HR: 11:35h
AN: AE41B-05    [PDF]
TI: Dynamics of Streamer-to-Leader Transition in Transient Luminous Events Between Thunderstorm Tops and the Lower Ionosphere
AU: * Pasko, V P
EM: vpasko@psu.edu
AF: Penn State University, 211 B EE East, University Park, PA 16802 United States
AB: Sprites commonly exhibit vertically oriented branched structures filling large volumes of atmosphere in the altitude range 40-90 km above thunderstorms [e.g., Gerken and Inan, JASTP, 65, 567, 2003]. These structures likely represent scaled by atmospheric pressure versions of weakly ionized thin channels of ionization, called streamers, which are commonly observed during initial stage of discharge development in short (several cm) gaps in relatively cold (i.e., 300 K) air at ground pressure [Raizer, Gas Discharge Physics, 1991, p. 334]. The appearance of sprites is very different from the lightning phenomenon occurring at near ground pressures, which deposits large amounts of energy in relatively small volumes leading to total single ionization of atoms [Raizer, 1992, p. 373]. Recent observations of transient luminous events (TLEs) emanating from thundercloud tops, exhibiting some lightning like features near the cloud tops, and then propagating upward through the altitude range typically occupied by sprites [Wescott et al., JGR, 106, 21549, 2001; Pasko et al., Nature, 416, 152, 2002; Su et al., Nature, 423, 974, 2003] represent an opportunity to study a transition from lightning-like structures at thundercloud altitudes to large scale filamentary sprite-like structures at the lower ionospheric altitudes. It is likely that this type of TLEs originates from a streamer zone of conventional lightning leaders and represents a "final jump stage" of the leader process, when the streamer zone of a leader makes contact with the opposite electrode (i.e., ionosphere) [Pasko and George, JGR, 107, doi:10.1029/2002JA009473, 2002]. The understanding of streamer-to-leader transition and scaling of this transition as a function of atmospheric pressure therefore represents a first neccessary step toward understanding of complex dynamics of optical features observed in these events. In this talk we will present results of modeling studies, which allow investigation of effective time scales of the initial stage of air heating in streamer channels up to 5000 K at which the thermal ionization becomes important. The model is zero-dimensional and accounts for time dynamics of air heating and ionization at a fixed point inside of the streamer channel. The model is derived from previous studies conducted for similar purposes at ground pressure [Lowke, J. Phys. D: Appl. Phys., 25, 202, 1992; Naidis, J. Phys. D: Appl. Phys., 32, 2649, 1999; Vidal et al., IEEE Trans. Plasma Sci., 30, 1339, 2002]. In the first part of the talk for calibration purposes we will present comparisons of our model and available laboratory data on time scales of air heating in streamer channels at ground and near ground pressures [Larsson, J. Phys. D: Appl. Phys., 31, 1998] following approach described in [Naidis, 1999]. In the second part we will present results corresponding to a range of air pressures, electric fields and electron densities representative of conditions in streamer channels at TLE altitudes. The obtained transition times indicate that the re-brightening events observed by Pasko et al. [2002] and trailing jet events observed by Su et al. [2003], which terminate at altitudes around 50-60 km, likely correspond to the final section of an upward leader, which was not able to complete the next step due to a dramatic increase in air heating time scales above 50 km.
DE: 2427 Ionosphere/atmosphere interactions (0335)
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
SC: Atmospheric and Space Electricity [AE]
MN: 2003 Fall Meeting