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

Abstract

[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 E k , where E k 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 E k 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 $10 18 s À1 , 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 gala...

High‐Energy Emissions Induced by Air Density Fluctuations of Discharges

“…Hence, the effective electric field is strongly enhanced only in a small region around the symmetry axis. Various measurements have shown that electric fields in streamer discharges can reach field strengths of up to ≈10 E k (Kim et al, 2004; Pancheshnyi et al, 2000; Spyrou & Manassis, 1989) consistent with results of streamer simulations and analytic estimates (Chanrion & Neubert, 2008; Köhn et al, 2018; Liu & Pasko, 2004; Moss et al, 2006; Naidis, 2009; Qin & Pasko, 2014; Tholin & Bourdon, 2013). In the vicinity of lightning leader tips, calculations have shown that the enhanced electric field can exceed several times the breakdown field (Köhn, Diniz, Harakeh, 2017; Köhn & Ebert, 2015).…”

“…One is that seed electrons from cosmic ray ionization of the atmosphere are born with energies in the runaway regime and are further accelerated by the ambient electric field in a cloud, forming a relativistic runaway electron avalanche (RREA; Babich et al, 2012; Dwyer, 2003; Gurevich et al, 1992; Gurevich & Karashtin, 2013; Wilson, 1925) including the feedback mechanism where high‐energy electrons produce high‐energy gamma rays through the bremsstrahlung process which subsequently produces secondary electrons and positrons through photoionization, Compton scattering, or pair production (Dwyer, 2003, 2007; Kutsyk et al, 2011; Skeltved et al, 2014). The other is that thermal (cold) electrons are accelerated into the runaway regime in the high, but very localized, field of streamer tips as well as by the enhanced electric fields in the vicinity of lightning leader tips (Babich et al, 2015; Celestin & Pasko, 2011; Chanrion & Neubert, 2008; Köhn et al, 2014; Köhn & Ebert, 2015; Köhn, Diniz, Harakeh, 2017) and subsequently turn into relativistic RREAs (Carlson et al, 2010, 2006; Köhn, Diniz, Harakeh, 2017; Moss et al, 2006). In the following we explore the streamer mechanism.…”

Physical Processes Related to Discharges in Planetary Atmospheres

“…However, the two methods have been checked against each other in the case of RB with discrepancies in the avalanche rates of only a few to tens of percent . Good agreement between discharge characteristics obtained by both methods has also been demonstrated in the conventional breakdown regime (e.g., Moss et al 2006, and references therein). Our discussions below will focus on the Boltzmann treatment of electron transport.…”

“…The possibility of two kinds of TGFs corresponding to low and high altitude sources can be envisaged. A lightning leader as a source of TGFs is predicted by Moss et al (2006) who show that thermal electrons can be accelerated in the leader streamer zone up to energies of several hundreds of keV and possibly up to several tens of MeV. This mechanism then predicts that some TGFs can be produced by high altitude leader processes.…”

Deviations from the local field approximation in negative streamer heads

“…The detection of x-rays emanating from lightning strokes [32,33,34,35] indicates that electrons can gain very high energies within early stages of the lightning event. Therefore runaway electrons within streamer and leader processes should be investigated.…”