A Preliminary Morphology of Optical Transients Above Thunderstorms

Poster A32C-04, Fall AGU Meeting

J T Desroschers, M J Heavner, D L Hampton, D D Sentman, E M Wescott

Abstract

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

Observations

  • 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 (VLF) signature.
  • 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 of 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 emission. 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.

    Conclusions

    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 over South 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 studies.
    Presenting Author Matt Heavner, part of the University of Alaska Fairbanks Red Sprites and Blue Jets research team.