A model for electric field enhancement in lightning leader tips to levels allowing X‐ray and γ ray emissions

We investigate the emergence of space stems ahead of negative leaders. These are luminous spots that appear ahead of an advancing leader mediating the leader's stepped propagation. We show that space stems start as regions of locally depleted conductivity that form in the streamers of the corona around the leader. An attachment instability enhances the electric field leading to strongly inhomogeneous, bright, and locally warmer regions ahead of the leader that explain the existing observations. Since the attachment instability is only triggered by fields above 10 kV/cm and internal electric fields are lower in positive than in negative streamers, our results explain why, although common in negative leaders, space stems, and stepping are hardly observed if not absent in positive leaders. Further work is required to fully explain the streamer to leader transition, which requires an electric current persisting for timescales longer than the typical attachment time of electrons, around 100 ns.

Section: Discussionmentioning

confidence: 52%

Section: Discussionmentioning

confidence: 96%

Spontaneous Emergence of Space Stems Ahead of Negative Leaders in Lightning and Long Sparks

Malagón-Romero¹,

Luque²

2019

“…We model the effects of small‐scale air density perturbations and ionization by a preceding streamer as suggested in Babich et al (2015), Eichwald et al (1996), Navarro‐González et al (2001), Hill and Robb (1968), Kacem et al (2013), Liu and Zhang (2014), Marode et al (1979), Villagrán‐Muniz et al (2003), and Plooster (1970). We choose sinusoidal air density perturbation in the radial direction with the minimum on the axis ( r = 0) and the maximum at the outer boundary ( r = L r )

n_{j} (r) = n_{0} ((), 1 - ξ_{j} \cos (rπ / L_{r})); j = 0, \dots, 4

where n 0 =2.55·10 25 m −3 is the background neutral density at sea level and ξ j = 0−4 is the perturbation amplitude of the j 'th profile simulated.…”

Section: Methodsmentioning

confidence: 99%

“…They were first observed in 1994 from the Compton Gamma‐ray Observatory satellite (Fishman et al, 1994) and later confirmed by other space‐based observations (Briggs et al, 2010; Marisaldi et al, 2010; Smith et al, 2005). X‐rays have also been observed at closer proximity to the source with sensors on the ground and on balloons (Dwyer, 2004; Mallick et al, 2012; Moore et al, 2001) and in high‐voltage laboratory discharge experiments of long sparks (Babich et al, 2015; Dwyer et al, 2005; Kochkin et al, 2014, 2016). The X‐ray and γ ‐ray are produced by high‐energy, runaway electrons through the bremsstrahlung process; however, the acceleration process of these electrons is still under debate.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

High‐Energy Emissions Induced by Air Density Fluctuations of Discharges

Köhn

Chanrion

Neubert

2018

Self Cite

Bursts of X‐rays and γ‐rays are observed from lightning and laboratory sparks. They are bremsstrahlung from energetic electrons interacting with neutral air molecules, but it is still unclear how the electrons achieve the required energies. It has been proposed that the enhanced electric field of streamers, found in the corona of leader tips, may account for the acceleration; however, their efficiency is questioned because of the relatively low production rate found in simulations. Here we emphasize that streamers usually are simulated with the assumption of homogeneous gas, which may not be the case on the small temporal and spatial scales of discharges. Since the streamer properties strongly depend on the reduced electric field E/n, where n is the neutral number density, fluctuations may potentially have a significant effect. To explore what might be expected if the assumption of homogeneity is relaxed, we conducted simple numerical experiments based on simulations of streamers in a neutral gas with a radial gradient in the neutral density, assumed to be created, for instance, by a previous spark. We also studied the effects of background electron density from previous discharges. We find that X‐radiation and γ‐radiation are enhanced when the on‐axis air density is reduced by more than ∼25%. Pre‐ionization tends to reduce the streamer field and thereby the production rate of high‐energy electrons; however, the reduction is modest. The simulations suggest that fluctuations in the neutral densities, on the temporal and spacial scales of streamers, may be important for electron acceleration and bremsstrahlung radiation.

Increase of the electric field in head‐on collisions between negative and positive streamers

Ihaddadene

Célestin

2015

Head‐on collisions between negative and positive streamer discharges have recently been suggested to be responsible for the production of high electric fields leading to X‐rays emissions. Using a plasma fluid approach, we model head‐on collisions between negative and positive streamers. We observe the occurrence of a very strong electric field at the location of the streamer collision. However, the enhancement of the field produces a strong increase in the electron density, which leads to a collapse of the field over only a few picoseconds. Using a Monte Carlo model, we have verified that this process is therefore not responsible for the acceleration of a significant number of electrons to energy >1 keV. We conclude that no significant X‐ray emission could be produced by the head‐on encounter of nonthermal streamer discharges. Moreover, we quantify the optical emissions produced in the streamer collision.