A Preliminary Morphology of Optical Transients Above Thunderstorms
Poster A32C-04, Fall AGU Meeting
Intensive campaigns to investigate
thunderstorm-related optical emmissions occurring in the middle and
upper atmosphere, and associated phenomena, have been carried out by
several groups during the previous three years. Observations over the
Central United States have been the main focus, with additional
observations over Central and South America. Several types of optical
phenomena have already been identified: red sprites, blue jets, and
airglow enhancements. We will present still frame and video examples
of the wide variety of optical emissions that have been observed above
thunderstorms. Information regarding the associated thunderstorm
conditions will be included where available.
This poster available at http://elf.gi.alaska.edu/fagu95/morph.html
Sprites Recorded over
North, Central, and South America. Storms over the Central United
States produced numerous sprites, so that well over 400 were recorded
during a three week period in 1994. South and Central American
thunderstorms, while as electrically active, did not produce the same
number of sprites (approximately two dozen over nearly four weeks of
airplane flights). Sprites have a terminal altitude 87 km,
measured using triangulation from two planes. The sprites last for
less than one video frame (17 msec). Work this summer has shown the
red color of sprites is due to N_2 first positive emissions.
The sprites are associated with positive strokes
(concurrent lightning strokes are recorded by the National Lightning
Detection Network). Sprites do have an associated Very Low Frequency
Jets Recorded primarily
over a very
intense electrical storm (over Arkansas) with large (2.75 inch) hail
reported, jets have a lifetime of between 200 and 300 milliseconds.
The terminal altitude of jets is 40 km (triangulated).
Jets are weakly associated with negative
strokes--statistically significant more strokes occur before jets than
after jets (NLDN data). Jets have no apparent VLF signature. The
source of the blue emissions of jets is unkown.
Starters Blue starters are
associated with blue jets. The terminal altitude is 21 km
(triangulated). Starters last for two or three video frames (less than 100
ms). The starters are correlated
with negative lightning strokes in the same manner as blue jets.
jets. There is no apparent VLF signature associated with starters.
The source of the blue emissions is unkown.
Airglow Enhancements These occur over
sprite producing thunderstorms, both in conjunction with
sprites and alone. The airglow enhancements were first reported from
shuttle observations. A VLF signature is associated with the airglow
enhancement, which have a red color similar to sprites so possibly
due to N_2 first positive emissions. The enhancements occur at
airglow layer (at or above 85 km).
Palm Tree A single
observation associated with large series of several groups of sprites
and a series of positive strokes. The triangulated terminal altitude
the event is 56 km. The red color observed in the color
camera is the same as a sprite, so perhaps N_2 first positive
VLF data has not been analysed in detail.
Candles One storm over
New Mexico (June 19, 1995) produced only `candles'. Upon review these
thin structures are observed with sprites and there are hints of thin
structures in sprites. N_2 first positive emission. A study of
ELF/VLF observations is pending.
Present analysis efforts involve a complete cataloging of optical
transients (and associated VLF measurements) associated with
thunderstorms. Several different types of observations have been
made. Sprites are the most common, associated with
every Mesoscale Convective System producing positives strokes. (With
the data from the National Lightning Detection Network to determine
the location of positive stroke activity, sprites
were observed every night of airplane flight over the Central United
States). However, there was a suprising dearth of sprite activity
and Central America. Airglow enhancements, as first reported from
space shuttle observations, seem to occur over many sprite producing
thunderstorms. Blue jets are apparently associated with thunderstorms
which are very electrically active and produce large hail.
The blue starters are apparently blue jets which fail to meet some
critical requirement for propagation upward to 40 km. The palm tree
example is interesting because it is of brief duration (like the
sprites), but definitely does not reach the terminal altitude
associated with sprites. Candles are apparently similar to sprites,
but do not have the detailed structure.
Differences in storm conditions, atmospheric conditions, or
ionospheric conditions could be the determining factor in the
appearance of optical transients observed between thunderstorm tops
and the ionosphere, leaving much room for both theory work and joint
Presenting Author Matt
Heavner, part of the University of Alaska Fairbanks Red Sprites and Blue Jets