The physics of lightning

[1] Modeling studies indicate that double-headed streamers originating from single electron avalanches in lightning-driven quasi-static electric fields at mesospheric altitudes accelerate and expand, reaching transverse scales from tens to a few hundreds of meters and propagation speeds up to one tenth of the speed of light, in good agreement with recent telescopic, high-speed video and multichannel photometric observations of sprites. The preionization of the medium ahead of a streamer by the ionizing UV photons originating from a region of high electric field in the streamer head (i.e., photoionization) significantly modifies the streamer scaling properties as a function of air pressure in comparison with those predicted by similarity laws. The photoionization leads to lower peak electric fields in the streamer head, lower streamer electron densities, wider initial streamer structures, and lower acceleration and expansion rates of streamers at sprite altitudes 40–90 km, when compared to the ground level. The primary reason for the observed differences is that the effective quenching altitude of the excited states of the molecular nitrogen b1Πu, b′1, and c′41 that give photoionizing radiation is about 24 km. The quenching of these states is therefore negligible at sprite altitudes, leading to a substantial enhancement of the electron-ion pair production ahead of the streamer tip because of the photoionization, when compared to the ground level. The maximum radius of the expanding streamers is predominantly controlled by the combination of the absorption cross section χmin = 3.5 × 10−2 cm−1 Torr−1 of the molecular oxygen (O2) at 1025 A and the partial pressure of O2 in air, Streamers exhibit branching when their radius becomes greater than Model results indicate a lower branching threshold radius for positive streamers in comparison with negative streamers, under otherwise identical ambient conditions. These results are in good agreement with recent results of high-speed photography of laboratory streamers in near-atmospheric pressure N2/O2 mixtures and similar morphology documented during recent telescopic and high-speed video observations of sprites.

/pdf/effects-of-photoionization-on-propagation-and-branching-of-356orhkr3k.pdf

Effects of photoionization on propagation and branching of positive and negative streamers in sprites

[1] Streamers are thin filamentary plasmas that can initiate spark discharges in relatively short (several centimeters) gaps at near ground pressures and are also known to act as the building blocks of streamer zones of lightning leaders. These streamers at ground pressure, after 1/N scaling with atmospheric air density N, appear to be fully analogous to those documented using telescopic imagers in transient luminous events (TLEs) termed sprites, which occur in the altitude range 40–90 km in the Earth's atmosphere above thunderstorms. It is also believed that the filamentary plasma structures observed in some other types of TLEs, which emanate from the tops of thunderclouds and are termed blue jets and gigantic jets, are directly linked to the processes in streamer zones of lightning leaders. Acceleration, expansion, and branching of streamers are commonly observed for a wide range of applied electric fields. Recent analysis of photoionization effects on the propagation of streamers indicates that very high electric field magnitudes ∼10 Ek, where Ek is the conventional breakdown threshold field defined by the equality of the ionization and dissociative attachment coefficients in air, are generated around the tips of streamers at the stage immediately preceding their branching. This paper describes the formulation of a Monte Carlo model, which is capable of describing electron dynamics in air, including the thermal runaway phenomena, under the influence of an external electric field of an arbitrary strength. Monte Carlo modeling results indicate that the ∼10 Ek fields are able to accelerate a fraction of low-energy (several eV) streamer tip electrons to energies of ∼2–8 keV. With total potential differences on the order of tens of MV available in streamer zones of lightning leaders, it is proposed that during a highly transient negative corona flash stage of the development of negative stepped leader, electrons with energies 2–8 keV ejected from streamer tips near the leader head can be further accelerated to energies of hundreds of keV and possibly to several tens of MeV, depending on the particular magnitude of the leader head potential. It is proposed that these energetic electrons may be responsible (through the “bremsstrahlung” process) for the generation of hard X rays observed from ground and satellites preceding lightning discharges or with no association with lightning discharges in cases when the leader process does not culminate in a return stroke. For a lightning leader carrying a current of 100 A, an initial flux of ∼2–8 keV thermal runaway electrons integrated over the cross-sectional area of the leader is estimated to be ∼1018 s−1, with the number of electrons accelerated to relativistic energies depending on the particular field magnitude and configuration in the leader streamer zone during the negative corona flash stage of the leader development. These thermal runaway electrons could provide an alternate source of relativistic seed electrons which were previously thought to require galactic cosmic rays. The duration of the negative corona flash and associated energetic radiation is estimated to be in the range from ∼1 μs to ∼1 ms depending mostly on the pressure-dependent size of the leader streamer zone.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011350

Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders

This review concerns the theory of the avalanche multiplication of high-energy (0.1 – 10 MeV) electrons in a neutral material, a newly discovered phenomenon known as runaway breakdown (RB). In atmospheric conditions RB takes place at electric fields an order of magnitude weaker than those needed for normal breakdown in air. Experimental work of the past few years has shown that RB determines the maximum electric field strength in thunderclouds and is behind a variety of phenomena newly observed in thunderstorm atmosphere, such as giant high-altitude discharges between thunderclouds and the ionosphere, anomalous amplifications of X-ray emission, intense bursts of gamma radiation, etc. These phenomena are becoming increasingly active areas of study. A necessary condition for the occurrence of runaway avalanche is the presence of high energy seed electrons. In the atmosphere, these are cosmic ray secondary electrons. Therefore, the observed effects reflect the close relationship between cosmic rays and electrodynamic processes in the thunderstorm atmosphere. The first laboratory results on RB are also presented. Further studies in this area may be of interest for high-current electronics.

https://iopscience.iop.org/article/10.1070/PU2001v044n11ABEH000939/pdf

Runaway breakdown and electric discharges in thunderstorms

For over a century, numerous undocumented reports have appeared about unusual large-scale luminous phenomena above thunderclouds1,2,3,4,5,6 and, more than 80 years ago, it was suggested that an electrical discharge could bridge the gap between a thundercloud and the upper atmosphere7,8. Since then, two classes of vertically extensive optical flashes above thunderclouds have been identified—sprites9,10,11 and blue jets12,13,14. Sprites initiate near the base of the ionosphere, develop very rapidly downwards at speeds which can exceed 107 m s-1 (ref. 15), and assume many different geometrical forms16,17,18,19. In contrast, blue jets develop upwards from cloud tops at speeds of the order of 105 m s-1 and are characterized by a blue conical shape12,13,14. But no experimental data related to sprites or blue jets have been reported which conclusively indicate that they establish a direct path of electrical contact between a thundercloud and the lower ionosphere. Here we report a video recording of a blue jet propagating upwards from a thundercloud to an altitude of about 70 km, taken at the Arecibo Observatory, Puerto Rico. Above an altitude of 42 km—normally the upper limit for blue jets and the lower terminal altitude for sprites—the flash exhibited some features normally observed in sprites. As we observed this phenomenon above a relatively small thunderstorm cell, we speculate that it may be common and therefore represent an unaccounted for component of the global electric circuit.

https://www.ursi.org/Proceedings/ProcGA02/papers/p0857.pdf

Electrical discharge from a thundercloud top to the lower ionosphere

A variety of storm top electrical discharges have been observed using several types of low-light imagers, film, and the human eye. Recently, a video recorded an unprecedented, bright blue upward discharge from a tropical thunderstorm top near Puerto Rico. The event reached the base of the ionosphere. The horizontal dimensions of cloud top discharges can range from 100 m to several kilometers. Upward extents vary from 100 m to 70 km. Shapes include “points” of light, upwardly flaring trumpets, and narrow, vertical, lightning-like channels, some topped with expanding blue, flame-like features. Visual appearances range from brilliant white lightning-like channels to a grainy, almost particulate appearing jets of dim blue light, and sometimes as a blue flame within which a brilliant white channel appears. The classical blue jet is at the lower limit of human night vision whereas some upward discharges have been clearly seen during daylight. Cloud top “pixies” last no longer than 16.7 ms, whereas upward lightn...

Upward Electrical Discharges From Thunderstorm Tops

Photometric measurements in the SPRITES ’95 & ’96 campaigns of nitrogen second positive (399.8 nm) and first negative (427.8 nm) emissions

Recent time-resolved multi-color photometric data obtained on one class of lightning-related transient upper-atmospheric electromagnetic events called sprites have confirmed an impulsive ionization emission during the sprite initiation. Data have also been obtained on some sprites which do not exhibit observable tendrils and which exhibit ionization emission that, if present, is below our detection limit. This suggests that some sprite events exhibit strong ionization while others do not. These results indicate that conditions causing sprite optical emissions are highly variable.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GL003672

Multi-color photometric measurements of ionization and energies in sprites

Blue-light imagery and photometry of sprites

The scientific community's perception of the middle atmosphere above thunderstorms as “uninteresting” changed completely in the last decade. Today, a host of lightning-related Transient Luminous Events (TLEs) have been identified, including sprites, blue jets, elves, sprite halos, and trolls. Others may remain to be discovered.

Aside from the intrinsic scientific issues arising from this linkage of tropospheric electrical phenomena with that of middle atmosphere, a number of practical questions emerge. What, if any threats might TLEs pose to aerospace operations above 20 km? Do sprites represent a heretofore undocumented source of middle atmospheric Nox? What role might they play in the global electrical circuit, as well as in the energetics of the upper atmosphere [Bering et al., 1998]? Might these phenomena impact satellite-based global monitoring and surveillance efforts?

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/00EO00278

R. A. Armstrong

Papers

Upward Electrical Discharges From Thunderstorm Tops

Photometric measurements in the SPRITES ’95 & ’96 campaigns of nitrogen second positive (399.8 nm) and first negative (427.8 nm) emissions

Multi-color photometric measurements of ionization and energies in sprites

Blue-light imagery and photometry of sprites

The hundred year hunt for the sprite