Observations have been made of a new terrestrial phenomenon: brief (approx. millisecond), intense flashes of gamma rays, observed with space-borne detectors. These flashes must originate at altitudes in the atmosphere above at least 30 km in order to be observable by orbiting detectors aboard the Compton Gamma-Ray Observatory (CGRO). At least a dozen events have been detected over the past 2 years. The photon spectra from the events are very hard and are consistent with bremsstrahlung emission from energetic (MeV) electrons. The most likely origin of these high energy electrons, while speculative at this time, is a rare type of high altitude electrical discharge above thunderstorm regions.

/pdf/discovery-of-intense-gamma-ray-flashes-of-atmospheric-origin-58ej84so5v.pdf

Discovery of Intense Gamma-Ray Flashes of Atmospheric Origin

The Pierre Auger Observatory, located on a vast, high plain in western Argentina, is the world's largest cosmic ray observatory. The objectives of the Observatory are to probe the origin and characteristics of cosmic rays above $10^{17}$ eV and to study the interactions of these, the most energetic particles observed in nature. The Auger design features an array of 1660 water-Cherenkov particle detector stations spread over 3000 km$^2$ overlooked by 24 air fluorescence telescopes. In addition, three high elevation fluorescence telescopes overlook a 23.5 km$^2$, 61 detector infill array. The Observatory has been in successful operation since completion in 2008 and has recorded data from an exposure exceeding 40,000 km$^2$ sr yr. This paper describes the design and performance of the detectors, related subsystems and infrastructure that make up the Auger Observatory.

https://pure.rug.nl/ws/files/133727110/1_s2.0_S0168900215008086_main.pdf

The Pierre Auger Cosmic Ray Observatory

An image of an unusual luminous electrical discharge over a thunderstorm 250 kilometers from the observing site has been obtained with a low-light-level television camera. The discharge began at the cloud tops at 14 kilometers and extended into the clear air 20 kilometers higher. The image, which had a duration of less than 30 milliseconds,resembled two jets or fountains and was probably caused by two localizd electric charge concentrations at the cloud tops. Large upward discharges may create a hazard for aircraft and rocket launches and, by penetrating into the ionosphere, may initiate whistler waves and other effects on a magnetospheric scale. Such upward electrical discharges may account for unexplained photometric observations of distant lightning events that showed a low rise rate of the luminous pulse and no electromagnetic sferic pulse of the type that accompanies cloud-to-earth lightning strokes. An unusually high rate of such photometric events was recorded during the night of 22 to 23 September 1989 during a storm associated with hurricane Hugo.

https://science.sciencemag.org/content/sci/249/4964/48.full.pdf

Television image of a large upward electrical discharge above a thunderstorm system.

The dual jet aircraft Sprites94 campaign yielded the first color imagery and unambiguously triangulated physical dimensions and heights of upper atmospheric optical emissions associated with thunderstorm systems. Low light level television images, in both color and in black and white (B/W), obtained during the campaign show that there are at least two distinctively different types of optical emissions spanning part or all of the distance between the anvil tops and the ionosphere. The first of these emissions, dubbed 'sprites' after their elusive nature, are luminous structures of brief (less than 16 ms) duration with a red main body that typically spans the latitude range 50-90 km, and possessing lateral dimensions of 5-30 km. Faint bluish tendrils often extend downward from the main body of sprites, occasionally appearing to reach cloud tops near 20 km. In this paper the principal characteristics of red sprites as observed during the Sprites94 campaign are described. The second distinctive type of emissions, 'blue jets,' are described in a companion paper (Wescott et al., this issue).

Preliminary results from the Sprites94 Aircraft Campaign: 1. Red sprites

Quasi-electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes lead to ambient electron heating (up to ∼5 eV average energy), ionization of neutrals, and excitation of optical emissions in the mesosphere/lower ionosphere. Model calculations predict the possibility of significant (several orders of magnitude) modification of the lower ionospheric conductivity in the form of depletions of electron density due to dissociative attachment to O2 molecules and/or in the form of enhancements of electron density due to breakdown ionization. Results indicate that the optical emission intensities of the 1st positive band of N2 corresponding to fast (∼ 1 ms) removal of 100–300 C of thundercloud charge from 10 km altitude are in good agreement with observations of the upper part (“head” and “hair” [Sentman et al., 1995]) of the sprites. The typical region of brightest optical emission has horizontal and vertical dimensions ∼10 km, centered at altitudes 70 km and is interpreted as the head of the sprite. The model also shows the formation of low intensity glow (“hair”) above this region due to the excitation of optical emissions at altitudes ∼ 85 km during ∼ 500 μs at the initial stage of the lightning discharge. Comparison of the optical emission intensities of the 1st and 2nd positive bands of N2, Meinel and 1st negative bands of , and 1st negative band of demonstrates that the 1st positive band of N2 is the dominating optical emission in the altitude range around ∼70 km, which accounts for the observed red color of sprites, in excellent agreement with recent spectroscopic observations of sprites. Results indicate that the optical emission levels are predominantly defined by the lightning discharge duration and the conductivity properties of the atmosphere/lower ionosphere (i.e., relaxation time of electric field in the conducting medium). The model demonstrates that for low ambient conductivities the lightning discharge duration can be significantly extended with no loss in production of optical emissions. The peak intensity of optical emissions is determined primarily by the value of the removed thundercloud charge and its altitude. The preexisting inhomogeneities in the mesospheric conductivity and the neutral density may contribute to the formation of a vertically striated fine structure of sprites and explain why sprites often repeatedly occur in the same place in the sky as well as their clustering. Comparison of the model results for different types of lightning discharges indicates that positive cloud to ground discharges lead to the largest electric fields and optical emissions at ionospheric altitudes since they are associated with the removal of larger amounts of charge from higher altitudes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JA03528

Sprites produced by quasi-electrostatic heating and ionization in the lower ionosphere

This report describes a transient luminosity observed at the altitude of the airglow layer (about 95 km) in coincidence with a lightning flash in a tropical oceanic thunderstorm directly beneath it. This event provides new evidence of direct coupling between lightning and ionospheric events. This luminous event in the ionosphere was the only one of its kind observed during an examination of several thousand images of lightning recorded under suitable viewing conditions with Space Shuttle cameras. Several possible mechanisms and interpretations are discussed briefly.

Lightning induced brightening in the airglow layer

An examination and analysis of video images of lightning, captured by the payload bay TV cameras of the space shuttle, provided a variety of examples of lightning in the stratosphere above thunderstorms. These images were obtained on several recent shuttle flights while conducting the Mesoscale Lightning Experiment (MLE). The images of stratospheric lightning illustrate the variety of filamentary and broad vertical discharges in the stratosphere that may accompany a lightning flash. A typical event is imaged as a single or multiple filament extending 30 to 40 km above a thunderstorm that is illuminated by a series of lightning strokes. Examples are found in temperate and tropical areas, over the oceans, and over the land.

Observations of lightning in the stratosphere

This paper documents the first observations of a new stratospheric electrical phenomenon associated with thunderstorms. On the night of 30 June (UT 1 July) 1994, 30 examples of these events, which we have called “blue starters,” were observed in a 6 m 44 s interval above the very energetic Arkansas thunderstorm where blue jets were first observed. The blue starters are distinguished from blue jets by a much lower terminal altitude. They are bright and blue in color, and protrude upward from the cloud top (17–18 km) to a maximum 25.5 km (83,655 ft.) in altitude. All blue starters events were recorded from two small areas near Texarkana, Texas/Arkansas where hail 7.0 cm in diameter was falling. Comparison to cloud-to-ground (CG) lightning flashes revealed: 1. Blue starters were not observed to be coincident with either positive or negative CG flashes, but they do occur in the same general area as negative CG flashes; 2. Cumulative distributions of the negative CG flashes in ±5 s before and after the starter and within a radius of 50 km shows a significant reduction for about 3 s following the event in the two cells where starters and jets were observed. The energy deficit is approximately 109 J. It is possible that blue starters are a short-lived streamer phenomenon.

Blue starters∷ Brief upward discharges from an intense Arkansas thunderstorm

Blue Jets: their relationship to lightning and very large hailfall, and their physical mechanisms for their production

A note of inquiry published in a magazine read by airplane pilots has elicited 15 new observations of a rare form of lightning that comes out of the cloud top, goes up vertically, and terminates in the clear air above. These confirm previous observations showing that this phenomenon usually occurs above very large and energetic thunderclouds. However, small clouds with tops at 15,000 ft mean sea level (5000 m) have been observed to exhibit this rare form of lightning also. One of the more spectacular observations was made over the severe storm system that produced the devastating tornadoes of April 11, 1965.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JD094iD11p13179%4010.1002/%28ISSN%292169-8996.ELECCONF2

O. H. Vaughan

Papers

Lightning induced brightening in the airglow layer

Observations of lightning in the stratosphere

Blue starters∷ Brief upward discharges from an intense Arkansas thunderstorm

Blue Jets: their relationship to lightning and very large hailfall, and their physical mechanisms for their production

Recent observations of lightning discharges from the top of a thundercloud into the clear air above