HR: 0800h
AN: AE31A-0154    [Abstracts]
TI: Numerical Study on Branching Properties of Sprite Streamers
AU: * Hiraki, Y
EM: hira@pat.geophys.tohoku.ac.jp
AF: Tohoku University, Aramaki Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Pasko, V
EM: vpp1@mail.psu.edu
AF: Penn State University, 211B EE East, University Park, PA 16802 United States
AU: Liu, N
EM: nul105@psu.edu
AF: Penn State University, 211B EE East, University Park, PA 16802 United States
AU: Fukunishi, H
EM: fuku@pat.geophys.tohoku.ac.jp
AF: Tohoku University, Aramaki Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AB: The complex morphology of sprites, dominated by highly branched and predominantly vertical filamentary structures, is well documented in the existing literature [e.g., {\it Gerken and Inan, JASTP, 65,} 567, 2003; {\it Moudry et al., JASTP, 65,} 509, 2003], and has recently been interpreted in terms of thin channels of ionization called streamers [{\it Liu and Pasko, JGR, 109,} A04301, 2004, and references therein]. The commonly observed, but not yet fully understood, tree-like structures in sprites are likely related to the branching properties of individual streamer channels. The branching phenomenon has received increasing attention in recent experimental and modeling studies of streamers at ground pressure [e.g., {\it Arrayas et al., PRL, 88,} 174502(R), 2002; {\it Yi and Williams, JPD (Appl. Phys.), 35,} 205, 2002; {\it Hallac et al., JPD, 36,} 2498, 2003]. However, there is still a disagreement on whether the branching of model streamers is a pseudo-branching (i.e. a consequence of numerical instability) or just correctly reflects the real splitting physics [{\it Pancheshnyi and Starikovskii, JPD, 36,} 2683, 2003]. Furthermore, the modeling results on branching morphology reported by different research groups remain highly controversial [{\it Kulikovsky, JPD, 33,} 1514, 2000; {\it Arrayas et al.,} 2002; {\it Rocco et al., PRE, 66,} 035102(R), 2002; {\it Hallac et al.,} 2003; {\it Liu and Pasko,} 2004; and references therein]. The purpose of this study is to clarify the physical nature of the streamer branching by means of numerical simulations using several numerical schemes. For this study we have developed a minimal model of streamers in a non-attaching nitrogen-like gas. The model allows studies of streamers propagating between planar electrodes in a strong applied electric field, and is similar to the model used by {\it Arrayas et al.} [2002]. Although this model only accounts for the effects of the ionization of neutral molecules and the transport of electrons, it has an advantage by minimizing the total number of involved physical processes and by allowing us to investigate more clearly the variations in branching morphology obtained by using different numerical schemes. In this talk we report modeling results of branching as a function of magnitudes of applied electric field and medium pre-ionization using the following numerical schemes to calculate the electron flux: (1) the 1st order upwind, (2) the 2nd order, (3) the 3rd order upwind-biased, (4) the improved Scharfetter-Gummel (ISG), and (5) the flux-corrected transport (FCT). In all studied cases, we observe the same pre-branching features of streamers (i.e., extremely high peak field and electron density in the head with a weak curvature) as reported previously by {\it Arrayas et al.} [2002], {\it Rocco et al.} [2002] and {\it Liu and Pasko} [2004]. The results obtained by different numerical schemes show slight variations in onset time and morphology of branching; however, it is shown in all considered cases that the branching is very sensitive to ambient electron density and that the expanding streamers can reach a branching stage for a wide range of applied electric field. These results agree with the earlier suggestion by {\it Liu and Pasko} [2004]. The results on two distinct types of branching morphology observed in our numerical experiments and their relationship to physical and numerical parameters of the model will be presented.
DE: 3324 Lightning
DE: 3337 Numerical modeling and data assimilation
DE: 3367 Theoretical modeling
DE: 3304 Atmospheric electricity
SC: Atmospheric and Space Electricity [AE]
MN: 2004 AGU Fall Meeting