The physics of lightning

[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

[1] We report the global transient luminous event (TLE) distributions and rates based on the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment onboard the FORMOSAT-2 satellite. ISUAL observations cover 45°S to 25°N latitude during the northern summer and 25°S to 45°N latitude during the northern winter. From July 2004 to June 2007, ISUAL recorded 5,434 elves, 633 sprites, 657 halos, and 13 gigantic jets. Surprisingly, elve is the dominant type of TLEs, while sprites/halos are a distant second. Elve occurrence rate jumps as the sea surface temperature exceeds 26 degrees Celsius, manifesting an ocean-atmosphere-ionosphere coupling. In the ISUAL survey, elves concentrate over the Caribbean Sea, South China Sea, east Indian Ocean, central Pacific Ocean, west Atlantic Ocean, and southwest Pacific Ocean; while sprites congregate over central Africa, Japan Sea, and west Atlantic Ocean. The ISUAL experiment observed global rates of 3.23, 0.50, 0.39, and 0.01 events per minute for elves, sprites, halos, and gigantic jets, respectively. Taking the instrumental detection sensitivity and the restricted survey area into account, the corrected global occurrence rates for sprites and elves likely are a factor of two and an order of magnitude higher, respectively. ISUAL observations also indicate that the relative frequency of high peak current lightning (>80 kA) is 10 times higher over the oceans than over the land. On the basis of the corrected ISUAL elve global occurrence rate, the total electron content at the lower ionosphere above elve hot zones was computed to be elevated by more than 5%.

https://ir.nctu.edu.tw:443/bitstream/11536/8465/1/000258515200004.pdf

Global distributions and occurrence rates of transient luminous events

Blue jets, gigantic jets, cloud-to-cloud discharges and cloud-to-ground lightning are all electrical discharges from thunderclouds. An analysis of numerical simulations and observations of these phenomena places them all in a unifying framework. Thunderstorms occasionally produce upward discharges, called blue jets and gigantic jets, that propagate out of the storm top towards or up to the ionosphere1,2,3,4. Whereas the various types of intracloud and cloud-to-ground lightning are reasonably well understood, the cause and nature of upward discharges remains a mystery. Here, we present a combination of observational and modelling results that indicate two principal ways in which upward discharges can be produced. The modelling indicates that blue jets occur as a result of electrical breakdown between the upper storm charge and the screening charge attracted to the cloud top; they are predicted to occur 5–10 s or less after a cloud-to-ground or intracloud discharge produces a sudden charge imbalance in the storm. An observation is presented of an upward discharge that supports this basic mechanism. In contrast, we find that gigantic jets begin as a normal intracloud discharge between dominant mid-level charge and a screening-depleted upper-level charge, that continues to propagate out of the top of the storm. Observational support for this mechanism comes from similarity with ‘bolt-from-the-blue’ discharges5 and from data on the polarity of gigantic jets6. We conclude that upward discharges are analogous to cloud-to-ground lightning. Our explanation provides a unifying view of how lightning escapes from a thundercloud.

/pdf/upward-electrical-discharges-from-thunderstorms-hppud9zhql.pdf

Upward electrical discharges from thunderstorms

An overview of the general phenomenology and physical mechanism of large-scale electrical discharges termed 'sprites' observed at high altitude in the Earth's atmosphere above thunderstorms is presented. The primary emphasis is placed on summarizing available experimental data on various emissions documented to date from sprites and interpretation of these emissions in the context of similar data obtained from laboratory discharges, in particular the pulsed corona discharges, which are believed to be the closest pressure-scaled laboratory analogue of sprite discharges at high altitude. We also review some of the recent results on modelling of laboratory and sprite streamers emphasizing the importance of the photoionization effects for the understanding of the observed morphological features of streamers at different pressures in air and provide a comparison of emissions obtained from streamer models with results of recent satellite-based observations of sprites.

/pdf/red-sprite-discharges-in-the-atmosphere-at-high-altitude-the-8hxh5695sa.pdf

Red sprite discharges in the atmosphere at high altitude: the molecular physics and the similarity with laboratory discharges

Transient luminous events in the atmosphere, such as lighting-induced sprites and upwardly discharging blue jets, were discovered recently in the region between thunderclouds and the ionosphere. In the conventional picture, the main components of Earth's global electric circuit include thunderstorms, the conducting ionosphere, the downward fair-weather currents and the conducting Earth. Thunderstorms serve as one of the generators that drive current upward from cloud tops to the ionosphere, where the electric potential is hundreds of kilovolts higher than Earth's surface. It has not been clear, however, whether all the important components of the global circuit have even been identified. Here we report observations of five gigantic jets that establish a direct link between a thundercloud (altitude approximately 16 km) and the ionosphere at 90 km elevation. Extremely-low-frequency radio waves in four events were detected, while no cloud-to-ground lightning was observed to trigger these events. Our result indicates that the extremely-low-frequency waves were generated by negative cloud-to-ionosphere discharges, which would reduce the electrical potential between ionosphere and ground. Therefore, the conventional picture of the global electric circuit needs to be modified to include the contributions of gigantic jets and possibly sprites.

W. C. Lai

Papers

Gigantic jets between a thundercloud and the ionosphere.