HR: 0800h
AN: AE31A-0158    [Abstracts]
TI: Monte Carlo Model for Analysis of Runaway Electrons in Streamer Tips in Sprites
AU: * Moss, G D
EM: gdm131@psu.edu
AF: The Pennsylvania State University, Electrical Engineering East, University Park, PA 16802 United States
AU: Pasko, V P
EM: vpasko@psu.edu
AF: The Pennsylvania State University, Electrical Engineering East, University Park, PA 16802 United States
AU: Veronis, G
EM: gveronis@stanford.edu
AF: Stanford University, Packard Electrical Engineering BLD, Stanford, CA 94305 United States
AB: The observed filamentary structures in sprites [Gerken and Inan, JASTP, 65, 567, 2003] have recently been interpreted in terms of thin channels of ionization called streamers, which exhibit acceleration, expansion and branching [Liu and Pasko, JGR, 109, A04301, 2004]. The expanding streamers can reach a branching state for a wide range of applied electric fields and an extremely high peak field can be generated in the streamer tip immediately preceding the branching [Liu and Pasko, 2004]. The acceleration of electrons in tips of highly overvolted streamers [Babich, Sov. Phys. Dokl., 27, 215, 1982] has been proposed for interpretation of X-ray radiation observed in experiments reported by Tarasova et al. [Sov. Phys. Tech. Phys., 19, 351, 1974]. It has also been proposed that the high electric fields in streamer tips can accelerate electrons to energies of several keV, initiating electron runaway in relatively low ambient electric fields E$\sim$Ek [Pasko et al., GRL, 25, 2123, 1998], where Ek is the conventional breakdown threshold field. The recently reported X-ray emissions observed during the approach to ground stage of natural [Moore et al., GRL, 28, 2141, 2001] and triggered [Dwyer et al., Science, 299, 694, 2003; GRL, 31, L05118, 2004] lightning leaders may be related to the enhancement of electric field in the leader streamer zone [Dwyer et al., GRL, 31, L12102, 2004] leading to the generation of runaway electrons in streamer tips. In this talk we report results from a Monte Carlo model, which is capable of describing electron dynamics in air (including the runaway phenomena) under influence of an external electric field of arbitrary strength. The model is similar in technical details to the model previously developed for N2 by Tzeng and Kunhardt [Phys. Rev. A, 34, 2148, 1986] and Kunhardt and Tzeng [Phys. Rev. A, 34, 2158, 1986], and incorporates the following features: (1) the null collision method to determine time between collisions; (2) the remapping of the electron assembly to improve statistics for the high-energy tail of the electron distribution; (3) the differential ionization and scattering cross sections for realistic description of energy spectrum of secondary electrons and the forward scattering properties of electrons at high energies. The elastic collision cross sections of electrons on N2, O2 and Ar are determined from the most recent total cross sections available in the literature and forty-three inelastic collision cross sections obtained from A.V. Phelps [http://jilawww.colorado.edu/www/research/colldata.html]. At high electric fields ($>$5Ek) the model results are validated by comparisons with studies conducted for N2 by Tzeng and Kunhardt [1986] and more recently by Bakhov et al. [IEEE Trans. Plasma Sci., 28, 1254, 2000], and at low fields ($<$5Ek) by comparisons with available data from swarm experiments in air and solutions of Boltzmann equation based on two-term spherical harmonic expansion of the electron distribution function [Morgan and Penetrante, Comp. Phys. Comm., 58, 127, 1990].
DE: 3324 Lightning
DE: 2427 Ionosphere/atmosphere interactions (0335)
DE: 2435 Ionospheric disturbances
DE: 3304 Atmospheric electricity
SC: Atmospheric and Space Electricity [AE]
MN: 2004 AGU Fall Meeting