Runaway Electron Avalanche Research Articles

The criterion for self-sustaining production of relativistic runaway electron avalanches by the positron feedback in thunderstorms

Relativistic runaway electron avalanches (RREA) occurring in thunderstorm large-scale electric fields are one of the sources of atmospheric gamma radiation. Relativistic feedback is the generation of RREAs by positrons or backscattered gamma rays of the previously produced RREAs. In strong electric fields, relativistic feedback can make RREAs self-sustainable that hypothetically can cause a terrestrial gamma-ray flash (TGF). This paper introduces a kinetic approach to study the positron relativistic feedback, which is predominant for the directly observed conditions within thunderstorms. The criterion for self-sustaining production of RREAs by the positron feedback in thunderstorms is derived. The discovered criterion allows one to obtain the parameters of the thunderstorm electric field required for self-sustaining RREA development with positron feedback for any altitude. It is shown that the derived conditions of self-sustainable RREA development are not achieved for the parameters of electric field observed in real thunderclouds.

Confinement of passing and trapped runaway electrons in the simulation of an ITER current quench

Runaway electrons (REs) present a high-priority R&D issue for ITER but little is known about the extent to which RE generation is affected by the stochastic field intrinsic to disrupting plasmas. RE generation can be modelled with reduced kinetic models and there has been recent progress in involving losses due to field stochasticity, either via a loss-time parameter or radial transport coefficients which can be estimated by tracing test electrons in 3D fields. We evaluate these terms in ITER using a recent JOREK 3D MHD simulation of plasma disruption to provide the stochastic magnetic fields where RE markers are traced with the built-in particle tracing module. While the MHD simulation modelled only the current quench phase, the case is MHD unstable and exhibits similar relaxation as would be expected during the thermal quench. Therefore, the RE simulations can be considered beginning right after the thermal quench but before the MHD relaxation is complete. The plasma is found to become fully stochastic for 8 ms and the resulting transport is sufficient to overcome RE avalanche before flux surfaces are reformed. We also study transport mechanisms for trapped REs and find those to be deconfined as well during this phase. While the results presented here are not sufficient to assess the magnitude of the formed RE beam, we show that significant RE losses could be expected to arise due to field stochasticity.

Was there an upward atmospheric discharge in the Tunguska catastrophe?

This work deals with the issue of the light emission that took place during the Tunguska catastrophe of 1908. According to eyewitnesses, after the disappearance of the flying object over the horizon, a column of light rose above the place of its fall. This pillar was visible on a sunny morning from a distance of ∼500 km and reached a height of ∼80 km above the ground. The duration of the existence of this column of light was estimated at 6–7 s. The thermal effect of radiation from this column was great: at a distance of ∼30 km from the epicenter, live needles caught fire, and at a distance of 65 km, the effect was close to a burn of human skin. The model of the radiation source is most consistent with the level of thermal impact on the environment if this source was located at an altitude of 30 km above the ground. According to estimates, the energy of this radiation exceeded 5·1022 erg. This work discusses the assumption that during the Tunguska catastrophe there was a powerful upward atmospheric discharge, which was a consequence of the formation of numerous relativistic runaway electron avalanches.

The synergy of the cosmic ray and high energy atmospheric physics: Particle bursts observed by arrays of particle detectors

Particle bursts detected on the earth's surface during thunderstorms by various particle detectors originated from the relativistic runaway electron avalanches (RREAs) initiated by free electrons accelerated in the strong atmospheric electric fields. Two oppositely directed dipoles in the thundercloud accelerate electrons in the direction of the earth's surface, and to the open space. The particle bursts observed by orbiting gamma ray observatories are called terrestrial gamma ray flashes (TGFs, with energies of several MeV, only sometimes reaching tens of MeV); ones registered by particle detectors located on the ground – are called thunderstorm ground enhancements (TGEs, with energies, usually reaching 40-50 MeV). Balloons and aircraft in the troposphere register gamma ray glows (with energies of several MeV). Recently, high-energy atmospheric physics includes also, so-called, downward TGFs (DTGFs), intense particle bursts with a duration of a few milliseconds.Well-known extensive air showers (EASs) originate from the interactions of galactic protons and fully-stripped nuclei with the atmosphere atoms. EAS particles have very dense cores around the shower axes. However, high-energy particles in the EAS cores comprise a very thin disc of (a few tens of ns), and a particle detector traversed by an EAS core will not register a particle burst, but only one very large pulse. Only neutron monitor, by collecting delayed thermal neutrons from EAS core particle interactions with soil, can register particle bursts. We discuss the relation between short particle bursts available from the largest particle arrays with EAS phenomena. We demonstrate that the neutron monitors can extend the EAS “lifetime” up to a few milliseconds, a time comparable with DTGFs duration. The possibility to use the network of neutron monitors for high-energy cosmic ray research is also deliberated.Plain Language Summary: Short and extended particle bursts are registered in space, the troposphere, and the earth's surface. Coordinated monitoring of the particle fluxes, near-surface electric fields, and lightning flashes makes it possible to formulate a hypothesis on the origin of intense bursts and their relation to extensive air showers and atmospheric discharges. Analysis of the observational data and possible origination scenarios of particle bursts allows us to conclude that the bursts can be explained by the electron acceleration in the thunderous atmosphere and by gigantic showers developed in the terrestrial atmosphere by high-energy protons and fully-stripped nuclei accelerated in Galaxy.

Modeling Low‐Frequency Radio Emissions From Terrestrial Gamma Ray Flash Sources

AbstractRelativistic runaway electron avalanches (RREAs) occur when electrons in electric fields in air reach energies above which they gain more energy from the electric field than they lose to collisions with the surrounding atmosphere. RREAs are known to happen in the electric fields in thunderstorms, and are considered to be the mechanism responsible for producing Terrestrial Gamma‐ray Flashes (TGFs). As RREAs propagate, they leave a trail of low‐energy electrons and positive and negative ions behind. These populations of charged particles will carry currents as they move in the thunderstorm electric field. In the present work, we model the charged species left behind by the propagating RREA, and the resulting radio emissions in the context of injection of thermal runaway seed electrons by a leader. We find that for certain initial conditions, these radio emissions match the slow low‐frequency (LF) pulses that have previously been observed concurrently with TGFs. This confirms that the slow LF pulses are likely generated directly by the TGF source itself, as has been previously suggested using a different TGF production model. Slow LF pulses may therefore potentially be used to infer characteristic properties of TGF sources.

Atmospheric Electron Spatial Range Extended by Thundercloud Electric Field Below the Relativistic Runaway Electron Avalanche Threshold

AbstractCosmic‐ray shower, hitting the atmosphere all the time, includes high‐energy electrons. Electric fields in thunderstorms accelerate these electrons so that they can reach farther distances with producing secondary electrons. The strong field above 0.284 MV m−1 causes the Relativistic Runaway Electron Avalanches (RREA) process with an exponential increase of electrons. Even with lower electric fields below this RREA threshold which are usually measured in thunderstorms, an increase in electrons' kinetic energy and production of secondary electrons can occur without avalanche processes, known as Modification Of Spectra (MOS) of the cosmic‐ray shower. Both phenomena are related to observed gamma ray glow, which is bremsstrahlung emission from accelerated electrons in thunderstorms. We calculate an extended range and kinetic energy of electrons below the RREA threshold using an analytical evaluation and Geant4 Monte Carlo simulations. For example, at 0.280 MV m−1 slightly below the RREA threshold, 3 MeV electrons gain 100% of their original kinetic energy through the passage of the continuous slowing down approximation (CSDA) range defined at the null electric field, which is consistent with results in previous studies as the transition to the RREA process. Even at a lower field of 0.260 MV m−1, the energy recovery by the field allows an injected 20 MeV electron beam to extend its average electron range from 73 m, in the null field configuration, to 562 m. We have performed analytical calculation and Geant4 simulations of atmospheric electron behavior for initial electron energy of 0.1–100 MeV and an electric field below 0.290 MV m−1.

The Criterion for Infinite Positron Feedback in Dynamics of Relativistic Runaway Electron Avalanches

Relativistic runaway electron avalanches (RREA) accelerated by thunderstorm large-scale electric fields are one of the sources of atmospheric gamma radiation. In strong electric fields, RREAs can multiply by the relativistic feedback. Infinite relativistic feedback makes avalanches self-sustainable and hypothetically can cause a terrestrial gamma-ray flash (TGF). This paper introduces a kinetic approach to study the relativistic feedback caused by positrons since positron feedback dominates for the directly observed electric field strengths. With this approach, the criterion for infinite positron feedback within thunderstorms is derived. Discovered criterion allows obtaining the thunderstorm electric field parameters required for infinite positron feedback for any altitude. The possibility of derived thunderstorm conditions is discussed.

Relativistic Runaway Electron Avalanches Within Complex Thunderstorm Electric Field Structures

AbstractRelativistic runaway electron avalanches (RREAs) are generally accepted as a source of thunderstorms gamma ray radiation. Avalanches can multiply in the electric field via the relativistic feedback mechanism based on processes with gamma rays and positrons. This study shows that a non‐uniform electric field geometry can lead to the new RREAs multiplication mechanism—“reactor feedback,” due to the exchange of high‐energy particles between different accelerating regions within a thundercloud. A new method for the numerical simulation of RREA dynamics within heterogeneous electric field structures is proposed. Under the assumption that reactor feedback can produce a terrestrial gamma ray flash (TGF), necessary conditions for TGF occurrence in the system with the reactor feedback are derived using the developed analytical description and the numerical simulation. Observable properties of TGFs influenced by the proposed mechanism are discussed.

Effect of bremsstrahlung on the characteristic growth length of an avalanche of runaway electrons

In avalanches of runaway electrons, which are formed in high-altitude atmospheric discharges, the average electron energy is several megaelectronvolts. For such high-energy electrons, the loss of energy by bremsstrahlung is significant when they collide with gas atoms. The aim of this study was to evaluate the effect of bremsstrahlung on the exponential growth length of a runaway electron avalanche. It has been shown that taking account of the bremsstrahlung produced by the electrons of an avalanche gives corrections of no more than a few percent to the avalanche exponential growth length. The corrections are maximum for electric field strengths close to the threshold below which the avalanche electrons fail to become continuously accelerated. The effect of bremsstrahlung decreases with increasing electric field strength. The higher the atomic number of the gas in which an avalanche of runaway electrons propagates, the greater the corrections.

Terrestrial gamma-ray flashes initiated by positive leaders

Terrestrial gamma-ray flashes (TGFs) are powerful submillisecond bursts of gamma rays produced by thunderstorms. To date, most TGFs have been observed by spacecraft in low-Earth orbit and have been found to be associated with negative intracloud lightning leaders. In recent years, TGFs have also been measured on the ground as downward beams originating from the overhead storms. While the majority of these ground-level TGFs appear to be associated with negative lightning leaders, similar to the TGFs seen from space, others are associated with upward-propagating positive leaders. In this paper, Runaway Electron Avalanche Model Monte Carlo simulations, modified to include low-energy electron and ion currents and self-consistent electric fields, are used to model TGF production by the relativistic feedback mechanism initiated by positive leaders. It is found that intense bursts of gamma rays are produced by positive leaders, similar to the observed ground-level TGFs. It is also found that these events produce dangerous radiation doses in excess of 1 Sievert and so may be of concern for aviation safety.

Tropospheric dispersive phase anomalies during heavy rain detected by L-band InSAR and their interpretation

The split-spectrum method (SSM) can largely isolate and correct for the ionospheric contribution in the L-band interferometric synthetic aperture radar (InSAR). The standard SSM is performed on the assumption of only the first-order ionospheric dispersive effect, which is proportional to the total electron content (TEC). It is also known that during extreme atmospheric events, either originated from the ionosphere or in the troposphere, other dispersive effects do exist and potentially provide new insights into the dynamics of the atmosphere, but there have been few detection reports of such signals by InSAR. We apply L-band InSAR into heavy rain cases and examine the applicability and limitation of the standard SSM. Since no events such as earthquakes to cause surface deformation took place, the non-dispersive component is apparently attributable to the large amount of water vapor associated with heavy rain, whereas there are spotty anomalies in the dispersive component that are closely correlated with the heavy rain area. The ionosonde and Global Navigation Satellite System (GNSS) rate of total electron content index (ROTI) map both show little anomalies during the heavy rain, which suggests few ionospheric disturbances. Therefore, we interpret that the spotty anomalies in the dispersive component of the standard SSM during heavy rain are originated in the troposphere. While we can consider two physical mechanisms, one is runaway electron avalanche and the other is the dispersive effect due to rain, comparison with the observations from the ground-based lightning detection network and rain gauge data, we conclude that the rain dispersive effect is spatiotemporally favorable. We further propose a formulation to examine if another dispersive phase than the first-order TEC effect is present and apply it to the heavy rain cases as well as two extreme ionospheric sporadic-E events. Our formulation successfully isolates the presence of another dispersive phase during heavy rain that is in positive correlation with the local rain rate. In comparison with other dispersive phases during Sporadic-E episodes, the dispersive heavy rain phases seem to have the same order of magnitude with the ionospheric higher order effects.

Measurement of TGE particle energy spectra: An insight in the cloud charge structure

In this letter, we present for the first time the method to get insight into the charge structure of the thundercloud using simultaneously measured electron and gamma-ray energy spectra of the thunderstorm ground enhancement (TGE). We discuss and compare two main scenarios that can support the initiation of the relativistic runaway electron avalanches (RREA) in the thundercloud. One of the scenarios includes the emergence of the lower positive charged layer (LPCR). The LPCR is a short-living charge structure sitting on hydrometeors, which exhibit accidental charge reversals depending on temperature changes. LPCR can sustain a downwards directed near-surface electric field for several minutes, before falling down with precipitation. Recently, we launched on Aragats a large spectrometer, which is capable to measure energy spectra of electrons and gamma-rays in the energy range from 4 to 100 MeV. Comparison of energy spectra of electrons and gamma-rays makes it possible to scrutinize the LPCR emergence and estimate the height where the strong electric field that originates the RREA terminates.

Toroidal modeling of runaway avalanche in DIII-D discharges

A toroidal modeling tool is developed to study the runaway electron (RE) avalanche production process in tokamak plasmas, by coupling the Rosenbluth–Putvinski avalanche model (Rosenbluth and Putvinski 1997 Nucl. Fusion 37 1355) with an n = 0 magneto-hydrodynamic (MHD) solver. Initial value numerical simulations are carried out for two DIII-D discharges with different plasma shapes (one near circular, and the other with high elongation). It is found that, assuming the same level of about 1% seed current level, the Rosenbluth–Putvinski model somewhat underestimates the RE plateau current for the circular-shaped plasma, as compared with that measured in DIII-D experiments. For an elongated, higher current plasma, simulations find strong runaway current avalanche production despite the lack of measured plateau RE current in experiments. A possible reason for this discrepancy is a lack of additional RE dissipation physics in the present two-dimensional model. Systematic scans of the plasma boundary shape, at fixed pre-disruption plasma current, find that the plasma elongation helps to reduce the RE avalanche production, confirming recent results obtained with an analytic model (Fülöp et al 2020 J. Plasma Phys. 86 474860101). The effect of the plasma triangularity (either positive or negative), on the other hand, has a minor effect. On the physics side, the avalanche process involves two competing mechanisms associated with the electric field. On the one hand, a stronger electric field produces a higher instantaneous avalanche growth rate. On the other hand, a fast growing RE current quickly reduces the fraction of the conduction current together with the electric field, and hence a faster dissipation of the poloidal flux. As a final result of these two dynamic processes, the runaway plateau current is not always the largest with the strongest initial electric field. These results lay the foundation for future self-consistent inclusion of the MHD dynamics and the RE amplification process.

Constraining Spectral Models of a Terrestrial Gamma‐Ray Flash From a Terrestrial Electron Beam Observation by the Atmosphere‐Space Interactions Monitor

AbstractTerrestrial Gamma ray Flashes (TGFs) are short flashes of high energy photons, produced by thunderstorms. When interacting with the atmosphere, they produce relativistic electrons and positrons, and a part gets bounded to geomagnetic field lines and travels large distances in space. This phenomenon is called a Terrestrial Electron Beam (TEB). The Atmosphere‐Space Interactions Monitor (ASIM) mounted on‐board the International Space Station detected a new TEB event on March 24, 2019, originating from the tropical cyclone Johanina. Using ASIM's low energy detector, the TEB energy spectrum is resolved down to 50 keV. We provide a method to constrain the TGF source spectrum based on the detected TEB spectrum. Applied to this event, it shows that only fully developed Relativistic Runaway Electron Avalanche spectra are compatible with the observation. More specifically, assuming a TGF spectrum , the compatible models have ϵ ≥ 6.5 MeV (E is the photon energy and ϵ is the cut‐off energy). We could not exclude models with ϵ of 8 and 10 MeV.

On the origin of particle flux enhancements during winter months at Aragats

We consider the particle flux enhancement occurring during the Winter months on Aragats. We demonstrate that these enhancements originate from 222Radon chain isotopes gamma radiation only. The relativistic runaway electron avalanche is possible on Aragats only in Spring-Autumn seasons. We measure with NaI ORTEC spectrometer the gamma ray radiation of the Radon progeny and use multidetector measurements to confirm the origin of the enhanced particle flux.

Generation Possibility of Gamma‐Ray Glows Induced by Photonuclear Reactions

AbstractRelativistic 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 timescale of gamma‐ray glows and even provide energetic particles as seeds for RREAs of gamma‐ray glows.

Groups of narrow bipolar events within thunderstorms

This investigation is focused on groups of Narrow Bipolar Events (NBEs), defined as NBEs that occurred within 10 km horizontally and ±660 ms of a located, large-amplitude NBE from a dataset of positive NBEs that occurred in Mississippi thunderstorms. In two months only 15 groups were found, with a total of 31 positive and 4 negative NBEs. Each group had 2 to 5 NBEs; four groups had both positive and negative polarity NBEs. About half of the NBEs had typical values for range-normalized fast antenna (FA) electric field change magnitudes (4–15 V/m) and typical VHF powers (1000–45,000 W), but 17 NBEs had FA magnitudes 0.2–2.5 V/m, and 17 NBEs had VHF powers 30–900 W. Seven weak NBEs had FA magnitudes of 0.2–1.0 V/m and VHF powers of 30–100 W. These findings indicate that weak NBEs are more common than previously thought. None of the NBEs in groups initiated a lightning flash, and (with one possible exception) none of the later NBEs in a group were initiated by earlier NBEs in the group. The data of the NBE groups are consistent with the turbulence-extensive air shower (EAS)/relativistic runaway electron avalanche (RREA) mechanism, which states that each NBE occurs in a separate 1-km3 volume containing many small regions with electric field ≥3 MV/(m∙atm); an EAS/RREA passing through the 1-km3 volume initiates the positive streamers that comprise the NBE. Relative to thunderstorm radar reflectivity, 23 NBEs occurred in or above the reflectivity core, 10 NBEs occurred high in the storm anvil, and 2 NBEs occurred beside the storm core. We speculate that the occurrence of many of the NBE groups was associated with dynamically intense convection.

Observations of the Origin of Downward Terrestrial Gamma‐Ray Flashes

AbstractIn 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.

Possible Mechanisms of Anomalous Electromagnetic Radiation in the Earth’s Atmosphere

An analysis is performed of a sharp increase in the intensity of atmospheric radiation recorded at wavelengths of 8.5 mm, 3.3 mm, and 8–12 μm during a magnetic storm on December 14, 2006. The measurements are made in the city of Chita, where the event coincided with the arrival of an atmospheric front and respective cooling of the air. Causes of the phenomenon that are associated with the acceleration of electrons in the electric fields of the atmosphere are considered. Studies of the phenomenon are relevant to both atmospheric physics and remote sensing. Nonthermal electromagnetic radiation of the microwave, infrared, and visible ranges can be used to detect and study such atmospheric processes as runaway electron avalanches, photonuclear reactions, electrization, thunderstorms, magnetic storms, and precursors of natural catastrophic and anomalous phenomena.

A stratospheric and satellite CubeSat format probe for detecting relativistic runaway electron avalanches

In planetary atmospheres, runaway electron avalanches could happen due to large scale electric fields, which accelerate electrons to energies about 0,1 – 10 MeV. This phenomenon is not fully understood. Nowadays, most of the satellite data is obtained on low orbits. However, runaway breakdown can also occur at altitudes less than 30 km. In this case, most of the radiation is scattered without reaching the satellites on high orbits. The formation of charged particles in the atmosphere can affect the results of numerous experiments. Therefore, it is important to have the most proper model of this phenomenon. Project goal is to create a stratospheric CubeSat format probe capable of detecting these events at an altitude of about 30 km and above. The purpose of the experiment is to observe changes in the fluxes of both high-energy electrons and radiation, as well as an analysis of possible correlations of the measured parameters. We developed a concept of the probe and created a detector prototype, consisting of a thick polystyrene scintillation counter wrapped in mylar and two SiPM SensL MicroSB-30035-X13 readout.