HR: 1340h
AN: AE23A-0905    [Abstracts]
TI: Charge Transfer Characteristics and Initiation Mechanisms of Long Delayed Sprites
AU: * Li, J
EM: jl108@ee.duke.edu
AF: Electrical and Computer Engineering Department, Duke University, 129 Hudson Hall, Duke University, Durham, NC 27708, United States
AU: Cummer, S A
EM: cummer@ee.duke.edu
AF: Electrical and Computer Engineering Department, Duke University, 129 Hudson Hall, Duke University, Durham, NC 27708, United States
AU: Lyons, W A
EM: walyons@frii.com
AF: FMA Research, Inc., FMA Research, Inc., Yucca Ridge Field Station, Ft. Collins, CO 80524, United States
AU: Nelson, T E
EM: tnelson@frii.com
AF: FMA Research, Inc., FMA Research, Inc., Yucca Ridge Field Station, Ft. Collins, CO 80524, United States
AB: Simultaneous measurements of high altitude optical emissions and the magnetic field produced by sprite-associated lightning discharges enable a close examination of the link between low altitude lightning process and high altitude sprite process. In this work, we report results of the coordinated analysis of high speed (1000--10000 frames per second) sprite video and wideband (0.1 Hz to 30 kHz) magnetic field measurements made simultaneously at the Yucca Ridge Field Station and Duke University during the June through August 2005 campaign period. During the observation period, the high speed camera detected 83 sprite events in 67 TLE sequences, which are caused by the same number of +CGs. 46% of these sprite events are delayed more than 10 ms after the lightning return stroke. With the estimated lightning source current moment waveform, we computed the continuing current amplitude and total charge transfer characteristics of the long delayed sprites (>10 ms delay). Our calculation shows the total charge moment change of the long delayed sprites can vary from several hundred C km to more than ten thousand C km. All the long delayed sprites are related with intense continuing current bigger than 2 kA. This continuing current provides about 50% to 90% of the total charge transfer. However, a bigger continuing current does not necessarily mean a shorter time delay. This indicates that other processes also involved in the sprite initiation for long delayed sprites. In our observations, the sferic burst, a high frequency noise caused by intra-cloud activity, is always accompanied by a slow intensification in the lightning source current before the time of sprite initiation. Thus we used the lightning source current as an input and employed a 2-D FDTD model to numerically simulate the electric field at different altitudes and compare it with the breakdown field. Including the effect of the electron mobility dependence on electric field, the simulation results showed that the slow intensification itself plays an important role in sprite initiation. For events analyzed, the predicted altitude from the FDTD simulation agree well with the measurement from high speed video images. Both the measurements and the simulations indicate that sprites with long time delay tend to initiate at a lower altitude comparing to short delayed sprites.
DE: 3324 Lightning
DE: 3360 Remote sensing
SC: Atmospheric and Space Electricity [AE]
MN: 2007 Fall Meeting