Date:  Wednesday, January 24, 1996
Time:  4:15 PM ­ Refreshments at 4:00
Location:  ERL 126


PULSED RADAR MEASUREMENTS 
DURING THE SPRITES'95 CAMPAIGN IN COLORADO

Roland T. Tsunoda
Geoscience and Engineering Center,
SRI International,
Menlo Park, CA 94025

Abstract

Considerable interest has been generated in the geophysical community by the
recent discovery of transient optical emissions (called red sprites and blue
jets) that occur at high altitudes above thunderclouds. In trying to understand
the underlying processes that lead to these emissions,  one of the key
questions is whether ionization is produced and how much.  A difficulty with
trying to experimentally answer this question is the availability of an
appropriate sensor.  Because the amount of ionization produced is thought to
be relatively small (<104 el/cm3),  it seems that only radio sensors that
operate at VLF frequencies can succeed.  We have taken the approach that
refractive index fluctuations are likely to be associated with the heating of
electrons and,  if so,  those fluctuations could lead to underdense scattering
at higher radio frequencies.  With this approach,  we fielded a 24.38 MHz
pulsed radar last summer to participate in the Sprites'95 campaign,  conducted
from the vicinity of Fort Collins, Colorado.

Although we have not yet had the opportunity to compare our radar echoes with
the occurrences of red sprites and blue jets we have found interesting
preliminary results.  In particular,  we have obtained radar echoes at high
altitudes that appear to be related to lightning discharges and not simply to
meteors.  These radar echoes are characterized by rapid rise and decay times
and complex amplitude variations that appear to be modulations associated with
lightning strokes.  We point out how a 25-MHz radar can play a valuable role
in investigating high-altitude lightning-related processes including the
appearances of red sprites and blue jets.