[Seminar] Cosmic Rays and Lightning

[Dr. Breese Quinn]

Colloquium
Lewis Hall, Room 101
Tuesday, 9 Novemver 2004
4:00 pm

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. [1992] 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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