Sprite observations from the space shuttle during the Mediterranean Israeli dust experiment (MEIDEX)

Abstract

The Mediterranean Israeli dust experiment (MEIDEX) flew on-board the space shuttle in winter 2003, in a 39°-inclination orbit for 16 days, passing over the major thunderstorm regions on Earth. The primary science instrument of the MEIDEX payload is a Xybion IMC-201 image-intensified radiometric camera with six narrow band filters, boresighted with a wide-FOV color video camera. During the nightside of the orbit there will be dedicated observations toward the Earth's limb above areas of active thunderstorms, in an effort to image transient luminous events (TLEs) from space. Optical observations from space will be conducted with the filter that matches the observed wide peak centered at that typifies red sprites, and also with the 380 and filters for recording blue jets. Observations will consist of a continuous recording of the Earth's limb, from the direction of the dusk terminator towards the nightside. Areas of high convective activity will be forecasted and uplinked to the crew before the observation. The astronaut will direct the camera toward areas with lightning activity, observed visually through the windows and on monitors in the crew cabin. Simultaneously with the optical observations from space, dedicated ground measurements will be conducted on a global scale. Two field sites in the Negev Desert in Israel will be used to collect electromagnetic data in the ELF and VLF frequency range. Additional ground stations in Germany, Hungary, USA, Antarctica, Chile, South Africa, Australia, Taiwan and Japan will also record Schumann resonance and VLF signals. The coordinated measurements from various locations on Earth and from space will enable us to triangulate the location and determine the polarity and charge moment of the parent lightning of the optically observed TLEs. The success of the campaign will further clarify the geographical distribution of Sprites, Elves and Jets.

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Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642
www.elsevier.com/locate/jastp
Sprite observations from the space shuttle during the
Mediterranean Israeli dust experiment (MEIDEX)
Yoav Yair∗;1, Colin Price, Zev Levin, Joachim Joseph, Peter Israelevitch, Adam Devir,
Meir Moalem2, Baruch Ziv1, Mustafa Asfur
Department of Geophysics and Planetary Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
This research was made possible by the devotion and enthusiasm of the seven astronauts of the space shuttle Columbia in mission STS-107
Abstract
The Mediterranean Israeli dust experiment (MEIDEX) ?ew on-board the space shuttle in winter 2003, in a 39◦-inclination
orbit for 16 days, passing over the major thunderstorm regions on Earth. The primary science instrument of the MEIDEX
payload is a Xybion IMC-201 image-intensi?ed radiometric camera with six narrow band ?lters, boresighted with a wide-FOV
color video camera. During the nightside of the orbit there will be dedicated observations toward the Earth’s limb above areas
of active thunderstorms, in an e?ort to image transient luminous events (TLEs) from space. Optical observations from space
will be conducted with the 665 nm ?lter that matches the observed wide peak centered at 670 nm that typi?es red sprites, and
also with the 380 and 470 nm ?lters for recording blue jets. Observations will consist of a continuous recording of the Earth’s
limb, from the direction of the dusk terminator towards the nightside. Areas of high convective activity will be forecasted and
uplinked to the crew before the observation. The astronaut will direct the camera toward areas with lightning activity, observed
visually through the windows and on monitors in the crew cabin. Simultaneously with the optical observations from space,
dedicated ground measurements will be conducted on a global scale. Two ?eld sites in the Negev Desert in Israel will be
used to collect electromagnetic data in the ELF and VLF frequency range. Additional ground stations in Germany, Hungary,
USA, Antarctica, Chile, South Africa, Australia, Taiwan and Japan will also record Schumann resonance and VLF signals.
The coordinated measurements from various locations on Earth and from space will enable us to triangulate the location and
determine the polarity and charge moment of the parent lightning of the optically observed TLEs. The success of the campaign
will further clarify the geographical distribution of Sprites, Elves and Jets.
c ? 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Sprites; Transient luminous events; Space-based observations
1. Introduction
Transient luminous events (TLE) constitute a wide
range of optical and electromagnetic phenomena, which
occur from the tops of active thunderclouds to heights of
∗Corresponding author. Research and Projects Department, The
Open University of Israel, 16 Klausner St. Ramat Aviv, Tel-Aviv,
Israel.
1Also with: Department of Natural and Life Sciences, The Open
University of Israel, 16 Klausner St., Ramat Aviv, Tel-Aviv 61394,
Israel.
2IAF, Space Branch.
40–90 km. Although rather common and even marginally
visible to the naked eye under the right viewing condi-
tions, TLE were not observed or accounted for, with the
exception of reports by high ?ying pilots (Vaughan and
Vonnegut, 1988). Nicknamed “red sprites”, “elves” and
“blue jets”, these discharges were ?rst reported by Franz
et al. (1990). Since their discovery they have been pho-
tographed from ground stations (Lyons, 1994), aircraft
(Sentman and Wescott, 1993) and the space shuttle (Boeck
et al., 1995). Several theories have been suggested to ac-
count for the generation of TLE (see review in Sentman
and Wescott, 1993) but none seems to be conclusive and
to engulf the variety of phenomena observed (Huang et al.,
1364-6826/03/$ - see front matter c ? 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S1364-6826(02)00332-2

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636Y. Yair et al./Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642
1999). In the last couple of years, the importance of TLE to
the global electrical circuit and to the chemistry of the upper
atmosphere (including, possibly, the ozone layer) has been
recognized. Studies focus on the relation between the lower
thundercloud and its activity to sprite formation (Lyons et
al., 1999) and on the morphology of sprites (Moudry et al.,
1999; Gerken et al., 2000). Others focus on the relation be-
tween sprites, Q-bursts and positive ground ?ashes (PGF)
(Boccippio et al., 1995), the relationship between the par-
ent lightning and the resultant TLE (Barrington-Leigh and
Inan, 2000), and on the ionization processes associated with
sprites (Armstrong et al., 2000). Lately, a unique discharge
was observed by Pasko et al. (2002) which showed a direct
link between the top of the thunderstorm and the lower
ionosphere. This event emphasizes the importance of the
role the TLE play in the global electrical circuit.
The ?rst images of TLE from space were obtained dur-
ing the mesoscale lightning experiment (MLE, Vonnegut
et al., 1985) that was conducted between 1989 and 1991,
by using the space shuttle’s payload-bay cameras (Boeck
et al., 1998). These are standard low-light monochrome
video cameras that are usually operated for monitoring ac-
tivities within the shuttle cargo bay, and are not calibrated in
any accurate way. The ?ight controllers at NASA Johnson
Space Center (JSC) targeted the night limb and observed
lightning activity all over the eclipse side of the earth, with-
out any crew involvement. An analysis of hundreds of hours
of observations yielded 17 events of vertical ?ashes from
cloud top toward the ionosphere (Boeck et al., 1994). These
events were approximately geo-located by using stars and
ground lights, and were found to occur over Africa, South
America, USA, Australia, Borneo and the Paci?c Ocean.
Termed “stratospheric lightning”, these events had a mean
duration of 0:13 s (3.8 video frames) and occurred 0:358 s
after the visible (parent) lightning in the cloud deck be-
low. The oblique view of the illumination inside the cloud
(caused by a strong lightning ?ash) from the shuttle pro-
vided the ?rst unambiguous optical link between the parent
stroke and the subsequent TLE.
While innovative and unique, these space-based observa-
tions were rather limited and lacked the accuracy needed to
determine the optical energy, spectral properties and relation
to the parent storm. MEIDEX o?ers a signi?cant improve-
ment for space-based TLE observations, using an absolutely
calibrated camera combined with ground VLF/ELF-based
geo-locating systems.
2. Mission description
The Mediterranean Israeli dust experiment (MEIDEX)
was conducted from the space shuttle Columbia, in
its STS-107 research ?ight in winter 2003. It fo-
cused on the study of desert dust transport from the
Sahara, its sources, sinks and climatic e?ects. MEIDEX is
a daytime experiment and is based on the remote sensing
of back-scattered light in the NIR–VIS–UV from aerosol
particles. It was operated by the shuttle crew, with the con-
trol of a science team located at the Payload Control and
Command Center in NASA’s GSFC (in Greenbelt, MD).
An instrumented airplane conducted simultaneous in situ
measurements of the aerosols beneath, or in the vicinity of,
the shuttle track. The mission lasted for 16 days, in a 39◦in-
clination orbit at an altitude of 150 NM (280 km). In order
to increase the scienti?c yield of the payload, it was sug-
gested to add nighttime limb observations on the emissions
from TLE. The orbit enabled the coverage of most of the
lightning generating regions on Earth and greatly enhanced
the ability to create a global picture of sprite activity.
3. The MEIDEX payload
The main science instrument in MEIDEX is a Xybion
radiometric camera model IMC-201, equipped with a
rotating ?lter wheel with six narrow-band ?lters. The wave-
lengths of these ?lters were chosen so as to coincide with
those that are used by the total ozone measuring spec-
trometer (TOMS) and the moderate resolution imaging
spectroradiometer (MODIS) instrument on-board the Terra
satellite for aerosol observations. The central wavelengths
are 340, 380, 470, 555, 665 and 860 nm. The respective
full-width at half-maximum (FWHM) of these ?lters are 4,
4, 30, 30, 50 and 40 nm, respectively. Xybion cameras of
other models were used for ground measurements of sprites
by Lyons (1996) and Lyons et al. (1999) and in the Leonid
MAC campaign for imaging meteors, when they serendip-
itously recorded sprites and elves (Yano et al., 2001).
Spectral data from earlier studies show that ?ve out of six
wavelengths chosen for MEIDEX are good for sprite ob-
servations (Hampton et al., 1996; Armstrong et al., 1999).
Table 1 summarizes the key emission lines that are close to
the MEIDEX ?lters.
The camera is equipped with a 50 mm Hamamatsu UV
lens, adjusted with a special ba?e to mitigate stray light
fromenteringtheoptics.TheFOVofthecameraisrectangu-
lar, 10:76◦vertical and 14:04◦horizontal (diagonal 17:86◦).
The CCD has 486 over 704 pixels, where each pixel corre-
sponds to 1:365×10−7s. When observing the limb, the spa-
tial resolution will be at least 0:66 km (H) × 0:73 km (V).
Any event that occurs within the ?eld of view and closer
to the shuttle than the limb will be resolved even better.
The expected resolution of the optical observations provides
the ability to map the morphology of various types of TLE
(C-sprites, V-sprites, Beads, Fuzz, Fingers, carrots, etc.) that
was revealed during airborne and ground-based telescopic
observations (Gerken et al., 2000; Lyons et al., 2000). The
video format of the IMC-201 camera is NTSC which means
that it produces its video output at 30 Hz (33:3 ms=frame).
The camera will be operated in two modes: (a) A “run-
ning mode”, where the ?lters are sequentially changed at the
frame rate (33:3 ms=?lter). This means that the complete

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Y. Yair et al./Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642637
Table 1
The MEIDEX camera ?lters and key emission lines (in nm)
MEIDEX ?lter (nm)Emission line (nm)Type
380
470
555
665
860
380.5
470.9
557.7
662.4
857.4
N2(2P) O-2
N2+(1N) 2-O
O(I) Airglow
N2(1P) 6-3
N2(1P) 8-8
six-?lter sequence will take 200 ms. The exposure time with
each ?lter is automatically increased from 50 ns to 4 ms
(in 50 ns increments) until the maximal light level in each
frame has reached a predetermined level. Due to the low
light levels emitted by TLE it seems inevitable that the max-
imal exposure time of 4 ms will be attained for all ?lters.
(b) A “locked mode”: in this mode the ?lter wheel is locked
to one speci?c ?lter and all images are taken with this ?lter.
The exposure time for each ?lter is automatically increased
but now from 50 ns to 33:3 ms (in 50 ns increments) until
the maximal light level in each frame reaches a predeter-
mined level.
Boresighted with the Xybion is a second, low-light level
color video camera with a wide FOV of ±60◦. This cam-
era will serve as a view?nder for the crew and will assist
in real-time observations. Both cameras are mounted on a
single-axis gimbal which has a ±22◦cross-track scanning
ability. The data from both cameras are routed to digital
recording devices within the payload and the crew-cabin,
and will also be downlinked in real time to the ground for
quick analysis. The entire payload is housed inside a 5 ft3
canister,?lledwithdrynitrogenandsealedwitha16??coated
quartz window. The canister will be mounted on a cross-bay
structure in the aft part of the shuttle cargo bay (Fig. 1), as
part of a large array of experiments named Freestar (http://
sspp.gsfc.nasa.gov/hh/freestar/experiments page.html).
Upper End Plate w/
Quartz Window
Gimbal Motor
& Worm Drive
Worm Gear
SEKAI WFOV
Camera
Lower End Plate
Lens and
Baffle
Xybion Camera
Avionics Mount Plate
w/ electronics
Fig. 1. A schematic drawing of the MEIDEX payload.
4. Camera calibration
The major advantage of MEIDEX over the early shuttle
observations of sprites and elves is due to the use of an ab-
solutely calibrated instrument. The radiometric calibration
of the Xybion IMC-201 camera was performed in the Lab-
oratory for Atmospheres at Goddard Space Flight Center,
MD. The basic procedure was to get an absolute radiomet-
ric calibration against a calibrated source. This was done by
measuring the constant spectral radiance N (W=sr=cm2=nm)
of an aperture of an integrating sphere with di?erent expo-
sure times, measured in millisecond, for all the six ?lters
that are mounted on the ?lter wheel. The product (Nt) has an
almost linear correlation with the video signal of the aper-
ture expressed in gray-level units [GL0]. The polynomial ?t-
Nt = f3(GL0) shows that such a ?t has a residuals ¡1%
over most of the dynamic range of the camera. This was
shown for all six ?lters. By normalizing this polynomial de-
pendence for all ?lters, one can show that the radiometric
response of the camera is the same for all ?lters (Fig. 2).
Prolonged activation during the thermal quali?cation tests
of the payload has shown that the CCD temperature changes
with time. In addition, the orbital attitudes of the space shut-
tle will change continually throughout the mission, and will
put external thermal constraints on the payload. This neces-
sitated a calibration of the temperature e?ect on the absolute
calibration. The response of the camera was measured and
enables us to obtain a correction factor as a function of the
temperature. The correction slope varies between 0:1%=◦C
and 0:3%=◦C (Fig. 3).
Lastly, a ?at-?eld calibration was performed in order to
derive the pixel-to-pixel non-uniformity correction. This
was achieved by the use of an integrating sphere that has
a rather large aperture with a constant spectral radiance
N (W=sr=cm2=nm) all over its aperture. The images of this
aperture are corrected ?rst to a polynomial surface that is

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638Y. Yair et al./Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642
Fig. 2. Normalized response of the IMC-201 camera.
XYBION Temperature Response
0.92
0.94
0.96
0.98
1
1.02
1.04
202530 3540
Temperature [C]
Normalized Response
Filter 1
Filter 2
Filter 3
Filter 4
Filter 5
Filter 6
Fig. 3. Temperature response of the IMC-201 camera.
?lter dependent. This correction reduces the non-uniformity
from a distribution with ∼15% FWHM to a distribution with
∼5% FWHM. The residual non-uniformity is caused by
pixel-to-pixel non-uniformity that is constant for all ?lters.
By removing this non-uniformity the distribution of pixel
response to uniform radiance is reduced to distribution with
∼1% FWHM.
These calibrations were completed at GSFC and will be
repeated once more when the payload is integrated to the
shuttle prior to launch. This will ensure that the Xybion
IMC-201 will conduct measurement of radiance levels that
can vary by 5 orders of magnitude with an absolute accuracy
of 1%.
5. Observations from the space shuttle
We have de?ned special regions-of-interest (ROI) for
TLE observations and these will be conducted when the
shuttle trajectory is within those areas or looking toward
them as the shuttle travels away into the nightside. The
ROI include South America, North Australia and Indone-
sia, Southeast Asia, China, the Sea of Japan, the Continental
USA and the Gulf of Mexico. The speci?c timing of an ob-
servation at any geographic location would depend on the
mission time-line and the availability of shuttle resources.
The calculated distance from the shuttle to the Earth’s limb
is approximately 1900 km. With a ?eld of view of 17:86◦,
we cover a vertical slab of the atmosphere that is 400 km
high. Sprites, Elves and Jets that appear in the region be-
tween cloud tops and a height of 80–100 km can therefore
be recorded on a reasonably large section of the digital im-
age. Observations will consist of continuous recording of the
Earth’s limb from the direction of the dusk terminator while
?ying towards the nightside, preferably before midnight lo-
cal time at the observed area (Fig. 4). This will ensure
that some lightning activity will still be present, as gener-
ally local thunderstorms subside in the early morning hours.
TLE sometimes have a horizontal dimension of 100 km, and
may also occur in short intervals. Against the dark back-
ground of Earth’s limb they should be relatively easy to
discern. The crew will be able to visually observe lightning

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Y. Yair et al./Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642639
Fig. 4. Geometry of shuttle sprite observations, with the payload bay directed toward the dusk limb while ?ying into the nightside.
activity through the shuttle window and on the video mon-
itor display, and to re-direct the camera FOV as necessary
toward that region by manipulating the truss. This will en-
hance the probability of intercepting TLE over active storm
centers.
The overall TLE duration changes according to type, and
can be of the order of 1–50 ms (Lyons et al., 2000), a
factor that determines the total number of spectral images
that will be obtained in each individual event. Boeck et al.
(1994) have used a single video frame (17 ms) for analy-
sis, even though they captured more frames in each event.
This duration is su?cient to determine the exposure time
for a pre-selected ?lter in the locked mode. The primary ?l-
ter for the observations will be the 665 nm ?lter. The half
point of this ?lter (640–690 nm) match very well the wide
peak centered at 670 nm that was observed by Hampton
et al. (1996). A second choice would be the use of the blue
?lters in 380 and 470 nm. Limb observations from the shut-
tle have the bene?t of avoiding the loss of blue light (due
to Rayleigh scattering) that is experienced by ground obser-
vations. This may improve the detection of blue jets, which
so far have been infrequently documented. Night time ob-
servations may be a?ected by Mie-scattered light from the
lightning ?ash in the cloud below. This light is sometimes
obscured by the anvil of the thundercloud, and may possi-
bly be avoided altogether by careful pointing of the cam-
era (spatial separation). Since blue jets last on the order of
200 ms, the scattered lightning light may be avoided tem-
porally as well.
The observations in the red and blue spectral regions are
valuable to the understanding of the mechanism that gener-
ates sprites and blue jets. Hampton et al. (1996) detected
the presence and relative intensities of the N2 1P lines.
These were in good agreement with auroral intensities, sug-
gesting that sprites are formed by electron impact excitation
(a mechanism proposed by several theoretical models
(Pasko et al., 2002; Milikh et al., 1995)). Our measure-
ments will record the absolute intensity of sprites in the 640
–690 nm spectral region and this will enable the calculation
of the emitted energy. Such input is essential for validating
theories of sprite formation.
6. Forecast of thunderstorm centers for sprite
observations from space
The global occurrence rate of sprites was estimated to
be of the order of several per minute, and they are usually
associated with extremely powerful ground ?ashes. Sprites
are expected to occur above intensive thunderstorms, es-
pecially in the tropics and over the continental USA, in
association with mesoscale convective systems (MCSs) in
particular (Williams, 1998). An MCS is a cluster of cumu-
lonimbi with a collective diameter of hundreds of kilome-
ters, which is frequent in tropical regions but also occurs
in mid-latitudes. In order to observe sprites from space and
to photograph them e?ectively, the astronauts need to have
in advance information concerning the location and inten-
sity of MCSs. While the vast majority of sprite observations
were obtained in summertime continental thunderstorms in
the US, they have lately been observed in winter storms in
Japan (Takahashi, 2001), Taiwan (Su et al., 2001), and Eu-
rope (Neubert et al., 2001). The storm types in these cases
di?ered from the “classical” MCSs, often having convec-
tive cells embedded in cold fronts of mid-latitude cyclones.
Since the shuttle completes one revolution of the Earth ev-
ery 90 min, and can observe any given target usefully for
only a few minutes, the need for an accurate forecast of the
geographical location of major storm centers is crucial dur-
ing the mission.

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640Y. Yair et al./Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642
We found that the most e?cient way to get information
regarding spatial distribution and intensity of MCSs and
major convective regions is via signi?cant weather maps
producedbyaviationcenters.Thesemapscontainvisualrep-
resentation of signi?cant atmospheric phenomena that may
a?ectaviation,suchasthelocationsofturbulence,icing,vol-
canic eruptions, jet streams and the distribution of potential
thunderstorms. The maps are derived subjectively by trained
forecasters, based on satellite imageries and forecasted maps
from the available models. For each region where thunder-
storms are expected, the levels of the cloud tops are denoted,
together with an abbreviated notion regarding their intensity.
When the regions of expected thunderstorm activity meet
the de?nition of an MCS we regard the region as a candidate
for sprites observations. In addition, we look for the temper-
ature maps of the −70◦C isotherm, which was shown to cor-
relate well with storms that, based on their electromagnetic
signals, produce strong ground ?ashes which are conducive
to sprites (Fullekrug and Price, 2002). The accuracy of the
location prediction of MCSs in the signi?cant weather maps
was evaluated by comparisons with satellite imagery (Ziv
et al., 2002), and was found to be higher than 90%. During
the mission, a global map will be produced daily, in which
the regions where MCSs are predicted will be designated,
with special notations to indicate intensive activity and ar-
eas where the cloud tops are expected to exceed 45;000 ft.
This information will help the crew to adjust the camera
line-of-site according to the forecasted activity and increase
the probability of successful observations.
7. Ground-based ELF/VLF measurements
During the 16-day mission, there will be extensive ground
measurements of ELF and VLF signals that are associated
with strong lightning discharges that are conducive to sprite
formation. Two ?eld sites in the Negev Desert in Israel
will be used to collect data related to sprite activity, in
order to allow geo-location of the parent lightning of an
optically registered TLE. The ELF instruments are located
at Tel Aviv University’s astronomical observatory near the
town of Mitzpe Ramon. The station has two horizontal mag-
neticinductioncoils,andoneverticalelectricalball-antenna.
The three components of the electromagnetic ?eld are sam-
pled in the 1–50 Hz range, using a notch ?lter at 50 Hz
(Fig. 5). The magnetic induction coils can detect ?eld vari-
ations at the pico-Tesla (10−12T) level. During the STEPS
campaign a sampling frequency of 1 kHz was used. The raw
data were continuously collected during a 6-h period from
0200–0800 UT each day.
The VLF antenna is very similar in size and sensitiv-
ity to the Palmer Station, Antarctica, antenna operated by
Stanford University (Reising et al., 1996). It has two or-
thogonal triangular loop-antenna 9 m high, with a baseline
of 18 m. The sensitivity of the system in the broadband
range (0.1–50 kHz) is 6 ?V=m. The dynamic range of the
Fig. 5. Change of the electric ?eld received at the Negev station
from a sprite-generating lightning ?ash that was observed over
China.
antenna/preamp is approximately 100 dB, implying that the
signal would clip if it is greater than 6 V=m. The broadband
VLF data are recorded during the same 6-h period on digital
audiotapes (DAT) and later digitized at 100 kHz to analyze
the signals. Both ELF and VLF antennae have GPS clocks
for temporally correlating with the sprites observed from
other stations and from space. The VLF antenna is located
at the Ben Gurion University Desert Research Institute at
Sde Boker, some 20 km from the ELF station.
The timing of the sprite events detected in the Negev
Desert is independently calculated using the ELF vertical
electric ?eld changes. We look at the derivative of the elec-
tric ?eld time series (dE=dt) and register all occurrences
where the derivative (slope) of the E-?eld is greater than
a speci?able number of standard deviations (SD) from
the mean. This threshold method supplies a list of times
where this threshold is ?rst exceeded, the polarity of the
impulses, and their amplitudes. Using the VLF data for
direction ?nding has a much greater accuracy since the tem-
poral resolution (sampling freq: = 100 kHz) of the original
lightning waveform is much better-preserved (Fullekrug
et al., 1996; Fullekrug and Constable, 2000). On the other
hand, the waveform is not well resolved in the ELF data
(sampling freq:=1 kHz), and can lead to signi?cant bearing
errors, translating into large absolute errors in location when
the distance from the lightning source exceeds 10 Mm.
Hence, the ratio of the VLF horizontal magnetic compo-
nents is used to obtain the azimuth of the incoming sferics.
Price et al. (2001) have shown that signals from
sprite-producing storms that occur in the USA can be
geo-located based on the ELF/VLF signals received in
Israel. The distance between the source lightning and the
observing station was calculated by using the impedance
method developed by Burke and Jones (1995). The ELF

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Y. Yair et al./Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642641
transients produced by lightning discharges travel in the
Earth-ionosphere waveguide producing standing waves,
at the characteristic Schumann resonances (SR) of 8, 14,
20 Hz (Schumann, 1952). The di?erent modes of these
standing waves have di?erent amplitudes in the magnetic
and electric components, depending only on the distance
between the source and receiver. Therefore, looking at
the ratio between the electric and magnetic spectra of the
Schumann resonances allows one to calculate the distance
of the observing station from the source (Kemp and Jones,
1971). The spatial resolution (D) achieved using this
method depends on the length of the ELF pulse used for
calculating the spectrum (D∼f=N). Using a time interval
of 1:5 s (N = 1500; f = 1 kHz) results in a distance res-
olution of 1000 km. A 1 s interval (N = 1000) results in
500 km resolution, while a 0:5 s pulse (N = 500) gives a
location resolution of 250 km. For our analysis we plan
to use t = 0:5 s. It should be noted that any increase of
sampling frequency above 100 Hz does not add any more
information to the ELF spectrum. The combination of the
VLF azimuth and the ELF distance estimation allows the
location of the source to be determined.
The coverage of the space shuttle orbit o?ers a unique
opportunity for simultaneous measurements of TLE-related
signals on a global scale. Indeed, during MEIEDX, several
ground stations will be taking data in the ELF and VLF
range, and an e?ort will be made to perform ground-based
optical observations as well. Stations in Germany, Hungary,
Japan and the USA will participate in gathering SR data
that will be used later to correlate and geo-locate the parent
lightning ?ash.
8. Summary
The sprite campaign planned during the Mediterranean
Israeli Dust Experiment (MEIDEX) o?ered a unique oppor-
tunity to conduct global space-borne observations of TLE,
with calibrated instruments. The spectrum, intensity, mor-
phologyandevolutioncanbestudiedinhighresolutionfrom
above most of the atmosphere, o?ering a potential for new
discoveries. The combination with a simultaneous extensive
ground campaign that will measure ELF and VLF signals
from sprite-producing ?ashes will most likely enhance our
understanding of the spatial and temporal variation in TLE
occurrence over the entire planet (high latitudes excluded).
The results of the MEIDEX-sprite campaign can be used
as a benchmark for conducting future space-based obser-
vations from orbiting platforms, such as the International
Space Station.
Acknowledgements
MEIDEX is a joint project between the Israeli Space
Agency (ISA) and NASA. We wish to thank Scott Janz
and Ernest Hilsenrath of the Laboratory for Atmospheres
at NASA/GSFC for their help in the calibrations of the
Xybion cameras. Special thanks to the Hitchhiker team
at NASA/GSFC: Tom Dixon, Michael Wright, Katie
Barthelme, Charles Knapp, Ken Harbert, and to the STS-107
mission planners at NASA/JSC, for making this mission
possible.
References
Armstrong, R.J., Susczynsky, D., Lyons, W., Nelson, T., 2000.
Multi-color photometric measurements of ionization and energy
in sprites. Geophysical Research Letters 27, 653–656.
Barrington-Leigh,C.P.,Inan,
by positive and negative lightning discharges, Geophysical
Research Letters 26, 6.
Boccippio, D.J., Williams, E.R., Heckman, S.J., Lyons, W.A.,
Baker, I.T., Boldi, B., 1995. Sprites, ELF transients and positive
ground strokes. Science 269, 1088–1091.
Boeck, W.L., Vaughan, O.H., Blakeslee, R.J., Vonnegut, B.,
Brook, M., McKune, J., 1995. Observations of lightning in
the stratosphere. Journal of Geophysical Research 100 (D1),
1465–1475.
Boeck, W.L., Vaughan Jr., O.H., Blakeslee, R.J., Vonnegut, B.,
Brook, M., 1998. The role of the space shuttle videotapes in the
discovery of sprites, jets and elves. Journal of Atmospheric and
Solar-Terrestrial Physics 60, 669–677.
Burke, C., Jones, D., 1995. Global radiolocation in the lower
ELF frequency range. Journal of Geophysical Research 100,
263–271.
Franz, R.D., Nemzek, R.J., Winckler, J.R., 1990. Television images
of a large upward electrical discharge above a thunderstorm
system. Science 249, 48.
Fullekrug, M., Constable, S., 2000. Global triangulation of intense
lightning discharges. Geophysical Research Letters 27, 333–336.
Fullekrug, M., Price, C., 2002. Estimation of sprite occurrences in
Central Africa. Meteorologische Zeitschrift 11 (2), 99–104.
Fullekrug, M., Reising, S.C., Lyons, W.A., 1996. On the accuracy
of arrival azimuth determination of sprite-associated lightning
?ashes by earth-ionosphere cavity resonances. Geophysical
Research Letters 23, 3691–3694.
Gerken, E.A., Inan, U.S., Barrington-Leigh, C.P., 2000. Telescopic
observations of sprites. Geophysical Research Letters 27,
2637–2640.
Hampton, D.L., Heavner, M.J., Wescott, E.M., Sentman, D.D.,
1996. Optical spectral characteristics of sprites. Geophysical
Research Letters 23 (1), 82–92.
Huang, E., Williams, E.R., Boldi, R., Heckman, S., Lyons, W.A.,
Taylor, M., Nelson, T.E., Wong, C., 1999. Criteria for sprites
and elves based on Schumann resonance observations. Journal
of Geophysical Research 104, 16943–16964.
Kemp, T., Jones, D., 1971. A new technique for the analysis of
transient ELF disturbances within the earth-ionosphere cavity.
Journal of Atmospheric and Terrestrial Physics 33, 567–572.
Lyons, W.A., 1994. Characteristics of luminous structures in the
stratosphere above thunderstorms as imaged by low-light video.
Geophysical Research Letters 21, 875.
Lyons, W., 1996. Sprite observations above the U.S. High Plains
in relation to their parent thunderstorm systems. Journal of
Geophysical Research 101 (29), 641–652.
U.S.,2000.Elvestriggered

Page 8

642Y. Yair et al./Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 635–642
Lyons, W.A., Nelson, T.E., Armstorng, R.A., Williams, E.R.,
Suzcynsky, D.M., Rtabely, R., Taylor, M., Gardner, L., 1999.
Characteristics of thunderstorms and lightning which produce
mesospheric transient luminous events. In: Proceedings of
the 11th International Conference on Atmospheric Electricity,
Guntersville, AL, pp. 84–87.
Lyons, W.A., Armstrong, R.A., Bering III, E.A., Williams, E.R.,
2000. The hundred year hunt for the sprite. Eos 81 (33), 373–
377.
Milikh, G., Papadopoulos, K., Chang, C., 1995. On the physics of
high altitude lightning. Geophysical Research Letters 22, 85–88.
Moudry, D.R., Heavner, M.J., Sentman, D.D., Wescott, E.M.,
1999. Detailed morphology of sprites. Poster presented at
the 11th International Conference on Atmospheric Electricity,
Guntersville, AL.
Neubert, T., Allin, T.H., Stenbaek-Nielsen, H., Blanc, E.,
2001. Sprites over Europe. Geophysical Research Letters 28,
3585–3588.
Pasko, V.P., Stanley, M.A., Mathews, J.D., Umran S. Inan, Troy
G. Wood, 2002. Electrical discharge from a thundercloud top to
the lower ionosphere. Nature 416, 152–154.
Price, C., Asfur, M., Lyons, W.A., Nelson, T., 2001. An improved
ELF/VLF method for globally geolocating sprite-producing
lightning. Geophysical Research Letters 29 (3), 11–14.
Reising, S., Inan, U., Bell, T., Lyons, W., 1996. Evidence for
continuing current in sprite-producing cloud-to-ground lightning.
Geophysical Research Letters 23, 3639–3642.
Schumann, O.W., 1952. Uber die strahlungslosen Eigenschwin-
gungen einer leitenden Kugel, die von einer Lufschicht und einer
Ionospharen-hulle umgeben ist. Zeitschrift fur Naturforschung
7A, 149.
Sentman, D.D., Wescott, E.M., 1993. Video observations of
upper atmospheric optical ?ashes recorded from an aircraft.
Geophysical Research Letters 20, 2857.
Su, H., Chen, A.B., Hsu, R., 2001. Observation of sprites over
the Asian continent. In: Proceedings of the AP-RASC Meeting,
Tokyo, August 1–4, 2001, p. 134.
Takahashi, Y., 2001. Activities of sprites and elves in winter season.
In: Proceedings of the AP-RASC Meeting, Tokyo, August 1–4,
2001, p. 134.
Vaughan, O.H., Vonnegut, B., 1988. Recent observations of
lightning discharges from the top of a thundercloud into the
clear air above. In: Proceedings of the Eighth International
Conference on Atmospheric Electricity, Uppsala, Sweden,
pp. 383–387.
Vonnegut, B., Vaughan Jr., O.H., Brook, M., Krehbiel, P., 1985.
Mesoscale observations of lightning from the space shuttle.
Bulletin of the American Meteorological Society 66, 20–29.
Williams,E., 1998.Thepositive
sprite-producinglightning.
Solar-Terrestrial Physics 60, 689–692.
Yano, H., Abe, S., Takahashi, Y., 2001. High de?nition TV imagery
of Elves and Sprites over the Mediterranean Sea during the 1999
Leonid meteor shower peak. In: Proceedings of the AP-RASC
Meeting, Tokyo, August 1–4, 2001, p. 133.
Ziv, B., Yair, Y., Pressman, K., Fullekrug, M., 2002. Veri?cation
of the Aviation Center global forecasts of Mesoscale Convective
Systems. Jour. App. Meteor., submitted.
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