Olivier Chanrion

A PIC-MCC code for simulation of streamer propagation in air

A particle code has been developed to study the distribution and acceleration of electrons in electric discharges in air. The code can follow the evolution of a discharge from the initial stage of a single free electron in a background electric field to the formation of an electron avalanche and its transition into a streamer. The code is in 2D axi-symmetric coordinates, allowing quasi 3D simulations during the initial stages of streamer formation. This is important for realistic simulations of problems where space charge fields are essential such as in streamer formation. The charged particles are followed in a Cartesian mesh and the electric field is updated with Poissons equation from the charged particle densities. Collisional processes between electrons and air molecules are simulated with a Monte Carlo technique, according to cross section probabilities. The code also includes photoionisation processes of air molecules by photons emitted by excited constituents. The paper describes the code and presents some results of streamer development at 70km altitude in the mesosphere where electrical discharges (sprites) are generated above severe thunderstorms and at ~10km relevant for lightning and thundercloud electrification. The code is used to study acceleration of thermal seed electrons in streamers and to understand the conditions under which electrons may reach energies in the runaway regime. This is the first study in air, with a particle model with realistic spatial dependencies of the electrostatic field. It is shown that at 1atm pressure the electric field must exceed ~7.5 times the breakdown field to observe runaway electrons in a constant electric field. This value is close to the field where the electric force on an electron equals the maximum frictional force on an electron - found at ~100eV. It is also found that this value is reached in a negative streamer tip at 10km altitude when the background electric field equals ~3 times the breakdown field. At higher altitudes, the background electric field must be relatively larger to create a similar field in a streamer tip because of increased influence of photoionisation. It is shown that the role of photoionization increases with altitude and the effect is to decrease the space charge fields and increase the streamer propagation velocity. Finally, effects of electrons in the runaway regime on negative streamer dynamics are presented. It is shown the energetic electrons create enhanced ionization in front of negative streamers. The simulations suggest that the thermal runaway mechanism may operate at lower altitudes and be associated with lightning and thundercloud electrification while the mechanism is unlikely to be important in sprite generation at higher altitudes in the mesosphere.

Production of runaway electrons by negative streamer discharges

[1] In this paper we estimate the probability that cold electrons can be accelerated by an ambient electric field into the runaway regime, and discuss the implications for negative streamer formation. The study is motivated by the discovery of ms duration bursts of g‐rays from the atmosphere above thunderstorms, the so‐called Terrestrial Gamma‐Ray Flashes. The radiation is thought to be bremsstrahlung from energetic (MeV) electrons accelerated in a thunderstorm discharge. The observation goes against conventional wisdom that discharges in air are carried by electrons with energies below a few tens of eV. Instead the relativistic runaway electron discharge has been proposed which requires a lower threshold electric field; however, seed electrons must be born with energies in the runaway regime. In this work we study the fundamental problem of electron acceleration in a conventional discharge and the conditions on the electric field for the acceleration of electrons into the runaway regime. We use particle codes to describe the process of stochastic acceleration and introduce a novel technique that improves the statistics of the relatively few electrons that reach high energies. The calculation of probabilities for electrons to reach energies in the runaway regime shows that even with modest fields, electrons can be energized in negative streamer tips intothe runaway regime, creating abeamed distribution in front of thestreamer that affects its propagation. The results reported here suggest that theories of negative streamers and spark propagation should be reexamined with an improved characterization of the kinetic effects of electrons. Citation: Chanrion, O., and T. Neubert (2010), Production of runaway electrons by negative streamer discharges, J. Geophys. Res., 115, A00E32, doi:10.1029/2009JA014774.

The planetary rate of sprite events

We propose a new formula to calculate the planetary rate of sprite events, based on observations with sprite detectors. This formula uses the number of detected sprites, the detection efficiency and the false alarm rate of the detector and spatial and temporal effectiveness functions. The role of these elements in the formula is discussed for optical and non-optical recordings. We use the formula to calculate an average planetary rate of sprite events of similar to 2.8 per minute with an accuracy of a factor similar to 2 - 3 by use of observations reported in the literature. The proposed formula can be used to calculate the occurrence rate of any physical event detected by remote sensing.

Characteristics and conditions of production of transient luminous events observed over a maritime storm

On the night of 15/16 November 2007, cameras in southern France detected 30 transient luminous events (TLEs) over a storm located in the Corsican region (France). Among these TLEs, 19 were sprites, 6 were halos, and 5 were elves. For 26 of them, a positive “parent” cloud‐to‐ground lightning (P+CG) flash was identified. The peak current of the P+CG flashes for the sprites had an average value of 63 kA and had a maximum value of 125 kA. The flashes for the halos and the elves had average values of 272 and 351 kA, respectively, and they had maximum values of 312 and 384 kA, respectively. No TLEs were detected after negative CG flashes with very large peak currents. Among the 26 P+CG flashes, 23 were located in a stratiform region with reflectivity values lower than 45 dBZ. The CG flashes in this region were classified into two groups according to the time interval separating them from the following flash: one group with values less than 2 s and one with values greater than 2 s. About 79% of all CGs were produced in a sequence of at least two flashes less than 2 s apart. For 65.5% of the sequences, the first flash was positive with an average peak current of 73 kA, while the later +CG flashes in a sequence had much lower peak currents. Several triangulated sprites were found to be shifted from their P+CG flashes by about 10 to 50 km and preferentially downstream. The observations suggest that the P+CG flashes can initiate both sprites and other CG flashes in a storm.

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

A model is proposed capable of accounting for the local electric field increase in front of the lightning stepped leader up to magnitudes allowing front electrons to overcome the runaway energy threshold and thus to initiate relativistic runaway electron avalanches capable of generating X-ray and γ ray bursts observed in negative lightning leader. The model is based on an idea that an ionization wave, propagating in a preionized channel, is being focused, such that its front remains narrow and the front electric field is being enhanced. It is proposed that when a space leader segment, formed ahead of a negative lightning leader, connects to the leader, the electric potential of the leader is transferred through the space leader in an ionizing wave that continues into the partly ionized channels of preexisting streamers of the space leader. It is shown with numerical simulations that the ionization channels of streamers limit the lateral expansion of the ionization wave, thereby enhancing the peak electric field to values allowing an acceleration of low-energy electrons into the runaway regime where electrons efficiently generate bremsstrahlung. The results suggest that the inhomogeneous ionization environment at the new leader tip amplifies the production rate of energetic electrons relative to a homogeneous environment considered in the past studies.

Relativistic electron beams above thunderclouds

Non-luminous relativistic electron beams above thunderclouds have been detected by the radio signals of low frequency 40-400 kHz which they radiate. The electron beams occur 2-9 ms after positive cloud-to-ground light- ning discharges at heights between 22-72 km above thun- derclouds. Intense positive lightning discharges can also cause sprites which occur either above or prior to the elec- tron beam. One electron beam was detected without any lu- minous sprite which suggests that electron beams may also occur independently of sprites. Numerical simulations show that beams of electrons partially discharge the lightning elec- tric field above thunderclouds and thereby gain a mean en- ergy of 7 MeV to transport a total charge of 10 mC up- wards. The impulsive current 3◊ 10 3 Am 2 associated with relativistic electron beams above thunderclouds is di- rected downwards and needs to be considered as a novel ele- ment of the global atmospheric electric circuit.

Profuse activity of blue electrical discharges at the tops of thunderstorms

Thunderstorm clouds may reach the lower stratosphere, affecting the exchange of greenhouse gases between the troposphere and stratosphere. This region of the atmosphere is difficult to access experimentally, and our knowledge of the processes taking place here is incomplete. We recently recorded color video footage of thunderstorms over the Bay of Bengal from the International Space Station. The observations show a multitude of blue, kilometer-scale, discharges at the cloud top layer at ~18 km altitude and a pulsating blue discharge propagating into the stratosphere reaching ~40 km altitude. The emissions are related to the so-called blue jets, blue starters, and possibly pixies. The observations are the first of their kind and give a new perspective on the electrical activity at the top of tropical thunderstorms; further, they underscore that thunderstorm discharges directly perturb the chemistry of the stratosphere with possible implications for the Earths radiation balance.

Electron acceleration during streamer collisions in air

Abstract High‐voltage laboratory experiments show that discharges in air, generated over a gap of one meter with maximal voltage of 1 MV, may produce X‐rays with photon energies up to 1 MeV. It has been suggested that the photons are bremsstrahlung from electrons accelerated by the impulsive, enhanced field during collisions of negative and a positive streamers. To explore this process, we have conducted the first self‐consistent particle simulations of streamer encounters. Our simulation model is a 2‐D, cylindrically symmetric, particle‐in‐cell code tracing the electron dynamics and solving the space charge fields, with a Monte Carlo scheme accounting for collisions and ionization. We present the electron density, the electric field, and the velocity distribution as functions of space and time. Assuming a background electric field 1.5 times the breakdown field, we find that the electron density reaches 2·1021 m−3, the size of the encounter region is ∼3·10−12 m3 and that the field enhances to ∼9 times the breakdown field during ∼10−11 s. We further find that the radial component becomes comparable to the parallel component, which together with angular scattering leads to an almost isotropic distribution of electrons. This is consistent with laboratory observations that X‐rays are emitted isotropically. However, the maximum energy of electrons reached in the simulation is ∼600 eV, which is well below the energies required to explain observations. The reason is that the encounter region is small in size and duration. For the photon energies observed, the field must be enhanced in a larger region and/or for a longer time.

The influence of bremsstrahlung on electric discharge streamers in N2, O2 gas mixtures

Streamers are ionization filaments of electric gas discharges. Negative polarity streamers propagate primarily through electron impact ionization, whereas positive streamers in air develop through ionization of oxygen by UV photons emitted by excited nitrogen; however, experiments show that positive streamers may develop even for low oxygen concentrations. Here we explore if bremsstrahlung ionization facilitates positive streamer propagation. To discriminate between effects of UV and bremsstrahlung ionization, we simulate the formation of a double headed streamer at three different oxygen concentrations: no oxygen, 1 ppm O2 and 20% O2, as in air. At these oxygen levels, UV-relative to bremsstrahlung ionization is zero, small, and large. The simulations are conducted with a particle-in-cell code in a cylindrically symmetric configuration at ambient electric field magnitudes three times the conventional breakdown field. We find that bremsstrahlung induced ionization in air, contrary to expectations, reduces the propagation velocity of both positive and negative streamers by about 15%. At low oxygen levels, positive streamers stall; however, bremsstrahlung creates branching sub-streamers emerging from the streamer front that allow propagation of the streamer. Negative streamers propagate more readily forming branching sub-streamers. These results are in agreement with experiments. At both polarities, ionization patches are created ahead of the streamer front. Electrons with the highest energies are in the sub-streamer tips and the patches.

Positive streamer initiation from raindrops in thundercloud fields

The threshold field for the electric gas discharge in air is ≈26 kVcm atm , yet themaximum field measured (from balloons) is ≈3 kVcm atm . The question of how lightning is stimulated is therefore one of the outstanding problems in atmospheric electricity. According to the popular idea first suggested by Loeb and developed further by Phelps, lightning can be initiated from streamers developed in the enhanced electric field around hydrometeors. In our paper, we prove by numerical simulations that positive streamers are initiated, specifically, around charged water drops. The simulation model includes the kinetics of free electrons, and positive and negative ions, the electron impact ionization and photon ionization of the neutral atmospheric constituents, and the formation of space charge electric fields. Simulations were conducted at air pressure 0.4 atm, typical at thundercloud altitudes, and at different background electric fields, drop sizes, and charges. We show that the avalanche-to-streamer transition is possible near drops carrying 63–485pC in thundercloud fields with intensity of 10 kVcm atm 1 and 15kVcm atm 1 for drops sizes of 1mm and 0.5mm, respectively. Thus, the electric field required for the streamer formation is larger than the measured thunderstorm fields. Therefore, the results of simulations suggest that second mechanisms must operate to amplify the local field. Such mechanisms could be electric field space variations via collective effects of many hydrometeors or runaway breakdown.