Sprite Observations at Creighton

II. OBSERVATION EFFORTS AT CREIGHTON UNIVERSITY

our intent was to operate solely from a single location atop the building of the

Atmospheric Science Department in downtown Omaha. This location offered a good view

of potential thunderstorms from the Northwest , through the North, and through the East.

However, due to light effects from the greater Omaha Metropolitan area, we were unable

to adequately visualize distant thunderstorm imagery - we would be able to see only the

most brilliant flashes from C-I events. Later, a series of new sites was chosen. Our intent

was to find a location that had a ready electric power supply, as little city lighting as

available, and decent elevation, such as on a hill or tall building. I will continue with a

summary of the details surrounding our efforts.

A. Equipment used

of these cameras (and some other equipment) were provided by Los Alamos National

Laboratory for the purpose of recording C-I imagery. These cameras have a rotating

"filter wheel" - a wheel containing several different opaque optical filters. Each filter is

designed to only detect electromagnetic radiation at a certain wavelength. The primary

camera has six optical filters, with spectral characteristics as shown in Table 2. The basic

camera can be seen in figure 3. Our primary efforts focused around filter (or channel)

number 5, at 7774 angstroms - one of the atomic oxygen lines. We chose this channel

because it provided the best looking imagery when compared to the other five filters and

their respective imagery. The term "best looking" should be interpreted to mean best gray

shade contrast - the camera will eventually produce a digitized image from

Table 2:  XYBION IMC-201 Camera Spectral Characteristics 

the video input in an 8-bit graphic image (256 gray shades). Each of these images is the

exact same resolution - 756 (Horizontal) pixels by 485 (Vertical) pixels, so the only real

issue was the determination of adequate contrast. Channel 5 won easily: after trying to

image an active thunderstorm in late May 96, we noted that channels 1,2,3,4, and 6 were

not able to provide intelligible imagery. Thus, the decision was made to use this channel

on a consistent basis - in order to have some form of "baselined" imagery.

plate of the IMC-201, and the resultant imagery was sent to a VCR/TV combination .

The VCR recorded the imagery in standard VHS format, for post-analysis. Attempts were

made to record rough geographic and navigational details. With the camera mounted on a

tripod, the camera field of view (FOV) , azimuth (with respect magnetic north), elevation,

and rough position were recorded. Geographic notes were taken of surrounding

landmarks for future reference. Also, notes were taken on the time. No attempt was made

to correct the camera's internal clock, but a time difference of 7 minutes, 32 seconds was

noted (the system time, for example, may say 04:07:32 GMT when it is really 04:00:00

GMT). The primary difficulty we experienced lay in the initial set-up of the entire

camera/TV/VCR, which required some re-engineering.

VCR machine. This high quality machine gave us the chance to look at our imagery in a

slower manner, frame by frame. Our ability to make observations of the recorded sprite

imagery was improved greatly.

B. Method of Employment

the Atmospheric Science Department in downtown Omaha, and due to unfavorable

lighting conditions in downtown Omaha, our vantage point had limited utility. We set out

to find a series of different locations that would provide a better view with few

background lights from towns and cities, and a wide azimuthal look angle. One such

place was found in Bellevue, Nebraska, just south of Omaha. Our efforts consisted of

carrying the camera, and associated equipment from Creighton to the Bellevue Sprite

Observation Site (BSOS) , where we set up the camera on a hill located near 41o09'30"N

and 095o57'20"W (position only accurate to within +/- 10") at over 1110 feet above

mean sea level. At this location, we had a good view from the south-southwest, through

the west, to the north-northwest, with comparatively less city light interference from the

surrounding country towns. See Figure 4 for a view of the site. Our view to the west

southwest (azimuth: 240o magnetic) was especially good- there were no significant lights

to cause concern - just some repetitive aviation beacons, which were relatively low on the

horizon and easy to distinguish from other lights.



                                                   Figure 4

C. Results

imagery was captured in June. No attempt will be made to provide an exhaustive

discussion of these events, just an overview of two nights in particular: the 15th and 22nd

of August, 1996. I will discuss these two because they were clearly the most prolific

nights all summer. Of the two nights, the 15th was by far the most remarkable display of

all, thus some care will be taken with this night.

1. Multiple events from August 15, 1996

a) Synoptic Weather Scenario

        On 15 August 1996, 0600 GMT (Greenwich Mean Time), a low pressure system

was located over northern Michigan, with a strong cold front extending through Illinois,

northern Missouri, and southern Nebraska. In advance of this front, a series of

thunderstorms were developing. Cells were observed from Missouri, and thence into

southern Nebraska and Kansas (see figure 5 - provided by the National Weather Service) .


By 0400Z (0000 Eastern Time), a  vigorous group of cells had started in the eastern

Colorado, and another group was losing strength in central Kansas. This is seen in the 0235Z

Radar Summary [figure 6] from the National Weather Service.  and 7).

The cells that passed through the BSOS were all but dissipated by this time.



 


                                                           FIGURE 7

b) Location / Description of local conditions

front, a thunderstorm complex had passed through the vicinity of the BSOS three to four

hours prior (see Figure 6) . The area had rapidly cleared out, and there were stars in the

sky down to the horizon. No obscuration was evident at all. Noticing this improvement in

local visibility, we started reviewing radar summaries and satellite imagery. In addition,

we obtained lightning summaries from the National Lightning Detection Network

(NLDN) courtesy of the Air Force Global Weather Center   (AFGWC - Now referred

to as the HQ Air Force Weather Agency). AFGWC also provided us with a look at imagery

from the Defense Meteorological Satellite Program - their Operational Linescan (OLS)

system can see lightning flashes at night. This gave us an excellent feel for the location of

actual lightning. Unfortunately, due to Department od Defense data restrictions, we are unable

to show that imagery here on this WWW site currently (We are working to obtain permission

from the government to use and display this data).

        At this point, we heeded the advice of Heavner [1995], who advised only making

attempts at C-I observations with a totally clear sky and distant (75-600 km)

thunderstorms, and made haste to the BSOS. The IMC-201 camera and related equipment

were set up, and directed towards roughly 240 degrees magnetic (actual measurements of

the azimuth were taken after the fact). A diligent watch was kept on the TV monitor to

see if a C-I event occurred. The system clock on the IMC-201 was not reset, but the error

was noted: 7 minutes, 32 seconds too fast when compared to GMT. All references to time

will henceforth be made based on system time to avoid confusion, but please keep the

actual time difference in mind.

c) Imagery / Information

        This night was clearly the most prolific of them all. Only a few of these images

will be provided here: please see the following World Wide Web address for more

information: http://thor.creighton.edu/sprites.
 

        We will only show a few of the more interesting images here. More is available via the

internet at the above site.
 

        The first event for the purposes of this summary occurred at 05:16:12.22 (Figures

10 and 11) according to the IMC clock. This was one of the more spectacular sprite

clusters of the night. It lasted for several video frames. Note the classic sprite structure in

figure 10 - well defined head and hair. The tendrils are not as clearly defined, which is

explained by the fact that we were quite some distance from the complex - 600 km away,

based on correlated radar, satellite, and azimuthal data. The altitude of the tallest sprite

was 85.5 km based on elevation derived from the video monitor (Please see Appendix B

for details on altitude calculations). Note the stars in the background of this image: the

sky was clear enough to see such distant sprites. Note the large lights to the extreme left

and right of the image, on the horizon. A persistent feature in all of these images, these

lights are simply housing complexes, both in Papillion, Nebraska, and in the Capehart

section of Bellevue, Nebraska.



Click here to see this image, FIGURE 10 , full size

       Full Size: FIGURE 11

        The next event occurred at 05:21:04.73. Visible here, in Figure 12, is a sprite

head, with hair, and a strange looking disk-shaped feature above the hair. This is clearly

the elve-type event described by Lyons and Dowden. Comparison with Lyons [1994]

work tends to confirm this. The estimated altitude of the elve was 84.4 km, and the sprite

was 73.0 km (approximately).

        At 05:22:36.00 (see Figure 13), a line or group of sprites was observed well above

the horizon. This was the best example of a line of sprites from the whole summer. For

comparison, note the radio beacon to the right on the image (just above the "36"). Here,

the estimated altitude of this line was 70.4 km.


           Full Size: FIGURE 12

            Full Size: FIGURE 13

        After this time, a final, striking display came to finalize the observations for the

15th of August. At 05:25:12.53, another strange, amorphous, disc-shaped glow appeared

a few degrees above the horizon, as in figure 14. This is the second - and best - example

of an elve on this night. A mere instant later, an brilliant, seemingly explosive-looking

sprite (figure 15) followed the elve. The time interval is so short (less than 1/100th of a

second) that the video time counter had no opportunity to update, thus the same time is

displayed. The estimated altitude of the elve was 84.4 km. I made no attempt to compute

the sprite, because it is somewhat difficult to distinguish the bottom of the elve from the

top of the sprite.



                 Full Size: FIGURE 14 

           Full Size:  FIGURE 15

2. Events from August 22, 1996

a) Synoptic Weather Scenario

        On 22 August 1996, 0000 GMT (Greenwich Mean Time), a low pressure system

was located over the eastern Dakotas, with a cold front extending through South Dakota,

western Nebraska and Colorado (see figure 16). A series of thunderstorms was

developing along the frontal boundary. Cells were observed from eastern Colorado, and

thence into Kansas and southwestern Nebraska (see figure 17) .


 

b) Location / Description of local conditions

hint of the eastward cirrus anvil from the thunderstorms in western Nebraska was slowly

becoming visible. Having reviewed the radar summaries and satellite imagery, we made

the decision to quickly set up the camera and equipment before the anvil came too far

east. After the fact, we obtained NLDN imagery courtesy of the Air Force Global

Weather Center (AFGWC).

equipment were set up, and directed towards roughly 255 degrees magnetic (actual

measurements of the azimuth were taken after the fact). A somewhat less diligent watch

was kept on the TV monitor on this occasion: other efforts to record geographic

information was conducted while the video camera was playing. Also, a conventional

video camera was placed. alongside the IMC-201 in hopes of capturing a C-I event using

normal imagery.

c) Imagery / Information

distorted due to a minor problem with the VCR recorder, but the actual sprite head is

clearly identifiable. There are two other, smaller sprite heads on each side of the main

head. This sprite represents the typical sprite cluster observed during our observations.

Note the greater elevation at which this sprite was observed: the parent thunderstorm

complex was 412 km away - much closer than the August 15 event. The estimated

altitude for this event was 94.3 km.

diffuse; we almost didn't see this event until we took a careful look in the post-analysis

phase. In addition to being diffuse, is not very bright, making it even harder to detect.

This demonstrates that there may be many more sprites than expected, since some, at

least, will be essentially sub-visual. The estimated altitude for this event is 105.6 km.



                        FIGURE 18


                       FIGURE 19

3. Miscellaneous

somewhat uncertain as to whether or nor these events are correctly classified as one of the

previously discussed C-I events. We discovered that we are in desperate need of some

type of cross validation check to provide a more reliable way of assessing the "identity"

of various atmospheric phenomena - many of the possible C-I events in question may

very well be simple cases of lightning cloud flash/reflection. The cases described above

(15 and 22 August 1996) were clear cut, given the clarity of the sky at the time of those

observations.

III. LESSONS LEARNED / AREAS FOR FUTURE IMPROVEMENT

A. Multispectral Data

events. As such, we only conducted a limited effort in terms of the range of possible

imagery that could have been collected. Here, we will summarize the different types of

imagery that are recommended for further exploitation, in addition to brief explanation

regarding the utility of each.

1. Conventional visual data

consistently monitor the sky in a normal visual sense. It would have helped a great deal to

have conventional visual imagery - such as from a normal video system- in order to

compare with imagery on the 7774 angstrom filter. In addition to assisting in the more

accurate identification of some events, if would have more readily screened out situations

like cloud flash, etc. Along with this, none of the events noted on either the 15th or 22nd

of August were seen with the naked eye. This was due, primarily, to the need to watch the

equipment of a near continuous basis.

2. Low-light Video

performance from this equipment. After learning how the equipment worked, we had

difficulties making connections work with existing components. We were able to re-

engineer the equipment successfully, but a better designed LLTV camera is needed to

optimize our ability to observe sprites and their structure. Also, the resolution and

focusing power of the video lens left much to be desired. One exciting possibility is the

utilization of modified astronomical equipment (telescopes, etc.). This kind of work is

being conducted already at the Lagmuir Laboratory in New Mexico. Exciting imagery has

resulted from this effort. We will need to find a way to record such telescopic imagery for

archival purposes.

3. Multi-filter imagery

morphology and genesis is the ability to observe many different types of imagery at

different spectral increments. Part of this process needs to be the utilization of the six

different filter wheels on the XYBION IMC-201 camera. Again, since we were operating

with only one camera in the field, we did not have the flexibility to look at different

wavelengths (with a rotating filter wheel) without sacrificing the quality and consistency

of having the 7774 angstrom data. In the future, we will need two LLTV cameras

operating in conjunction: one to record 7774 angstrom imagery, and the other to record

multi-filter imagery. This will certainly aid in future analysis.

4. Image analysis

digitally analyze imagery. Unfortunately, we were unable to make this package run

correctly, thus forcing us to scrap that part of the project. We need to research a manner

in which to capture imagery in a digital fashion and perform image analysis. This function

will aid us in discriminating between real C-I events and cloud-flash (and other false C-I)

events.

B. Utility of Digital Radar data and NLDN information

cases. Without the radar data, we would have been unable to pinpoint the location of

possible sprite producing thunderstorms. We were only able to save synoptic scale radar

summaries from the WSI Corporation. We need the ability to quickly obtain and store

near real time radar data for post-analysis.

Similarly, the NLDN products were useful, but only in a limited fashion. Like the

radar data, the NLDN was able to provide an advanced warning with regard to the

possibility of C-I genesis. It would be highly valuable to obtain and store this data at a

higher temporal and spatial resolution. The data we had access to from AFGWC was of

limited value: the imagery consisted of 3 hourly lightning summaries, leaving us unable to

correlate the C-I events we observed with actual lightning strokes. This is, I believe, a key

in understanding the interactive roles played by thunderstorms and the upper atmosphere.

C. VLF data correlation

the changes caused by C-I events in the ionosphere. We now have such a device here at

Creighton, and future efforts will incorporate the VLF signal with the C-I imagery.

D. Additional resources needed

that, there are some realistic kinds of resources that are needed that would significantly

improve our C-I observation efforts.

1. Personnel

such as on the 15th of August. Had more personnel been available to handle various details

of camera setup, then possibly more imagery would have been captured last summer. In

addition to this point, keep in mind that more people will be needed to handle multiple

cameras and other equipment, if these efforts are to expand in any of the ways

prescribed above.

2. Power supply

outset, prior coordination had to be conducted to ensure the BSOS and the other sites

would have power available for the equipment. Though it may seem a rather banal issue,

it is a significant limiting factor in planning where to go for potential observation sites.

Clearly, access to a portable generator of some type would aid in this effort, but short of

this, an active effort must be put forth to solicit any organization (or private citizen) to

allow access to an electric power supply. Bear in mind also that much of the equipment

(the IMC-201 camera, in particular) is a sensitive to power anomalies.

3. Location and Number of Observation Sites

and others, as mentioned previously, took advantage of mountain ranges or aircraft to

observe C-I phenomena. It is not likely that most future observers will have an aircraft

available, so a careful search for site(s) on a high point, such as a hill / mountain is

needed. A given site is particularly desirable if it has a full view through 360 degrees of

azimuth, though most sites will not likely have such a helpful view. Therefore, we

developed a number of sites using the homes of private citizens (after gaining prior

permission) to observe as far as possible through 360 degrees. Any team trying to

conduct C-I observations needs to have this part of the problem researched before any

cameras are set up for recording.

E. Final thoughts on C-I Mechanisms.

Based on all the hypotheses presented, and the observational information

available, here is an overall hypothesis that we feel may explain sprites, in particular. We

have no distinct hypothesis on blue jets yet, and we need to obtain more information

regarding elves (thought the EMP theory seems plausible).

The entire consideration of sprites and blue jets requires us to take a step back.

Looking at figure 20, consider the overall charge distribution associated with the

cloud/atmosphere complex.



                Full Size: Figure 20

The charges are first noticed in the ionosphere, where a  variety of ions/electrons are

available in abundance. Next, the top of the thunderstorm  possesses an overall positive

charge, while the lower portion of the cloud (around 6 km) has a strong NEGATIVE charge.

Below the 0 deg C isotherm, we will observe a small  concentration of positive charge, and

the earth's surface will also possess a net positive charge.

careful balance of charges throughout the vertical. A KEY ASSUMPTION in this

argument is that the entire distribution of charges maintains an overall balance,

preventing a flow of charge in any direction. Of course, this is not a completely accurate

assumption: charge is being exchanged dynamically between the ionosphere and the

lower parts of the atmosphere. This assumption is only valid in the sense of time scale: in

other words, in the time span of a second or less, this statement is more or less acceptable.

Now, let us suppose that a discharge takes place between the negatively charged

region at 6 km to the ground. Consider, then, that the aforementioned balance of charge

has been violated, thus enhancing the electric field gradient between the top of the cloud

and the ionosphere relative to the lower part of this electric dipole complex. This allows

for a possible discharge to take place between the positively charged top of the cloud and

the ionosphere. It would be reasonable to argue that a discharge would take place in this

manner, since the electrical potential difference between cloud/ionosphere area would be

the same, while the potential difference between the cloud top and the lower part of the

cloud would be significantly decreased. This, then, is a possible scenario that we might

use to explain the appearance of sprites. Of course, the potential difference between the

cloud and will need to be high enough to cause dielectric breakdown of the intermediary

atmospheric gas - a distinctly feasible possibility. This hypothesis is inspired, in part,

from the work of Wilson [1925, 1956], though modified slightly.

We were able to store limited types of imagery for a variety of C-I phenomena. We

observed the events at different times of the year, and under somewhat differing

conditions. Specifically, we observed obvious Red Sprites, (possible) blue jets, and

certainly at least two instances of elves. We were able to compute rough altitudes for the

tops of these flashes, and correlated them with distinct meteorological phenomena.

There is now a clearer sense of how to capture this imagery, and an improved plan

of attack for the systematic pursuit of these strange "lights-in-the-sky". We know what

conditions are best suited for sprite observation. We have a good idea of the kind of new

data and imagery that will be needed in the future to better understand these phenomena.

Finally, we have a working hypothesis to explore for future validation with

respect to the formation of these C-I events. As data collection efforts improve, we hope

to refine our understanding of these atmospheric events.

 Contact: Paul McCrone - PaulJMC@aol.com
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