Relativistic Runaway Electron Avalanches Research Articles

Overestimation of Astrophysical Gamma-Ray Energies during Thunderstorms: Synergy of Galactic and Atmospheric Accelerators

Particle accelerators abound in space plasmas, saturating the cosmos with fully stripped nuclei and gamma rays, with energies surpassing the capabilities of human-made accelerators by orders of magnitude. Upon reaching Earth’s atmosphere, these particles trigger extensive air showers (EASs), generating millions of secondary cosmic rays of lower energies. Free electrons from EASs developing in the atmosphere are seeds for atmospheric electron accelerators. Strong atmospheric electric fields (AEFs) evolving during thunderstorms act as accelerators, amplifying the intensity of electrons many times, significantly enlarging the EAS size (number of electrons). Thus, the energy of the primary cosmic ray recovered by EAS size can be significantly overestimated. Recently discovered by high-altitude EAS arrays, PeVatron candidates (ultra–high-energy (UHE) astrophysical gamma-ray sources) must be carefully examined according to the atmospheric conditions during EAS detection. Large High Altitude Air Shower Observatory and High-Altitude Water Cherenkov Observatory arrays are located in regions of frequent thunderstorms, and an AEF’s strength can reach and surpass the critical strength to start relativistic runaway electron avalanches. A few registered UHE gamma rays from stellar sources can be registered at just this time when the AEF highly enhances the EAS size. Thunderstorm ground enhancements are copiously registered at mountain peaks of Eastern Europe, Germany, and Armenia, with energies well above the threshold energy of EAS array scintillators. Thus, the overestimation of the energy of primary particles is not an exotic process but a consequence of already well-established physical phenomena. Consequently, a report on each registered UHE gamma ray should include the recorded time and corresponding weather conditions.

On the Self‐Quenching of Relativistic Runaway Electron Avalanches Producing Terrestrial Gamma Ray Flashes

AbstractTerrestrial gamma ray flashes (TGFs) are short bursts of gamma rays occurring during thunderstorms. They are believed to be produced by relativistic runaway electron avalanches (RREAs). It is usually admitted that the number of high‐energy electrons produced in the brightest TGFs remains mostly confined within a range from 1017 to 1019. To understand the constraints in the development of RREAs, we perform self‐consistent simulations using a newly developed model with a finite acceleration region and various injection rates. We find that RREAs should naturally self‐quench for a fixed total number of runaway electrons, and hence a fixed number of bremsstrahlung photons. From the idea that TGF sources quench themselves, we derive a simple equation controlling the total number of runaway electrons. In this framework, the existence of a saturation in the electron density discovered in a previous work places a lower limit on TGF durations.

An enormous increase in atmospheric positron flux during a summer thunderstorm on Mount Aragats

On July 11, 2023, we observed a remarkable 500% increase in positron flux, coinciding with a significant Thunderstorm Ground Enhancement (TGE). The enhanced flux of electrons and gamma rays was attributed to relativistic runaway electron avalanches (RREAs) generated within the dipole formed between the main negatively charged layer in the middle of the thundercloud and the Lower Positively Charged Region (LPCR) at the bottom of the thundercloud. Concurrently, a substantial enhancement in the 511 keV gamma-ray flux resulting from electron-positron annihilation was recorded. This surge is intricately linked to the LPCR within the thundercloud. The emergence of the LPCR induces a polarity change in the atmospheric electric field (AEF) below the LPCR (fourth dipole), leading to the deceleration of electrons and the acceleration of positrons.Particle flux measurements were conducted using scintillation and NaI(TL) spectrometers. To mitigate the contamination of natural gamma radiation and refine the 511 keV flux measurements, the ORTEC spectrometer was shielded with a 4 cm thick lead filter. CORSIKA simulations corroborate the observed positron flux enhancement.Highlighting the synergy between high-energy physics in the atmosphere and astroparticle physics, we introduce a new scenario to elucidate the enigmatic large flux of galactic positrons measured by the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station (ISS).

Self Consistent Modeling of Relativistic Runaway Electron Beams Giving Rise to Terrestrial Gamma‐Rays Flashes

AbstractTerrestrial Gamma Ray Flashes (TGFs) are short bursts of gamma rays occurring during thunderstorms. They are believed to be produced by Relativistic Runaway Electron Avalanches (RREAs). In this paper, we present a new numerical model based on the Particle‐In‐Cell (PIC) method to simulate the interactions between the electromagnetic fields and the electron avalanche self‐consistently. The code uses a cylindrical Yee lattice to numerically solve the electromagnetic fields, a Monte Carlo approach to simulate collisions with air molecules, and a plasma fluid model to calculate the effects of low‐energy electrons and ions. The model is first tested through the reproduction of dispersion relations in a hot plasma. RREAs propagating under a homogeneous background electric field are then simulated. Owing to the self‐consistent nature of this description, we report here new physical properties such as saturation processes in the electron density and in the number of high‐energy electrons, detailed dynamical screening of the electric field in the ion trail of the avalanche, and the associated electric currents. We find that the saturation of RREAs is obtained when the numbers of high‐energy electrons and photons is consistent with those believed to be representative of TGF sources.

Clusters of Compact Intracloud Discharges (CIDs) in Overshooting Convective Surges

AbstractWe observed five clusters of upper‐level compact intracloud discharges (CIDs) moving positive charge up over land and over water in Florida. The clusters each contained 3 to 6 CIDs, and the overall cluster duration ranged from 27 to 58 s. On average, the CIDs in a given cluster occurred 11 s apart and were separated by a 3D distance of about 1.5 km. All the clustered CIDs were located above the tropopause and were likely associated with convective surges that penetrated the stratosphere. The average periodicity of CID occurrence within a cluster (every 11 s) was comparable to the periodicity at which the average cluster area is expected to be bombarded by ≥1016 eV cosmic‐ray particles (every 5 s). Each of such energetic particles gives rise to a cosmic ray shower (CRS) and, in the presence of sufficiently strong electric field over a sufficiently large distance, to a relativistic runaway electron avalanche (RREA). We infer that each of our upper‐level CIDs is likely to be caused by a CRS‐RREA traversing, at nearly the speed of light, the electrified overshooting convective surge and triggering, within a few microseconds, a multitude of streamer flashes along its path, over a distance of the order of hundreds of meters (as per the mechanism recently proposed for lightning initiation by Kostinskiy et al., 2020, https://doi.org/10.1029/2020JD033191). The upper‐level CID clustering was likely made possible by the recurring action of energetic cosmic rays and the rapid recovery of the negative screening charge layer at stratospheric altitudes.

Effects of thunderstorms electric field on secondary photons of cosmic ray at large high altitude air shower observatory

Large high altitude air shower observatory (LHAASO) is a complex of extensive air shower (EAS) detector arrays, located on the Mt. Haizi (29°21' N, 100°08' E) at an altitude of 4410 m a. s. l., Daocheng, Sichuan Province, China. The information about primary cosmic rays can be obtained by using data from secondary particles measured at LHAASO, with photons make up the majority among these secondary particles. During thunderstorms, the atmospheric electric field can affect secondary charged particles (mainly positrons and electrons), thus changing the information of photons on the ground. In this work, Monte Carlo simulations are performed to investigate the effects of near-ground thunderstorm electric fields on cosmic ray secondary photons at LHAASO. A simple model with a vertical and uniform atmospheric electric field in a layer of atmosphere is used in our simulations. During thunderstorms, the number and energy of photons are found to significantly change and strongly depend on the electric field strength. In a field of –1000 V/cm (below the threshold of the relativistic runaway electron avalanche (RREA) process), the number of photons is increased by 23%. Also, the spectrum of photons softens, and the increased number of photons with energy less than 2 MeV exceeds 29%. In an electric field of –1700 V/cm (above the threshold of the RREA process), the number of photons experiences exponential growth, with an increase of 279%. The spectrum of photons becomes softer than that at –1000 V/cm, and the increased number with energy less than 2 MeV is more than 361%. It is consistent with the theory of RREA. For these phenomena of photons at LHAASO, the main factor is that the number of positrons and electrons are increased due to the acceleration of negative electric field on electrons, with increase of 65% in –1000 V/cm and 992% in –1700 V/cm, and the spectrum of positrons and electrons soften. Newborn free positrons/electrons may undergo bremsstrahlung and deposit part of their energy into photons, causing the change of number and energy of photons to follow roughly the same pattern as positrons and electrons. The simulation results can provide the information for understanding the variations of the data detected by LHAASO during thunderstorms and the acceleration mechanisms of secondary charged particles caused by an atmospheric electric field.

Reconstruction of high energy thunderstorm radiation effects on soil matrix using Monte Carlo simulations

Due to their electromagnetic properties, thunderclouds can act as natural particle accelerators. Electrons accelerated in the thunderclouds can reach energies up to tens of MeV. Large populations of high energetic electrons formed by avalanche growth driven by electric fields in the Earth atmosphere called Relativistic Runaway Electron Avalanches (RREA) propagate through matter. They are decelerated and deflected in the course of collisions with particles in the atmosphere and emit gamma rays known as bremsstrahlung. The produced gamma rays can further trigger photonuclear reactions in the air and soil. This article reports on the work of project CRREAT (Research Centre of Cosmic Rays and Radiation Events in the Atmosphere), studying various lightning-related phenomena in various ways, both in situ and in the laboratory. This paper focuses on the simulation of the laboratory experiments at the Microtron accelerator in Prague and the neutron generator in Ostrava, where we irradiated various soil samples with 20 MeV electron beams. Experiments showed which radionuclides can be formed during the reactions of high-energy electrons with various soils and can be as targeted products in the thunderstorm radiation effect analysis. Radionuclides produced in exposed samples were measured using a highpurity germanium (HPGe) detector. A computer simulation was done with a simple source and sample geometry using the general-purpose 3D Monte Carlo code PHITS.

How can we simulate ionizing radiation at aviation altitudes from TGFs?

So-called thunderclouds, which are large dark clouds that are able to generate thunder and lightning, can act as natural particle accelerators, producing complex high-energy phenomena such as terrestrial gamma-ray flashes (TGFs) and gamma-ray glows. These events are often described through the mechanism of relativistic runaway electron avalanches (RREAs), cascades of high-energy electrons accelerated by atmospheric electric fields. Since the energies of the RREAs are up to several tens of MeV, they can also trigger nuclear reactions with atoms of the air and in the soil while entering the ground. Although these phenomena are intriguing, their lack of precise measurement and still not completely understood origins pose a significant challenge for assessing their impact on aviation safety. This paper introduces the project Research Centre of Cosmic Rays and Radiation Events in Atmosphere (CRREAT), aimed at providing measurements of TGFs, thunderstorm ground enhancements (TGEs), and other ionizing radiation phenomena during thunderstorms, as well as at aviation altitudes, stratosphere, and low Earth orbits (LEO). The paper argues that without accurate data on the origins and physical characteristics of TGEs and TGFs, it is impossible to reliably simulate their impact on aircraft crews and passengers. The paper also mentions how the general-purpose 3D Monte Carlo (MC) code PHITS can be used for future simulations and comparisons with measurements related to ionizing radiation phenomena in the atmosphere.

Ambient Conditions of Winter Thunderstorms in Japan to Reproduce Observed Gamma‐Ray Glow Energy Spectra

AbstractElectric field of thunderclouds modifies components and energy spectra of the cosmic‐ray air shower. In particular, thunderstorms accelerate charged particles, resulting in an enhancement of gamma‐ray fluxes on the ground, known as a gamma‐ray glow. This phenomenon has been observed in recent years by the Gamma‐Ray Observation of Winter THunderclouds collaboration from winter thunderstorms in the Hokuriku area of Japan. The present work examines the ambient conditions required to produce spectral features of the previously detected gamma‐ray glows, by using Monte Carlo simulations of particle interactions in the atmosphere. We focus on three parameters, the strength and length of the electric field, and the length of a null‐field attenuation region below the electrified region. The average spectrum of observed gamma‐ray glows in winter thunderstorms of Japan requires an electric field intensity close to 0.31 MV/m, slightly exceeding the Relativistic Runaway Electron Avalanche threshold of 0.284 MV/m. The vertical size of the electric field region should be comparable to 1 km. The estimated attenuation region size is 300–500 m, necessary to reduce the low‐energy photon flux of the average gamma‐ray glows. There is still a wide range of acceptable parameter sets with degeneracy to make a similar spectrum.

On the vertical and horizontal profiles of the atmospheric electric field during thunderstorms

We present the first results of a new experiment on Mt. Aragats for measuring the horizontal profile of atmospheric electric fields during thunderstorms. Networks of advanced particle spectrometers operated on the slopes of Mt. Aragats continuously measure fluxes of charged and neutral particles, periodically registering impulsive enhancements, so-called Thunderstorm Ground Enhancements (TGEs). This gives the possibility to estimate the strength of the electric field in the lower atmosphere. Relativistic Runaway Electron Avalanches (RREAs) are registered by the particle detectors located on the Earth's surface as TGEs sometimes exceeding the fair-weather fluxes up to a hundred times. The strong accelerating electric field can reach 1.7-2.2 kV/cm at altitudes 3-6 km, and extend down to 50-150 m above the Earth's surface. The horizontal extent of the electric field can reach 10 km and more.

Thunderstorm ground enhancements: Multivariate analysis of 12 years of observations

We present a survey of more than a half-thousand thunderstorm ground enhancements (TGEs, fluxes of electrons, and gamma rays associated with thunderstorms) registered in 2008--2022 at Aragats space environmental center (ASEC). We analyze correlations between various measured parameters characterizing TGEs measured on Aragats. Two special cases of TGE events are considered: one, terminated by nearby lightning flashes, and another one---with a sufficiently large ratio of electrons to gamma rays. On the basis of the analysis, we summarize the most important results obtained during 12 years of TGE study, which include the following statements: (i) TGEs originated from multiple relativistic runaway electron avalanches starting with seed electrons from the ambient population of cosmic rays, which enter an extended region of the electric field with strength exceeding the critical value; (ii) quite frequently, TGEs occur prior to lightning flashes and are terminated by them; (iii) the energy spectra of avalanche electrons observed on Aragats indicate that the strong electric field region can extend very low above the ground covering a large area on the earth's surface.

The horizontal profile of the atmospheric electric fields as measured during thunderstorms by the network of NaI spectrometers located on the slopes of Mt. Aragats

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 between modification of the cosmic ray electron energy spectra (MOS) and relativistic runaway electron avalanche (RREA) processes, on the energy of the seed electrons, and on the strength and elongation of an atmospheric electric field. The network of large NaI spectrometers on slopes of Mt. Aragats 24/7 monitored secondary particle fluxes from 2013 until now, highly contributed to the understanding of the ways how RREAs are developed in the atmosphere. In 2022 we enlarge the NaI network with 2 remote detectors located at altitudes 2000 and 1700 m, and 13 and 16 km apart from the Aragats station to investigate the horizontal profile of the atmospheric electric field. We found, that the previously estimated values of the regions in the atmosphere, where RREA emerges, were highly underestimated. In the present report, we describe the NaI particle detector's network and present the first results of the experiment demonstrating that the particle fluxes from the atmospheric electron accelerators can cover large areas on the earth's (up to tens of km2).

Development of the relativistic runaway avalanches in the lower atmosphere above mountain altitudes

The comparative analysis of three thunderstorms on Aragats in May 2021 demonstrates that relativistic runaway electron avalanches (RREAs) can reach very low altitudes above the Earth’s surface on mountain altitudes. RREAs which reach the Earth’s surface are registered by the particle detectors as thunderstorm ground enhancements (TGEs) —large enhancements of electron and gamma ray fluxes, sometimes exceeding the fair-weather background fluxes up to a hundred times. By comparing the energy spectra of electrons and gamma rays it is possible to estimate the height above the ground where an RREA terminates and avalanche particles exit the accelerating field. For the several TGEs registered on Aragats, this estimate varied between 50 and 150 meters. The threshold electric field can reach on heights of . When a lightning’s active zone is above particle detectors RREAs last tens of seconds to a few minutes, until lightning flashes terminate electron acceleration. If the lightning activity is far from the detector site, TGEs are extended for a few tens of minutes and smoothly decay and TGE has a more or less symmetrical shape.

Calculation of Parameters of a Relativistic Runaway Electron Avalanche by the Group Equation Method for Moments of the Electron Distribution Function

Calculation of Parameters of a Relativistic Runaway Electron Avalanche by the Group Equation Method for Moments of the Electron Distribution Function

Estimate of the source parameters of terrestrial gamma-ray flashes observed at low-Earth-orbit satellites

A special attention has been paid in the past decades to studies of terrestrial gamma-ray flashes (TGFs) observed above active thunderstorms. The physical mechanism of the TGF generation and its source location in the atmosphere have not been firmly established. A numerical modelling, such as Monte Carlo simulation, is commonly used to analyze the problem having regard to the complexity of basic electrodynamic and transport equations. Here, in contrast to previous numerical studies, we have constructed a suitably idealized analytical model of a point source of gamma-rays in a vertically inhomogeneous atmosphere. An energy-distribution of gamma photons is assumed to be determined by the energy spectrum of electron bremsstrahlung resulted from the relativistic runaway electron avalanche in a strong electric field. The absorption of photon energy due to photoelectric effect, Compton scattering and the electron-positron pair production have been accounted for approximately with an effective coefficient of energy absorption. A photon mean free path in the atmosphere is assumed to be dependent on its energy and altitude. A spatiotemporal distribution of gamma-ray flux density and photon fluence at a low-Earth orbit (LEO) are estimated as functions of the TGF source altitude, total number of photons emitted by the source, and other parameters. The model matches the LEO observations indicating that the TGF source is located at an altitude about 10−14 km. The same model can be applied for the description of the recently found downward-directed TGF detected by the large-area Telescope Array cosmic ray observatory.

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.

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.

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.