Date:  Wednesday, January 17, 1996
Time:  4:15 PM ­ Refreshments at 4:00
Location:  ERL 126


Optical Observations of Storm Induced Optical Phenomena in the 
Upper Atmosphere: Sprites and ELV-s

S.B. Mende
Lockheed Martin Palo Alto Research Laboratory,
Palo Alto, CA  94304

Abstract

Recent video camera observations of atmospheric regions above thunderstorms
show several optical features.  In the past few years since the discovery of
these features, considerable advances have been made towards understanding
some of the key mechanisms responsible for their production.  These and
related phenomena provide evidence for electrical coupling between the lower
atmosphere and the ionosphere. The frequently occurring vertically oriented
"Sprites" (spanning the 50-90 km region) are thought to be caused by the
quasi-static electric fields which are set up after a large lightning flash
discharges the thunder cloud, leaving a high potential between the cloud and 
the ionosphere. The horizontally elongated faint ELV-s are driven by the large
electromagnetic field pulses which are emitted from intense lightning
discharges. In the summer of 1995 the Lockheed Martin Palo Alto Research
Laboratories participated in the "Sprites 95" campaign and have taken low
light level video images of Sprites and ELV-s. These images reveal many
interesting morphological features of the phenomena,  including measurement of
their spatial extents. The Lockheed low light level imaging spectrograph
documented some of the first spectra of Sprite-emitted light. These spectra
showed that most of the electrons causing the excitation of the light in
Sprites have only a few eV of energy. Light detected in the scanning register
of one of the CCD cameras used in the 1995 field campaign showed that brighter
features such as Sprites produced streak images which could be measured and
that the time structure of Sprites could be obtained with a resolution of 60
microseconds. This allowed the studying of the temporal behavior pattern of
Sprites. Many of the images show that ELV-s often accompany Sprites, but they
sometimes occur by themselves.  Several key questions still remain unanswered
concerning the Sprites and ELV-s:  What are their spectral and altitude
profiles, and how do they show the changing of processes with altitude? Are
there any other spectral features besides N21PG and (if so) what are the
relative intensities of these other emissions compared to the N21PG? What is
the direction of development or decay of the Sprite in the vertical direction?
Does it start at the ionosphere?  What governs the spatial form of Sprites? Is
there a credible mechanism for the generation of gamma rays by Sprites, 
ELV-s or related phenomena?