HR: 15:10h
AN: AE13A-07    [Abstracts]
TI: Electrical Breakdown Inside a Thundercloud - Cosmic Rays vs Hydrometeor Effects
AU: * Sentman, D D
EM: dsentman@gi.alaska.edu
AF: University of Alaska, Geophysical Institute, Fairbanks, AK 99775 United States
AU: Christian, H J
EM: Hugh.Christian@nasa.gov
AF: NASA Marshall Space Flight Center, 320 Sparkman Dr., Huntsville, AL 35805 United States
AB: Recent work has suggested that relativistic electrons created by an extensive air shower incident on a highly charged thundercloud could provide the trigger for lightning discharges by means of the runaway breakdown mechanism [Gurevich, A., K. Zybin, and R.Roussel-Dupre, Phys. Lett. A, 254, 79, 1999]. One of the motivations for development of this theory is that electric fields measured inside a thundercloud rarely exceed values that are approximately an order of magnitude below the limit for conventional breakdown in air [Marshall, T., M. McCarthy, and W. Rust, J. Geophys. Res., 100, 2081, 1995], and in fact correspond roughly to the threshold values for runaway breakdown. Since the fields are also apparently too low for streamer propagation, as computed using a gas phase air and water vapor model [Phelps., C.T., and R.F. Griffiths, J. Appl. Phys., 47, 2929, 1976], it was concluded that conventional processes cannot account for the relatively small electric fields associated with lightning breakdown, but that cosmic ray initiated processes could lower the breakdown threshold to the observed values. Not addressed in these works were previous studies of the possible effects on conventional breakdown of various types and amounts of hydrometeors, i.e., supercooled water drops, ice crystals and graupel that populate the ambient breakdown environment in the interior of a thundercloud. Such effects are well established both experimentally and theoretically, and are a viable alternate hypothesis to account for the observed discrepancy between the theoretical and observed breakdown fields. For example, experimental measurements of the effects of hydrometeors on breakdown [Crabb, J., and J. Latham, Q.J. Roy. Met. Soc., 100, 191, 1974] show that the breakdown threshold is lowered to within the range of maximum electric fields observed in thunderclouds. Theoretical studies have also shown that significant effects occur in related corona formation in hydrometeors [Schroeder, V., M. Baker, and J. Latham, Q.J.R. Met. Soc., 125, 1681, 1999]. To date, no detailed experiments or theoretical work appear to have been conducted on streamer initiation in a hydrometeor-rich environment, either by conventional means or as a result of energetic electron triggering. Similarly, no data appear to exist on hydrometeor effects on streamer propagation. In this talk, published laboratory and theoretical investigations of conventional breakdown in a hydrometeor-rich environment are reviewed and compared with the results of gas-phase runaway theory. It is concluded that current data are too inconclusive to convincingly favor either mechanism, and that further detailed studies are required.
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3359 Radiative processes
DE: 7554 X-rays, gamma rays, and neutrinos
DE: 7807 Charged particle motion and acceleration
SC: Atmospheric and Space Electricity [AE]
MN: Fall Meeting 2005