Date:  Wednesday, February 21, 1996
Time:  4:15 PM ­ Refreshments at 4:00
Location:  ERL 126


Runaway Electrons as a Source of Red Sprites
and Terrestrial Gamma Ray Bursts in the Mesosphere

Dr. T. F. Bell
STARLab, Dettt. of Electrical Engineering,
Stanford University

Abstract

Luminous glows, known as Red Sprites, following positive cloud-to-ground (CG)
lightning discharges and occurring at altitudes ranging from cloud-tops to as
high as 90 km, have been detected by imaging systems on the ground [Franz
et al., 1990; Lyons, 1994], on the Space Shuttle [Boeck et al., 1992], and on
aircraft [Sentman et al., 1995; Wescott et al., 1995].

This type of upward discharge has sharp features and vertical structure, with
response primarily in the red region of the spectrum, exhibiting short (~ 1 ms)
duration, and generally detached from the cloud-tops but extending to as high
as 95 km altitude [Sentman et al., 1995].

Recently Pasko et al. [1995] proposed that Red Sprites are produced by the
heating of mesospheric electrons by large quasi-electrostatic (QE) thundercloud
fields. These QE fields appear following a positive CG lightning discharge,
and their duration at any altitude is controlled by the local relaxation time
of the E field. This new mechanism accounts for many of the observed aspects
of these discharges.

According to the QE model, the electric field necessary to produce Sprites is
of the order of the electric field Ec necessary to produce ordinary air
breakdown. However, Ec is roughly an order of magnitude larger than the
threshold field necessary to produce ~ 1 MeV runaway electrons. Thus there is
reason to believe that 1 MeV runaway electrons may be involved in the
Red Sprites phenomenon.

Our discussion will focus on some of the consequences of the existence of a
beam of 1 MeV runaway electrons in the mesosphere and lower ionosphere. These
include the production of optical emissions and terrestrial gamma ray bursts.