[Seminar] Cosmic Rays and Lightning
Dr. Breese Quinn
quinn at phy.olemiss.edu
Mon Nov 8 08:04:48 CST 2004
Lewis Hall, Room 101
Tuesday, 9 Novemver 2004
Dr. Thomas Marshall
Dept. of Physics & Astronomy
University of Mississippi
"Do Cosmic Rays Initiate Lightning Flashes?"
In 1925 C. T. R. Wilson (Nobel Laureate in Physics for developing the
cloud chamber) first suggested that an energetic electron in a strong
thunderstorm electric field would gain more energy from the field than
it loses in collisions; such electrons are now called "runaway"
electrons. Gurevich et al.  suggested that an avalanche of
runaway electrons, called runaway breakdown, might initiate a lightning
flash. They suggested that runaway electrons have energies on the order
of 1 MeV. The 'seed' electron for such an avalanche is assumed to be a
cosmic ray secondary. In this colloquium I first review the way in
which runaway breakdown is hypothesized to occur. Then I present recent
in-cloud electric field (E) measurements and inferred lightning
initiation locations of three cloud-to-ground lightning flashes. These
data are among the first to identify a thunderstorm region in which the
preflash E exceeded various theoretical runaway electron threshold
values. The maximum measured E in the region was 186 kV m-1 at 5.77 km
altitude, which for runaway electrons is equivalent to 370 kV m-1 at sea
level; this E value is 130-183% of various estimations of the runaway
breakdown threshold. In addition, the volume where E exceeded the
runaway thresholds was estimated to be 1-4 km3, with a vertical depth of
1000 - 1200 m. At least within part of this volume (and perhaps in most
of it) the characteristic scale height for exponential growth of runaway
electrons was 100 m or less. Thus, the main result of this study is
that for these three flashes the conditions necessary for runaway
breakdown existed, so cosmic rays could have initiated the flashes. I
will also show a few examples of unusual electric discharges inside
clouds that may also have been initiated by cosmic rays.
Can those who are registered for Physics 510, please sign the roll
sheet when you arrive?
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