Runaway Air Breakdown Research Articles

The initiation of lightning by runaway air breakdown

A mechanism for lightning initiation by the sustained runaway breakdown of air is presented. Unlike earlier models that rely upon large cosmic‐ray air showers, this mechanism uses the runaway electrons produced by the steady background of atmospheric cosmic‐rays to amplify non‐uniformities in the electric field. The ionization of air from the runaway electrons creates a region of discharge that propagates in the opposite direction of the electrons, enhancing the electric field in front of it to the point where a conventional breakdown can occur. As the discharged region grows, positron feedback can become important, dramatically increasing the flux of runaway electrons and increasing the propagation speed of the discharged region up to 106 m/s. The ambient electric field strength needed for this mechanism is within the range of values observed in thunderclouds and is substantially lower than that required by most other mechanisms.

The feedback mechanism of runaway air breakdown

The feedback mechanism presented in the paper published by Dwyer (2003) is examined in the context of runaway breakdown and thunderstorm electrical conditions. A detailed comparison between previous kinetic and Monte Carlo simulations performed by the authors and others and those of Dwyer are presented. The role of positrons and bremsstrahlung photons in contributing to the runaway avalanche rate is discussed. An alternative and simplified treatment of the feedback mechanism is presented. Our preliminary results are in basic agreement with Dwyer and indicate that positron feedback is more important. In addition, we find that there is a temporal ‘stepping’ effect associated with the feedback process and that this effect could also lead to a spatial stepping reminiscent of stepped leaders.

Optical characteristics of blue jets produced by runaway air breakdown, simulation results

The results of numerical calculations of the intensity and spectra of optical emissions from blue jets produced by runaway air breakdown in the atmosphere are presented. It is found that a positive runaway streamer develops in the altitude range 20–34 km following an intracloud discharge that possesses a continuing current of ∼1.7 kA. The ionization front of the runaway streamer propagates upward with a velocity ∼90 km/s and produces optical emissions with a maximum intensity ∼400 kR and a duration ∼53 ms. The comparison between theory and observation yields good agreement for such important blue jet characteristics as maximum intensity of optical emissions, color, front velocity, duration, maximum radius and vertical dimensions and supports the viability of runaway air breakdown as a driving mechanism for this particular type of high altitude discharge.

Optical characteristics of red sprites produced by runaway air breakdown

The results of numerical calculations of intensity and spectra of optical emissions from red sprites produced by runaway air breakdown in the atmosphere are presented. It is shown that the optical emissions from red sprites consist of two components: (1) short‐term (t ≈ 0.3–2 ms) emissions produced as a result of dissipation of an energetic electron beam in air; 2) long‐term (t ≈ 2–10 ms) emissions produced by a population of low‐energy electrons in an electric field. The long‐term optical emissions are calculated for all low‐energy electrons, including the secondary low‐energy electrons produced by the relativistic electron beam, ambient background electrons, and electrons produced as a result of regular breakdown. The theoretical results are compared with observational data.

Simulations of high-altitude discharges initiated by runaway breakdown

Detailed 2D hydrodynamic and quasi-electrostatic simulations of high-altitude discharges driven by runaway air breakdown are presented for four cases, corresponding to sprites initiated by positive cloud-to-ground lightning strikes in which 200 C of charge is neutralized at an altitude of 11.5 km in 10, 7, 5 and 3 ms. We find that the computed optical emissions agree well with low-light level camera images of sprites, both in terms of the overall intensity and spatial distribution of the emissions. Our results show the presence of blue emissions extending down to 40 km (blue tendrils) and red sprite tops extending from 50 to 77 km. Simulated spectra show that N 2 1st positive emissions dominate in the wavelength range from 550 to 850 nm, in good agreement with observations. Strong radio pulses with durations of ∼300 μs and peak electric field amplitudes ranging from 20 to 75 V/m at an altitude of 80 km and an approximate distance from the discharge of 50 km were computed. The magnitude and duration of these pulses is sufficient to cause breakdown and heating of the lower ionosphere (80–95 km) and leads us to suggest that sprites may also launch the EMP responsible for the production of elves. The computed values for the γ-ray fluxes are in agreement with observations of γ-ray bursts of atmospheric origin and the peak secondary electron densities which we obtain are in good agreement with recent measurements of HF echoes at mesospheric heights and associated with lightning.

Ground‐based search for X rays generated by thunderstorms and lightning

A series of ground‐based experiments have been performed to investigate gamma ray/X ray count rate increases that are associated with thunderstorm and lightning activity. NaI scintillation detectors were configured in a long‐timescale channel to monitor long‐term count rate variations (on timescales of 1–104 s) as a function of time, pressure, temperature, humidity, electric field, rainfall, and general thunderstorm activity and in a short‐timescale channel that was triggered by an electric field change meter to monitor correlations of individual counts (on timescales of 10−7–10−2 s) with particular phases of a lightning flash. Long‐timescale results typically show count rate increases of a few percent to as much as 100% above background levels during thunderstorm activity and are likely due primarily to gamma ray emissions from radon daughter‐ion decay as the daughter ions are precipitated to the ground by rainfall. The production of bremsstrahlung X rays by a thunderstorm runaway electron mechanism cannot be ruled out by this data, but the data indicate that if the mechanism is in operation, the bremsstrahlung flux at the ground is at best intermittent and/or barely discernible above background levels. Short‐timescale results do not show any evidence for the production of X rays by individual lightning flashes. However, these results are inconclusive since only 10 cases of lightning flashes within a 1 km distance were recorded. The results of the overall study are compared to previous studies which claim positive correlations of count rate increases with thunderstorm and lightning activity. They are also discussed in terms of runaway electron acceleration in thunderstorm electric fields and in terms of the runaway air breakdown mechanism for lightning initiation. A review of the physics of and previous studies of X ray emissions from thunderstorms and lightning is presented in the introduction.

High altitude discharges and gamma‐ray flashes: A manifestation of runaway air breakdown

γ‐ray flashes of atmospheric origin as well as blue jets and red sprites are naturally explained by high‐altitude discharges produced by runaway air breakdown. We present the first detailed model of the development of upward propagating discharges and compute optical and γ‐ray emissions that are in excellent agreement with observations. According to our theory, such discharges represent the first known manifestation of runaway air breakdown, a fundamental new process in plasma physics.

On runaway breakdown and upward propagating discharges

The origins of mysterious γ ray and radio flashes recently detected by satellite‐based instruments passing over thunderstorms are examined in the context of upward propagating discharges initiated by runaway air breakdown. Preliminary calculations normalized by the recent optical measurements of so‐called sprites indicate that the runaway mechanism may well be the source of these emissions. If this is true, then upward discharges represent the first known manifestation of a fundamental, new process in plasma physics.

Nonuniform runaway air-breakdown

The spatial structure of an individual runaway breakdown pulse, stimulated by a seed energetic electron, is determined.

Kinetic theory of runaway air breakdown.

The kinetic theory for an air breakdown mechanism advanced in a previous paper [Phys. Lett. A 165, 463 (1992)] is developed. The relevant form of the Boltzmann equation is derived and the particle orbits in both velocity space and configuration space are computed. A numerical solution of the Boltzmann equation, assuming a spatially uniform electric field, is obtained and the temporal evolution of the electron velocity distribution function is described. The results of our analysis are used to estimate the magnitude of potential x-ray emissions from discharges in thunderstorms.