HR: 11:50h
AN: AE52A-06    [Abstracts]
TI: Optical Emissions of Sprite Streamers in Weak Electric Fields
AU: * Liu, N
EM: nul105@psu.edu
AF: The Pennsylvania State University, Department of Electrical Engineering, Communications and Space Sciences Laboratory (CSSL), University Park, PA 16802 United States
AU: Pasko, V P
EM: vpasko@psu.edu
AF: The Pennsylvania State University, Department of Electrical Engineering, Communications and Space Sciences Laboratory (CSSL), University Park, PA 16802 United States
AB: Sprites commonly consist of large numbers of needle-shaped filaments of ionization [e.g., Gerken and Inan, JASTP, 65, 567, 2003] and typically initiate at altitudes 70-75 km in a form of upward and downward propagating streamers [Stanley et al., GRL, 26, 3201, 1999; Stenbaek-Nielsen et al., GRL, 27, 3829, 2000; McHarg et al., JGR, 107, 1364, 2002; Moudry et al., JASTP, 65, 509, 2003]. The strong electric fields E exceeding the conventional breakdown threshold field Ek are needed for initiation of sprite streamers from single electron avalanches and recent modeling studies indicate that streamers propagating in fields E$>$Ek experience strong acceleration and expansion in good agreement with the above cited observations [Liu and Pasko, JGR, 109, A04301, 2004]. The initiated streamers are capable of propagating in fields substantially lower than Ek [Allen and Ghaffar, J. Phys. D: Appl. Phys., 28, 331, 1995] and it is expected that a significant part of sprite optical output comes from regions with E$<$Ek above and well below the initiation altitude. Principal approaches to remote sensing of electron energy distributions in sprites using absolute intensities and ratios of various emission bands arising from the excited electronic states of neutral and ionized molecular nitrogen have been extensively discussed in the existing literature [e.g., Armstrong et al., GRL, 27, 653, 2000; Takahashi et al., Adv. Space Res., 26, 1205, 2000; Morrill et al., GRL, 29, 100, 2002; Pasko and George, 107, 1458, 2002; Chern et al., JASTP, 65, 647, 2003; Miyasato et al., JASTP, 65, 573, 2003] and understanding of optical emissions produced by streamers propagating in weak electric fields represents an important component of related studies needed for correct interpretation of the existing experimental data. In this talk we will report results on application of time dependent optical emission model developed in [Liu and Pasko, 2004] to studies of sprite streamers in weak electric fields (E$<$Ek). The results indicate, in particular, substantially lower values (typically a factor of two) of the peak electric fields around tips of streamers in weak fields (E$<$Ek) in comparison with those in strong fields (E$>$Ek). Additionally, the values of electric fields inside of the streamer channel are always well below Ek and since the excitation coefficients for optical emissions are very sensitive to the driving electric field magnitude most of the optical luminosity of streamers in this case arises from streamer tips, indicating that observed streamer filaments in many cases may be produced by time averaging of optical luminosity coming from localized regions around streamer tips as streamers move through an instrument's field of view. We will discuss pressure dependent differences of optical emissions at different sprite altitudes, and important similarities between observed sprite streamers and recent time resolved ($<$1 ns) imaging of laboratory streamers in point-to-plane discharge geometry conducted at ground and near ground pressures [van Veldhuizen et al., IEEE Trans. Plasma Sci., 30, 162, 2002; Yi and Williams, J. Phys. D. Appl. Phys., 35, 205, 2002].
DE: 3324 Lightning
DE: 2427 Ionosphere/atmosphere interactions (0335)
DE: 2435 Ionospheric disturbances
DE: 3304 Atmospheric electricity
DE: 0310 Airglow and aurora
SC: Atmospheric and Space Electricity [AE]
MN: 2004 AGU Fall Meeting