UW Sprite Balloon Experiment 2002

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What are sprites?

Ground Based Sprite Image from Fort Collins, CO 1995 (Courtesy of the Geophysical Institute, the University of Alaska)

Sprites are optical phenomena that occur above active thunderstorms.


      Reports of optical phenomena above thunderstorms where first published in scientific literature in the late 19th century (Toynbee and Mackenzie, 1886, Everett and Everett, 1903).  In the 1950’s the first airborne observation was reported from a commercial airlines pilot over Fiji (Wright, 1950). CTR Wilson, regarded as the father of lightning theory, was the first to try to describe these phenomena physically (Wilson, 1956). Not until 1989 were these phenomena captured on film.  A group from the University of Minnesota recorded a twin upward flash from distant cloud tops while testing a low light level TV camera intended for sounding rockets (Franz,et al 1989)

     The name "sprite" was first proposed by Dr. Davis Sentman from the University of Alaska and first used in literature by W.A. Lyons in 1994. He said that the name "sprite" was "well suited to describe their appearance, and is non-judgemental as to the physics of the phenomena". Sentman himself said that the name originated from their “fairy-like qualities” (Sentman and Wescott, 1995) and it is “a term that is succinct and whimsically evocative of their fleeting nature” (Sentman and Wescott, 1996) 


Various Sprite Images (Courtesy of the Geophysical Institute, the University of Alaska)

Sprite Properties

Sprites are associated with positive cloud-to-ground (CG) lightning discharge, and they last from about 5ms to 300ms(Rodger, 1999).  Sprites usually have a column-like composition, with a lot of fine structure.  They extend from about 30-40km to about 70-90km, with a primary body at about 50-60km.  Their horizontal extent is about 25-50km, with a bright core <10km wide (Rodger 1999).  The main upper portion of sprites are red in color, while the lower tendrils go from red to blue with decreasing altitude (Sentman et al. 1995). These colors are caused by the excitation of molecular nitrogen (the brightest lines in the spectrum are in the ranges 650-680nm and 750-780nm). Sprites are directly correlated with positive CG strokes from large thunderclouds (Sentman and Wescott, 1993; Lyons, 1994).  These tend to be large discharges (>kA).  Only 10% of all lightning is positive CG. 

Where are Sprites Observed?

Since 1989 Sprites have been observed in North America, Africa, Australia, and South and Central America. The space shuttle astronauts have also observed Sprites over both land and oceans, and temperate and tropical regions. Thus, Sprites seemingly can occur over any region as long as strong thunderstorms are present.


Locations where sprites have been reported. Triangles are single observations 
and squares are multiple (Lyons, 1997). 

What Causes Sprites?

 Here are three possible candidates for sprite production which I will briefly describe:

1.  Quasi-Electrostatic Field

2.  Runaway Electron Breakdown

3.  Electromagnetic Pulse (EMP)  (Almost ruled out for sprite production, but associated with a related phenomena called Elves)
 
 

Model 1: The Quasi-Electrostatic Field

1. The cloud charges up before the lightning discharge inducing a negative shielding layer

2. The positive CG removes positive charge but the negative shielding layer remains over a much longer time scale

3. The negative shielding layer remains after the discharge causing polarization in the atmosphere and a quasi-static E-field.  This can me likened to a giant parallel plate capacitor as shown above.  This strong E-field causes electrical breakdown producing sprites.

Model 2: Runaway Electron Breakdown 

       High energy electrons produced by cosmic rays are accelerated by the quasi-static electric field.  Through collisions, these electrons produce ions and new electrons.  Most of these new electrons thermalize due to collisions, but some accelerate in the electric field allowing runaway breakdown.  Since below 1MeV the stopping power decreases with increasing electron energy, the higher energy electrons are able to accelerate rather than thermalize. 


       Cartoon of Runaway Electron Breakdown (Courtesy of Star Laboratory, Stanford)

     In a sufficiently strong electric field, these accelerating electrons produce an avalanche effect that allows the number of electrons to grow exponentially, known as runaway breakdown.  Collisional excitation with the relativistic and secondary electrons produce optical emissions seen as sprites (Bell et al., 1995: Roussel-Dupre and Gurevich, 1996; Yukhimuk et al., 1998)


Energy Loss Due to Collisions (Stopping Power) vs. Electron Energy (Courtesy of Star Laboratory, Stanford)
 

Differentiating between the quasi-electrostatic field and runaway electron models

If we measure a low vertical E-field near the sprite, the quasi-electrostatic field model could be ruled out. The runaway electron model includes the seeding of high energy electrons by cosmic rays, thus it requires a weaker vertical E-field to produce sprites.  The magnitude of the E-field we measure will tell us which model is more valid.  Hence, the new HV detector I am designing is an important instrument for an effective experiment.  The detection of bremsstrahlung x-rays produced by cosmic rays would support the runaway electron model.
 
 

Model 3: Electromagnetic Pulse (EMP)

The electromagnetic pulse from the return stroke of large lightning strikes causes electrical breakdown of the high altitude atmosphere above the thunderstorm(Rowland et al., 1995; Valdiva et al., 1997).  Since the charge is transferred very quickly (0.1ms) during the return stroke, the EMP is a radiation and inductive field phenomena. Since sprites occur at least 5ms after the positive CG and the EMP lasts about 1ms after the positive CG,  the EMP model seems to better explain another related phenomena known as “Elves” which occur 0.35ms after the positive CG.

 
Jeremy Thomas | Oct. 10, 2002