HR: 11:50h
AN: AE32A-06 INVITED     [Abstracts]
TI: Lightning Initiation Mechanisms: A Review and New Data on Submicrosecond-Scale "Lightning Initiation Pulses"
AU: * Rakov, V A
EM: rakov@ece.ufl.edu
AF: Department of Electrical and Computer Engineering, University of Florida, 553 Engineering Building # 33 PO Box 116130, Gainesville, FL 32611-6130 United States
AU: DeCarlo, B A
EM: bdc2000@ufl.edu
AF: Department of Electrical and Computer Engineering, University of Florida, 553 Engineering Building # 33 PO Box 116130, Gainesville, FL 32611-6130 United States
AB: Maximum electric fields typically measured in thunderclouds are 100 to 200 kV/m (the highest measured value is 400 kV/m), which is lower than the expected conventional breakdown field, of the order of 1 MV/m. Two mechanisms of lightning initiation have been suggested to explain the lack of measured electric fields near the breakdown value in the cloud (leaving aside the issue of difficulty to detect localized and short-lived "igniting cells"). One relies on the emission of positive streamers from hydrometeors when the electric field exceeds 250 to 950 kV/m, and the other involves the runaway breakdown that occurs in a critical field assumed to be about 100 kV/m at an altitude of 6 km. Either of these two mechanisms permits, in principle, creation of an ionized region ("lightning seed") in the cloud that is capable of locally enhancing the electric field at its extremities. Such field enhancement is likely to be the main process leading to the formation (via conventional breakdown) of a "hot", self-propagating lightning leader channel. According to Gurevich et al. (2003), the formation of a field-enhancing conductor ("lightning seed") requires a cosmic-ray particle with energy of the order of 10 PeV, involves the runaway breakdown, and is associated with a current pulse having amplitude of 100 to 200 A. This current pulse is predicted to generate a bipolar electric field pulse with a characteristic full width of 0.2 to 0.4 microseconds (Gurevich et al. 2002). Gurevich et al. (2003) presented submicrosecond-full-width electric field pulses measured 5 to 20 km from lightning discharges that they interpreted as lightning initiation pulses. (Additional measurements of such pulses were obtained in a different location in 2004; Gurevich, personal communication.) Note that these "lightning initiation pulses" are more than an order of magnitude shorter than the narrow bipolar pulses (NBPs) that have characteristic full widths of a few tens of microseconds. The latter pulses apparently can be produced by discharges composed of the lightning initiation process only or be part of the initial breakdown of full-fledged cloud flashes. Multiple NBPs have been observed during the same lightning flash. We measured submicrosecond-scale electric field pulses similar to the "lightning initiation pulses" predicted and observed by Gurevich et al. (2002, 2003). It appears that the submicrosecond-scale pulses can occur both before and after the first ordinary (typically tens-of-microseconds-width) initial-breakdown pulse of the lightning flash. These new measurements, obtained in Florida, will be presented and their possible interpretation will be discussed. References Gurevich, A.V., Duncan, L.M., Karashtin, A.N., and Zybin, K.P. 2003. Radio emission of lightning initiation. Phys. Lett. A, 312: 228-37. Gurevich, A.V., Duncan, L.M., Medvedev, Yu. V., and Zybin, K.P. 2002. Radio emission due to simultaneous effect of runaway breakdown and extensive atmospheric showers. Phys. Lett. A, 301: 320-6.
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
SC: Atmospheric and Space Electricity [AE]
MN: Fall Meeting 2005