Sprite
Observations at Creighton
Sprite
Observations at Creighton
by Paul J. McCrone
ATS 797 Directed Independent Research
Prof. : Dr. Dean Morss
Date : 20 November 1996
Creighton University
Dept. of Atmospheric Sciences Omaha NE
Referred to as "Red Sprites", "Blue Jets" and "Elves", these events are characterized by
the emission of reddish/orange, blue, and green light between the top of thunderstorm
clouds and the ionosphere. Believed to be somewhat similar to the Aurora Borealis,
these lights in the sky are caused by the near visible emission of high energy electrons and
other ions colliding/interacting with gaseous species in the stratosphere and mesosphere.
in order to best increase our understanding of these phenomena. In addition, our intention
was to determine the most effective and meaningful way in which to gather data, and
observe these events. Included are descriptions of actual Sprite and Elve occurrences
during July through August 1996.
clouds (or any other kind of lightning - producing clouds). For the sake of simplicity,
these events will be referred to with a shortened term : "C-I event" (Cloud - to -
Ionosphere event). The term "Cloud - to - Ionosphere" is not meant to imply that the
emission starts at the cloud, then ends in the ionosphere; this issue is still unresolved.
Here, a brief description of each of the C-I events will be provided, including a review of
past observations
1. General Description
The term "Sprite" is actually an acronym, as follows [Jeong, 1996]:
"High speed photometer measurements show that the duration of sprites is only a few ms [milliseconds]. Current evidence strongly suggests that sprites preferentially occur in decaying portions of thunderstorms and are correlated with large positive cloud-to-ground lightning strokes. The optical intensity of sprite clusters, estimated by comparison with tabulated stellar intensities, is comparable to a moderately bright auroral arc. The optical energy is roughly 10-50 kJ [kilojoules] per event, with a corresponding optical power of 5-25 MW [megawatts]. Assuming that optical energy constitutes 1/1000 of the total for the event, the energy and power are on the order of 10-100 MJ [megajoules] and 5-50 GW [gigawatts], respectively."
"If sprites are only barely detectable by the unaided human eye, in intensified television images obtained from the ground and from aircraft they appear as dazzlingly complex structures that assume a variety of forms." [Heavner, 1995]
"hair" (a fainter, reddish glow above the head), and faint bluish / purple tendrils that
extend downward [Sentman et all, 1995]. See figure 1 for an example of a typical sprite [This Image is an
artists rendition of a picture taken by Matthew Heavner,1995]. Sprites also have a clear, distinctive Very Low
Frequency (VLF) signature, making them detectable even during daylight hours.
From the beginning of modern C-I research in 1990, many have compared the reddish
colors and hues of a sprite are reminiscent of the aurora. This suggested "...that the light
of a sprite, like that of the aurora, comes from oxygen or nitrogen molecules excited by
collisions with high energy electrons" [Kerr, 1995]
b) Blue Jets
The following discussion provides the best description of Blue Jets:
"Blue jets are a second high altitude optical phenomenon, distinct from sprites, observed above thunderstorms using low light television systems. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15 degrees full width at vertical speeds of roughly 100 km/s (Mach 300), fanning out and disappearing at heights of about 40-50 km. Their intensities are on the order of 800 kR [kilorayleighs] near the base, decreasing to about 10 kR near the upper terminus. These correspond to an estimated optical energy of about 4 kJ, a total energy of about 30 MJ, and an energy density on the order of a few mJ/m3. Blue jets are not aligned with the local magnetic field." [Heavner, 1995]
As the name implies, these C-I events normally appear blue in color. The average duration
of the apparent source of a blue jet is roughly 200 ms. The entire event lasts about 200-
300 ms, total. Often, they are observed to follow upward directed lightning strokes.
c) Elves
The term "Elves" is also an acronym, as follows [Jeong, 1996]:
as "brief (~ 1ms) brightenings of the airglow layer ... as a distinct phenomenon" and as
an "amorphous glowing region". Observers have noted a greenish hue with these events.
Others have described elves as follows
"Elves are bright but diffuse luminous disks at altitudes of 80-100 km formed 300 (microseconds) after the initiating cloud-ground (CG) lightning and lasting only about 1 ms." [Dowden et al, 1995].
Like sprites, elves have an identifiable VLF signature. As implied by the acronym,
current theories on the generation of elves are focused around the Electromagnetic Pulse
(EMP) from lightning discharges below.
2. Past Observations/History of Sprites in Science
events. One of the first known records of a C-I event was by Mackenzie [1886], when a
series of sprites, blue jets and possibly elves were observed by seamen near Jamaica.
C.T.R. Wilson [1956] reported observing such a display in his remarkable paper on
Atmospheric Electricity. Vonnegut [1980] also reported such emissions.
Numerous reports of these "strange lightning" events are described by the many
eyewitnesses documented in the work of Corliss [1982]. The accounts in Corliss' works
may contain numerous examples if C-I events, since the whole idea of sprites was not
widely known at that time. Many good examples of C-I events are likely misclassified as
either "Rocket Lightning" or "Auroras correlated with Thunderstorms". Another account
of "lightning to the ionosphere" was given by Vaughan and Vonnegut [1982] and Gales
[1982]. Regrettably, the work of Corliss and many others was regarded mainly as a
curiosity at the time.
The area of sprite research clearly took a new tack when Franz et al [1990]
reported the recorded television image of an "upward electrical discharge". Intending to
simply test a new low-light video camera for physics research, their work was a product
of nearly complete chance. It stimulated a variety of research organizations from the
university to government level to begin a systematic study of these events. The Franz
work is considered by most to mark the real beginning of C-I research.
3. Current Theories / Hypotheses
In each of the following discussions, the intention is to provide the reader with
some background into the early theoretical work that has been accomplished thus far. For
a full discussion of these theories, see the applicable authors. Key points from each theory
will be provided.
who developed an overall theory of thundercloud electrification, which is widely regarded
as a "definitive statement as to the electrical state of thunderstorms" [Wallace and Hobbs,
1977]
a) Wilson's Thundercloud Electric Field Theory
Wilson [1925] started a general discussion of "the electric field of a thundercloud"
by stating:
"The electric field of the cloud may cause ionization at great heights, the result being continuous or discontinuous discharge between the cloud and the upper atmosphere ... The charges separated in the thundercloud may re-combine directly by a short-circuiting discharge within the cloud or by continuous or discontinuous discharges through external circuits, one such circuit including the earth and the upper atmosphere..." [Wilson, 1925]
When Wilson referred to the "upper atmosphere", he was specifically pointing to the 60-
80 km range. This serves as the first hypothetical suggestion that electrical disturbances
were occurring at this range in the atmosphere as a direct result of active thunderstorms.
In his later, more comprehensive work on thundercloud electricity, Wilson [1956]
further went on to describe that "...as soon as the cloud has acquired an appreciable
positive electric moment ..., external ionization currents come into action in three
regions." Wilson goes on to explain these three regions, one of which is "... the current
between the top of the cloud and the ionosphere" [Wilson, 1956]. Wilson argued that this
is an important consideration, since "...the effect of the external ionization currents is to
increase each of the two main charges of the cloud...". The main thrust of Wilson's
material set forth here is based on a complex - but classical - electrodynamic analysis of
the entire cloud/atmosphere complex.
ATMOSPHERE". In his words:
"It is quite possible that a discharge between the top of the cloud and the ionosphere is a normal accompaniment of a lightning discharge to earth. Consider first, however, what will happen if no such upper discharge occurs." [Wilson, 1956]
Wilson provided an argument that C-I events must take place, since their absence would
cause a condition that is not observed normally - "...the regeneration of the main field of
the cloud , an accumulation of negative charge occurring where their fall is retarded or
stopped by the upward air stream". As further proof to his argument, the following
discussion was provided:
"There have been a number of reports of ordinary lightning discharges extending upwards from the top of a thundercloud. What is likely to be a more normal accompaniment of a discharge to earth, but one which is only likely to be visible under very special conditions, is a diffuse discharge between the top of the cloud and the upper atmosphere. ...many years ago I observed what appeared to be discharges of this kind from a thundercloud below the horizon There were diffuse fan-shaped flashes of greenish color extending up into a clear sky." [Wilson, 1956]
Another radical idea was offered: C-I events may actually cause (or initiate) normal intracloud
and/or cloud -to-ground lightning:
"It is, on the other hand, possible that a lightning discharge to earth from the base of the cloud may be initiated by a discharge above the cloud. A large potential difference between the top of the cloud and the upper atmosphere will drag down negative ions from the very large supply existing in the ionosphere."
"There is obviously a limit to the potential which can be reached in a thundercloud. At a height where the pressure is half an atmosphere the limit set by the sparking potential would be about 12 x 109 V [Volts]. A potential of one -tenth of this, giving the equivalent of a field of 3000 V/cm at normal pressure, could hardly fail to cause a discharge initiated by accelerated electrons in the upper atmosphere. The maximum potential attainable in a thundercloud cannot much exceed this limit of about 109V. [Wilson , 1956]
So , in summary, Wilson's theory not only predicted the existence of C-I events, but
actually argued that these are a profoundly important part of the entire electrification
process of a thundercloud. In addition, there appears to be a potential symbiotic cause-
and-effect relationship between conventional lightning and C-I events: each can possibly
cause the other.
b) Roussel-Dupre's theory of Cosmic Ray excitation / Gamma Ray emission
Measurements from NASA's Compton Gamma Ray Observatory (GRO) of
Gamma Rays from intense thunderstorms have led some scientists to speculate as to the
specific mechanism that actually stimulates the C-I event at the moment of initiation. In a
recent publication, Robert Roussel-Dupre' of Los Alamos National Laboratory has
commented :
"Usually, the electrons and ions in this region [above the thundercloud] will quickly recombine, annulling the [electric] field. However, if an electron is produced with enough energy to knock more of its kin from neighboring atoms, an avalanche of charge can ensue. The newly freed electrons, pulled upward along the field lines, bump into atmospheric molecules, which then fluoresce at visible wavelengths. Indeed, studies have shown that the red hues of sprites come from glowing nitrogen molecules (N2). The gamma rays come about as the high- speed electrons [sometimes referred to as "Beta Particles"] are deflected by other charged particles ...while radio pulses picked up by ALEXIS [a new experimental satellite] are generated at certain points in the electrons upward journey. " [Roth, 1996]
While there are numerous different ideas as to how the initial "seed electron" starts out,
Roussel-Dupre' provides a scenario where cosmic rays are given credit - "these high-
energy extraterrestrial particles have long been known to knock electrons from atoms and
molecules in the Earth's upper atmosphere." [Roth, 1996]
c) Meteorites
One of the more obscure hypotheses regarding C-I generation is the one offered
by Muller [1995] who argued:
"However, there is another possible triggering phenomena: meteors. Meteors ionize large paths of air, and even if the ionization path does not quite reach the thunderhead, the path could trigger [electrical] breakdown... We postulate that the mesosphere behaves like a triggered spark chamber. Downward lightning suddenly increases the electric field above a thunder cloud. Only when such lightning is roughly coincident with the arrival of a meteor above the cloud do we get a red sprite. Because the ionization lasts for several seconds, the coincidence does not have to be exact." [Muller, 1995]
d) Others
One well regarded theory regarding the appearance of C-I events is EMP (or
sferics). Theoretical calculations from researchers at Tohoku University have determined
that the time between lightning flash and elve appearance ( less than 1 microsecond ) fits
the right time interval for an electromagnetic pulse to reach the ionosphere, and possibly
cause an elve [Kerr, 1995]. Sprites, on the other hand, are not affected by this,
apparently. It has been determined that "....sprites lag the huge positive lightning stroke
triggering them by several milliseconds. That's much longer than it would take for the
electromagnetic pulse from a lightning stroke to arrive at the altitude of a sprite" [Kerr,
1995]. For the most part, sprites are considered to follow a electrostatic model that is
very close to that proposed by Wilson. The timing of a sprite with respect to a lightning
bolt tends to support this idea.
B. Why Study Cloud -to- Ionosphere events?
events. To start off, we will look at the immediate impact that a C-I event may have on
the middle atmosphere. After this, we will briefly touch on possible negative results for
aircraft operating above a thunderstorm. Finally, a review of some issues regarding
atmospheric electricity.
1. Possible Impacts on Chemistry of Upper Atmosphere
jets, and elves occur in an altitude range from as low as 40 km to as high as 110 km. This
incorporates the upper stratosphere , through the entire mesosphere. This includes the
lower end of the ionosphere, and specifically the D and E layers. See Table 1 below for
the heights of each of the ionospheric layers. Figure 2 [Iribarne and Cho, 1980] will
provide a quick way of placing each of these layers within the context of the entire
atmosphere.
Select here to see Figure
2 at full size
most common species . The key question is as follows: what happens chemically when
we add beta particles (high energy electrons, such as with a sprite) to this chemical
soup? While there are so many potential reactions to consider, look at the following
F2 from 250 to 500km O2+,N2+
reactions from Iribrane [1980]. Note that e represents a single electron, and M is any
kind of "third body" atom or molecule.
(I) O+
+ e + M ==> O + M*
- - - - - - - - - - - - - - - - - - - - -
- -
(II) O2+
+ e ==> O + O
- - - - - - - - - - - - - - - - - - - - -
- -
(III) NO+ + e ==> N + O
- - - - - - - - - - - - - - - - - - - - -
- -
(IV) N2+
+ e ==> N + N
- - - - - - - - - - - - - - - - - - - - -
- -
(V) O2
+ e + M ==> O2- + M*
- - - - - - - - - - - - - - - - - - - - -
- -
(VI) O3+O
==> 2O2
Note in particular equations (II), (III), and (IV). Here, we have dissociative
recombination - destroying molecular ions to produce neutral atomic species. Equation (I)
also clearly destroys ions as well. In the case of (I), the species M has energy added to it
- hence the "*" symbol to denote the increased energy. This added energy is due to the
recombination of the O+ ion with the electron. Equation (V) is a special case of the D
region in particular. The important item to note is the obvious impact that beta particles
have on ionospheric chemistry.
As a final comment on this issue, look at equation (VI). You may note that there
are no beta particles in this equation. Further, ozone is the first term. Note that this
equation provides one of the means for stratospheric ozone to be destroyed. Also, keep in
mind that the first five equations are still valid reactions even in the mid-stratosphere, so
that a sprite or blue jet - thrusting numerous beta particles upward - is causing neutral
atomic oxygen to be produced. This, in turn, affects ozone concentration. While there is
no intention (by the author) to cause a sudden concern for the "threat" to the ozone layer
that is imposed by a C-I event, it is worthwhile to note that there could be a series of
complex chemical reactions taking place that have a significant impact on middle and
upper atmospheric chemistry.
2. Impact on High Altitude Aircraft and Spacecraft
As was discussed during the Roussel-Dupre' theories, C-I events are associated
with gamma radiation, in addition to natural beta particle radiation. Neither of these
influences would be considered good for aircraft pilots/passengers, or astronauts
(although the astronaut is clearly better protected). Also , gamma radiation tends to cause
problems for highly sensitive electronic equipment. Solar events of a similar nature have,
in the past, caused serious problems for orbiting satellites - going so far as to reset key
pieces of equipment without human controllers even realizing it. It would not be difficult
to calculate the theoretical electrical field for an aircraft passing through such a system,
thus enabling planners to assess the potential difference to which aircraft, personnel, and
equipment would be subjected (See Appendix A for such an example).
3. Insight into electrical charge distribution / interaction in the Atmosphere
Much has already been written regarding the electrical charge distribution
associated with sprites. The work of Wilson and Roussel-Dupre' provide ample
motivation in this arena. As we continue our study of C-I events, we will undoubtedly
observe either the confirmation of Wilson's theories, or we may see new truths yet.
C. Current Observation Efforts
1. Video
a) Ground based observations
Some of the most compelling observations and discoveries have been made at a
NASA funded laboratory at the Yucca Ridge Field Station (YRFS) in northern Colorado.
This location is situated on a high mountain top, and provides excellent coverage for a
large area. Here, special Low Light Television (LLTV) imagery has been recorded.
LLTV normally consists of a special photocathode multiplier tube which enhances the
light emitted. Incoming photons are converted to electrons, then the energy of these
electrons is increased, then converted back into higher energy photons - thus yielding a
brighter image. Usually, LLTV equipment is configured with special optical filters. These
filters take advantage of energy emitted from a variety of atmospheric gaseous
constituents, such as N2, O2, etc. In our work, we primarily looked at imagery using a
filter for energy emitted at 7774 angstroms - a common frequency for atomic oxygen,
otherwise referred to as OI (which refers simply to conventional atomic oxygen. If this
were ionized oxygen {one electron missing}, then the symbol would be OII. Ionized
oxygen {two electrons missing}would be OIII, and so on [Harrison, 1969]). In addition
to conventional video work, spectrographic data has been collected, and has already
answered some of the questions that have been haunting researchers for some time
[Mende et al , 1995]. This work has been accomplished by the use of the slit
spectrograph. Chief among these discoveries has been the confirmation of the source of
the red color associated with sprites: electron collisions with N2 molecules in the
stratosphere and mesosphere.
b) Aircraft measurements
Researchers at the University of Alaska have also been conducting a variety of
observation efforts using LLTV and other special video and spectrographic equipment.
What is distinctive about this effort is the ability of two different aircraft - each observing
simultaneously - to accurately pinpoint the location of C-I events. By maintaining course
and speed relative to each other properly, each can be on a path normal to each other,
allowing for easy triangulation. Also, knowledge of azimuth and elevation - as well as
plane altitude - allow for more precise determinations of C-I altitude - an important item
to know in order to understand the overall interaction. [Sentman et al, 1994]
c) Spacecraft imagery / data
Even before the Franz paper of 1990, a number of groups have been actively
engaged in C-I event observations, analysis, and research in space. One actively
interested party is the NASA Marshall Space Flight Center, where work from Boeck et al
[1995], and the prolific writing of Vaughan [1982, 1989, 1992, 1995] have provided
much insight. Many of the NASA efforts have focused on visual observations from the
Space Shuttle.
Other promising data has come from the Compton Gamma Ray Observatory, in
orbit since 1991. The Compton Observatory has unexpectedly been bombarded by
gamma radiation from the planet surface. Referred to as "Terrestrial Gamma-ray Flashes
or (TGF's)", these are short (in the order of milliseconds) blasts of gamma ray radiation
that are clearly related to intense thunderstorm activity [Horsack, 1996]. C-I events, and
blue jets in particular, are suspected as possibly related to this type of event [Zimmer,
1995].
2. VLF detection
During the last two years, exciting work has been completed with respect to the
identification of sprites using a Very Low Frequency (VLF - 5 to 50 kHz ) omni-
directional signal receiver. Here, standard VLF radio waves are transmitted into the
ionosphere. These waves can travel long distances at this altitude, due to the
electromagnetic wave ducting that occurs in the ionosphere. The occurrence of a red
sprite has been shown to correlate to disturbances in the received VLF wave. The wave
guide is scattered a great deal by the C-I, causing a distinctive perturbation pattern for
VLF signals. Using only a single VLF receiver, it has been demonstrated that sprites can
be located within 100 km, and 90 degrees in azimuth. Adding a network of such receivers
can significantly improve the accuracy of these measurements. [Dowden et al, 1996].
Initial work has shown that it is possible to discern both sprites and elves in the VLF
signal [Dowden et al, 1995]
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Contact: Paul
McCrone - PaulJMC@aol.com