A relativistic runaway electron avalanche (RREA) is an avalanche growth of a population of relativistic electrons driven through a material (typically air) by an electric field. RREA has been hypothesized to be related to lightning initiation, [1] terrestrial gamma-ray flashes, [2] sprite lightning, [3] and spark development. [4] RREA is unique as it can occur at electric fields an order of magnitude lower than the dielectric strength of the material.
When an electric field is applied to a material, free electrons will drift slowly through the material as described by the electron mobility. For low-energy electrons, faster drift velocities result in more interactions with surrounding particles. These interactions create a form of friction that slow the electrons down. Thus, for low-energy cases, the electron velocities tend to stabilize.
At higher energies, above about 100 keV, these collisional events become less common as the mean free path of the electron rises. These higher-energy electrons thus see less frictional force as their velocity increases. In the presence of the same electric field, these electrons will continue accelerating, "running away".
As runaway electrons gain energy from an electric field, they occasionally collide with atoms in the material, knocking off secondary electrons. If the secondary electrons also have high enough energy to run away, they too accelerate to high energies, produce further secondary electrons, etc. As such, the total number of energetic electrons grows exponentially in an avalanche.
The dynamic friction function, shown in the Figure, takes into account only energy losses due to inelastic collisions and has a minimum of ~216 keV/cm at electron energy of ~1.23 MeV. More useful thresholds, however, must include also the effects due to electron momentum loss due to elastic collisions. In that case, an analytical estimate [5] gives the runaway threshold of ~282 keV/cm, which occurs at the electron energy of ~7 MeV. This result approximately agrees with numbers obtained from Monte Carlo simulations, of ~284 keV/cm [6] and 10 MeV [7] , respectively.
The RREA mechanism above only describes the growth of the avalanche. An initial energetic electron is needed to start the process. In ambient air, such energetic electrons typically come from cosmic rays. [8] In very strong electric fields, stronger than the maximum frictional force experienced by electrons, even low-energy ("cold" or "thermal") electrons can accelerate to relativistic energies, a process dubbed "thermal runaway." [9]
RREA avalanches generally move opposite the direction of the electric field. As such, after the avalanches leave the electric field region, frictional forces dominate, the electrons lose energy, and the process stops. There is the possibility, however, that photons or positrons produced by the avalanche will wander back to where the avalanche began and can produce new seeds for a second generation of avalanches. If the electric field region is large enough, the number of second-generation avalanches will exceed the number of first-generation avalanches and the number of avalanches itself grows exponentially. This avalanche of avalanches can produce extremely large populations of energetic electrons. This process eventually leads to the decay of the electric field below the level at which feedback is possible and therefore acts as a limit to the large-scale electric field strength. [6]
The large population of energetic electrons produced in RREA will produce a correspondingly large population of energetic photons by bremsstrahlung. These photons are proposed as the source of terrestrial gamma-ray flashes. Large RREA events in thunderstorms may also contribute rare but large radiation doses to commercial airline flights. [10] The American physicist Joseph Dwyer coined the term "dark lightning" for this phenomenon, [11] which is still the subject of research. [12]
In particle physics, bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into radiation, thus satisfying the law of conservation of energy. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the decelerated particles increases.
Compton scattering is the quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron. Specifically, when the photon hits electrons, it releases loosely bound electrons from the outer valence shells of atoms or molecules.
A Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others may be temporarily created. The belts are named after James Van Allen, who is credited with their discovery.
Lightning is a natural phenomenon formed by electrostatic discharges through the atmosphere between two electrically charged regions, either both in the atmosphere or one in the atmosphere and one on the ground, temporarily neutralizing these in a near-instantaneous release of an average of one gigajoule of energy. This discharge may produce a wide range of electromagnetic radiation, from heat created by the rapid movement of electrons, to brilliant flashes of visible light in the form of black-body radiation. Lightning causes thunder, a sound from the shock wave which develops as gases in the vicinity of the discharge experience a sudden increase in pressure. Lightning occurs commonly during thunderstorms as well as other types of energetic weather systems, but volcanic lightning can also occur during volcanic eruptions. Lightning is an atmospheric electrical phenomenon and contributes to the global atmospheric electrical circuit.
The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation. It is named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated with Geiger in developing the technique further in 1928 to produce a practical tube that could detect a number of different radiation types.
Ionizing radiation, including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel up to 99% of the speed of light, and the electromagnetic waves are on the high-energy portion of the electromagnetic spectrum.
The Compton Gamma Ray Observatory (CGRO) was a space observatory detecting photons with energies from 20 keV to 30 GeV, in Earth orbit from 1991 to 2000. The observatory featured four main telescopes in one spacecraft, covering X-rays and gamma rays, including various specialized sub-instruments and detectors. Following 14 years of effort, the observatory was launched from Space Shuttle Atlantis during STS-37 on April 5, 1991, and operated until its deorbit on June 4, 2000. It was deployed in low Earth orbit at 450 km (280 mi) to avoid the Van Allen radiation belt. It was the heaviest astrophysical payload ever flown at that time at 16,300 kilograms (35,900 lb).
In condensed matter physics, scintillation is the physical process where a material, called a scintillator, emits ultraviolet or visible light under excitation from high energy photons or energetic particles. See scintillator and scintillation counter for practical applications.
A terrestrial gamma-ray flash (TGF), also known as dark lightning, is a burst of gamma rays produced in Earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 million electronvolts. It is speculated that TGFs are caused by intense electric fields produced above or inside thunderstorms. Scientists have also detected energetic positrons and electrons produced by terrestrial gamma-ray flashes.
Gamma-ray spectroscopy is the qualitative study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Gamma-ray spectrometry, on the other hand, is the method used to acquire a quantitative spectrum measurement.
The Global Geospace Science (GGS) Wind satellite is a NASA science spacecraft designed to study radio waves and plasma that occur in the solar wind and in the Earth's magnetosphere. It was launched on 1 November 1994, at 09:31:00 UTC, from launch pad LC-17B at Cape Canaveral Air Force Station (CCAFS) in Merritt Island, Florida, aboard a McDonnell Douglas Delta II 7925-10 rocket. Wind was designed and manufactured by Martin Marietta Astro Space Division in East Windsor Township, New Jersey. The satellite is a spin-stabilized cylindrical satellite with a diameter of 2.4 m and a height of 1.8 m.
Runaway breakdown is a theory of lightning initiation proposed by Alex Gurevich in 1992.
IACT stands for imaging atmosphericCherenkov telescope or technique. It is a device or method to detect very-high-energy gamma ray photons in the photon energy range of 50 GeV to 50 TeV.
The term runaway electrons (RE) is used to denote electrons that undergo free fall acceleration into the realm of relativistic particles. REs may be classified as thermal or relativistic. The study of runaway electrons is thought to be fundamental to our understanding of High-Energy Atmospheric Physics. They are also seen in tokamak fusion devices, where they can damage the reactors.
Gamma-ray burst emission mechanisms are theories that explain how the energy from a gamma-ray burst progenitor is turned into radiation. These mechanisms are a major topic of research as of 2007. Neither the light curves nor the early-time spectra of GRBs show resemblance to the radiation emitted by any familiar physical process.
A gamma ray, also known as gamma radiation (symbol γ or ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays. With frequencies above 30 exahertz (3×1019 Hz), it imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel) alpha rays and beta rays in ascending order of penetrating power.
Gamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy.
Joseph R. Dwyer is an American physicist known for his lightning research. He is a professor of physics at the University of New Hampshire. Dwyer received his Ph.D. in physics from the University of Chicago in 1994 and worked on cosmic-ray physics and gamma-ray astronomy as a research scientist at Columbia University and the University of Maryland before joining the faculty at the Florida Institute of Technology in 2000. After moving to Melbourne, Florida, Dwyer became interested in lightning physics and his research now focuses on high-energy radiation production from thunderstorms and lightning. In 2002, Dwyer and collaborators discovered that rocket-triggered lightning produced large quantities of x-rays, allowing for first the time detailed studies of an atmospheric phenomenon known as runaway breakdown. In 2014, Dwyer left the Florida Institute of Technology and joined the University of New Hampshire.
In electromagnetism, a streamer discharge, also known as filamentary discharge, is a type of transient electric discharge which forms at the surface of a conductive electrode carrying a high voltage in an insulating medium such as air. Streamers are luminous writhing branching sparks, plasma channels composed of ionized air molecules, which repeatedly strike out from the electrode into the air.
TARANIS was an observation satellite of the French Space Agency (CNES) which would have studied the transient events produced in the Earth's atmospheric layer between 10 km (6.2 mi) and 100 km (62 mi) altitude. TARANIS was launched in November 2020 with SEOSat-Ingenio aboard Vega flight VV17 and would have been placed in a Sun-synchronous orbit at an altitude of 676 km, for a mission duration of two to four years, but the rocket failed shortly after launch.