of this paper we study the absorption of neutron beams in the atmosphere. We initiate neutron beams of initial energies of 350 keV, 10 MeV, and 20 MeV at source altitudes of 4 km and 16 km upward and downward and see that in all these cases neutrons reach ground altitudes and that the cross-section areas extend to several km(2). We estimate that for terrestrial gamma-ray flashes approximately between 10 and 2000 neutrons per ms and m(2) are possibly detectable at ground, at 6 km, or at 500 km altitude. In the second part of the paper we discuss a feedback model involving the generation and motion of electrons, positrons, neutrons, protons, and photons close to the vicinity of lightning leaders. In contrast to other feedback models, we do not consider large-scale thundercloud fields but enhanced fields of lightning le...
A pipeline to link meteorological information and TGFs detected by AGILE
Journal Of Geophysical Research: Space Physics, 2017
Terrestrial gamma ray flashes (TGFs) are brief (approximately hundreds of microseconds) intense gamma ray emissions coming from Earth's atmosphere (∼15 km above sea level), correlated with thunderstorms and atmospheric electric activity. Since their unexpected discovery in the early 1990s by the Burst And Transient Source Experiment/Compton Gamma Ray Observatory, TGFs have been further investigated by several satellites devoted to high-energy astrophysics. The Astrorivelatore Gamma ad Immagini LEggero (AGILE) mission turned out to be particularly suitable to detect these events, due to a very wide energy range (up to 100 MeV), an optimized triggering system, and a unique low-inclination near-equatorial orbit (2.5 ∘). We describe a detection system, developed for the AGILE satellite, whose aim is to provide real-time meteorological information on each detected TGF. We take advantage of data acquired by geostationary satellites to promptly identify the associated storm and follow its evolution in space and time, in order to study its previous onset and development. Data from Low-Earth Orbit meteorological satellites, such as the Global Precipitation Mission, as well as ground measurements from lightning detection networks, can be integrated in the pipeline. This system allows us a prompt characterization of the ground meteorological conditions at TGF time which will provide instrument-independent trigger validation, fill in a database for subsequent statistical analysis, and eventually, on a longer term perspective, serve as a real-time alert service open to the community.
Detection of multiple terrestrial gamma-ray flashes from thunderstorm systems
Journal Of Geophysical Research: Space Physics, 2016
Since their discovery, Terrestrial Gamma ray Flashes (TGFs) exhibited an evident correlation with thunderstorms and lightning activity. The fleeting nature of these events and the heavy absorption of gamma rays in the lowest atmospheric layers severely hamper the observation of this phenomenon, making us reveal just a small fraction of a probably much wider population. As each thunderstorm produces a large amount of lightning discharges during its lifetime, it is reasonable that even a large amount of TGFs are produced during the same event. However, detection of multiple TGFs coming from the same storm is difficult to perform, as it requires the constant monitoring of a spatially limited geographic region: this is not an easy task to perform for satellites on high-inclination orbits that make them experience nonnegligible latitudinal shifts at each orbital passage over a certain region, preventing the monitoring of a limited geographic region throughout successive overpasses. In this perspective, the quasi-equatorial (2.5 ∘) orbit of the Astrorivelatore Gamma ad Immagini LEggero (AGILE) satellite ensures a minimal latitudinal shift when flying over the same region at successive passages, allowing for the follow-up of thunderstorms in time. We exploit this feature of the AGILE satellite to search for multiple TGFs coming from the same geographic region and, in particular, from the same thunderstorm. We carry out this search on the AGILE TGF database (2009-2016), ending up with a sample of 79 systems producing more than one TGF, both during the same overpass and up to four overpasses after. Data acquired by geostationary meteorological satellites and cross correlation with radio sferics detected by World Wide Lightning Location Network are used to support this investigation. The AGILE satellite for the first time clearly establishes the multiple occurrences of TGFs from convective thunderstorms, both on timescales of minutes to several hours.
Generation Possibility of Gamma‐Ray Glows Induced by Photonuclear Reactions
Journal of Geophysical Research: Atmospheres
Relativistic runaway electron avalanches (RREAs) imply a large multiplication of high‐energy electrons (∼1 MeV). Two factors are necessary for this phenomenon: a high electric field sustained over a large distance and an energetic particle to serve as a seed. The former sustains particle energies as they keep colliding and lose energy randomly; and the latter serves as a multiplication starting point that promotes avalanches. RREA is usually connected to both terrestrial gamma‐ray flashes (TGFs) and gamma‐ray glows (also known as Thunderstorm Ground Enhancement (TGE) when detected at ground level) as possible generation mechanism of both events, but the current knowledge does not provide a clear relationship between these events (TGF and TGE), beyond their possible common source mechanism, still as they have different characteristics. In particular, their timescales differ by several orders of magnitude. This work shows that chain reactions by TGF byproducts can continue for the tim...
Mathematical modeling of light curves of RHESSI and AGILE terrestrial gamma-ray flashes
Astrophysics and Space Science, 2019
We consider different distribution functions as possible fits for the light curves (time profiles) of Terrestrial Gamma-ray Flashes (TGFs): the piecewise Gaussian and the piecewise exponential, which correspond to the assumption of exponential growth and decay of the electrons that emit the radiation, and the inverse Gaussian and Ornstein-Uhlenbeck functions, which correspond to the assumption of an exit-time process. We compute maximum likelihood estimations (MLEs) for each of these four functions for a 100 TGFs recorded by the RHESSI and AGILE satellites, and compared those values with MLEs of the lognormal distribution function, which has been very widely used. The analysis of time profiles of these TGFs shows that all five distributions fit the data equally well. The results obtained in this work, combined with the results obtained for the analysis of BATSE and FERMI TGFs (Abukhaled et al. in J. Geophys. Res. Space Phys. 119:5918-5930, 2014) show that the TGF data as recorded thus far by various satellite gammaray detectors, are not powerful enough to discriminate between those various mathematical functions, which, if one function were preferred, would identify some physical features of the phenomenon. Keywords Terrestrial Gamma-ray Flash • TGF • AGILE • RHESSI • Light curve fitting • Maximum likelihood • Density functions • Stochastic processes • Electron cascades • Thunderstorms B M.
Ultra high energy cosmic rays The intersection of the Cosmic and Energy Frontiers
Astroparticle Physics
Pin down the shower energy observable to use μ as a composition-only Reduce hadronic uncertainties interaction model E threshold (TBD) Other messengers Galactic * μ / em separation † * depending on analysis progress † depending on detector configuration Figure 1: Diagram summarizing the strong connections of UHECRs with particle physics and astrophysics, the fundamental objectives of the field (in orange) for the next two decades, and the complementarity of current and next-generation experiments in addressing them. observatories offer a unique probe of the dark matter mass spectrum near the scale of grand unified theories (GUTs). The origin of super-heavy dark matter (SHDM) particles can be connected to inflationary cosmologies and their decay to instanton-induced processes, which would produce a cosmic flux of ultra-high-energy (UHE) neutrinos and photons. While their non-observation sets restrictive constraints on the gauge couplings of the DM models, the unambiguous detection of a single UHE photon or neutrino would be a game changer in the quest to identify the DM properties. UHECR experiments could be also sensitive to interactions induced by macroscopic DM or nuclearites in the atmosphere, offering further windows to identify the nature of DM. Astrophysics at the Energy Frontier The ability to precisely measure both energy and mass composition on an event-by-event basis simultaneously is critical as together they would give access to each primary particle's rigidity as a new observable. Given the natural relationship between rigidity and magnetic deflection, rigidity-based measurements will facilitate revealing the nature and origin(s) of UHECRs and enable charged-particle astronomy, the ability to study individual (classes of) sources with UHECRs. At the highest energies, the classic approach of maximizing exposure and achieving good energy resolution and moderate mass discrimination may well be sufficient if the composition is pure or is bimodal comprising a mix of only protons and Fe nuclei, for example. We already know however that this is not the case at energies below the flux suppression. Thus, a purposely-built observatory combining excellent energy resolution and mass discrimination will be complementary to instruments with possibly larger exposure. It is also clear that both approaches will benefit from the reduction of systematic uncertainties between hadronic interaction models. UHECRs also have an important role to play in multi-messenger astrophysics, not only as cosmic messengers themselves but also as the source of UHE photons and neutrinos. v • Full-sky coverage with low cross-hemisphere systematic uncertainties is critical for astrophysical studies. To this end, next generation experiments should be space-based or multi-site. Common sites between experiments are encouraged. • Based on the productive results from inter-collaboration and inter-disciplinary work, we recommend the continued progress/formation of joint analyses between experiments and with other intersecting fields of research (e.g., magnetic fields). • The UHECR community should continue its efforts to advance diversity, equity, inclusion, and accessibility. It also needs to take steps to reduce its environmental impacts and improve open access to its data to reduce the scientific gap between countries.