Geophys. Res. Lett., 24, 5, pp. 583-586, 1997. Submitted November 19, 1996; Accepted February 4, 1997.
Rapid Lateral Expansion of Optical Luminosity in
Lightning-Induced Ionospheric Flashes Referred to as `Elves'
U. S. Inan, C. Barrington-Leigh, S. Hansen, V. S.
Glukhov, and T. F. Bell
STAR Laboratory, Stanford University, Stanford, CA 94305
R. Rairden
Lockheed-Martin Palo Alto Research Laboratory, Palo Alto, CA 94309
Data acquired by a new array of horizontally spaced photometers
boresighted with a low-light-level camera provide the first measurement of the
rapid lateral expansion of optical luminosity in lightning-induced ionospheric
flashes referred to as `elves', occurring over time scales substantially less
than 1 ms. The narrow individual fields-of-view of
(2.21.1) provide a spatial resolution of
20-km at a range of 500 km, enabling the documentation of
expansion occurring over a horizontal range of 200 km with a time
resolution of 30 micro-s. The observed dynamic features of elves are
consistent with a model in which the optical output is produced as a
result of heating by the electromagnetic pulse (EMP) from a lightning
discharge.
Introduction
Recent two-dimensional modeling of the interaction with the lower
ionosphere of intense electromagnetic pulses (EMPs) from lightning
discharges has indicated [Inan et al., 1996a]
that optical luminosities (produced at 85-95 km altitudes as a
result of heating by the EMP fields) as observed from a distance
would appear to expand laterally at speeds faster than the speed of
light. This apparent spatial expansion was predicted to occur
during the few-hundred-microsecond temporal duration of the optical
flashes, often referred to as elves [Fukunishi et al., 1996]. Noting
that the measurement of the sub-millisecond dynamics of the phenomenon would
constitute an excellent test of the EMP-heating mechanism, a novel high speed
photometric array (named the `Fly's Eye') was designed and built at
Stanford University and was operated at the Yucca Ridge field station in
Colorado during July 1996. In this paper, we present the observed features
of two different cases which clearly demonstrate the rapid lateral expansion
of the optical luminosity. Most aspects of the data from the two events
uncovered so far are consistent with the EMP-heating mechanism as the root
cause of elves.
Instrumentation
The `Fly's Eye' instrument consists of an array of Hamamatsu
HC124-01 integrated photomultiplier assemblies, sensitive in the
range 185 nm to 800 nm, with light shields, lenses, and
precision-cut focal-plane
screens arranged to provide the sky-view depicted in Figure 1a.
Each numbered box in the top row corresponds to the 2.2 wide
by 1.1 high sensitive region (`pixel') of one photometer. Inset in
Figure 1a shows the horizontal angular dependence of the sensitivity of one of
the nine pixels. Photometer 11 (P11) has a 6.6 by 3.3 field
of view. Figure 1 shows the observation geometry for the case of the
examples presented in this paper, with the inset showing the viewing
altitudes of P1 through P9 and P11.
 |
| FIGURE 1. (a) The video camera and photometer array views of the sky during
the 07:17:38 UT event on July 22, 1996. The cluster of sprites with columnar
structure is at the center. The single video field (33 ms long, ending at
07:17:38.792 UT) has been enhanced (by subtracting a previous field and by
thresholding its intensity) to highlight the
diffuse emissions constituting the elves. The dashed line indicates
the horizon. The inset shows the measured
horizontal angular response
function A(phi) for one of the top-row photometers. (b) The
observation geometry for the two cases reported here. The inset shows the
viewing altitudes of P1 through P9 and P11. |
The photometer array was boresighted with an image intensified
(Varo Noctron V) black and white Pulnix video system with a field of
view of
20. The video frames were time coded
with a GPS-based timing system and recorded on standard VHS
videotape. A clinometer was used to automatically record the viewing
elevation. Signals from all the photometers, the clinometer, another
GPS receiver, and a broadband (300 Hz to 20 kHz) very low frequency
(VLF) receiver were sampled and recorded with
30 micro-s resolution. The absolute sensitivity of one
photometer was calibrated for white light at one gain level,
and dealer-supplied relative sensitivities of the photomultiplier
tubes and data for wavelength and gain level dependence were used
to calibrate the other photometers for other wavelengths and
gain levels. Many
nearby lightning flashes were recorded nearly simultaneously on all
channels, providing a calibration of system timing.
Observations
On July 22, 1996, a large mesoscale convective system
650 km southeast of the Yucca Ridge (YR) Field Site
(4040N, 10456W) produced many
sprites and was observed at YR unimpeded by any
intervening clouds. At this distance the ground under the storm was
35 km below the horizon from YR so that neither the
cloud-to-ground (CG) nor intracloud (IC) flashes produced by the
storm, nor their cloud-scattered light, were visible from YR.
Figure 1 shows the first video frame of a sprite event
observed coincident in time (within 30 ms) with a positive CG
discharge of estimated peak-current +150 kA occurring at 669 km
from YR at 07:17:38.767 UT, as recorded by the National
Lightning Detection Network (NLDN).
At 07:17:38.769 UT the onset of an intense VLF radio atmospheric
(`sferic' for short) was observed (Fig. 2 top panel) followed
in 150 micro-s (in agreement with path length difference as discussed
below) by a bright pulse in the center of the top row of photometers (Fig. 2,
middle panel). Photometer 11 detected the same event (Fig. 2, lower panel) but
also showed a less intense but longer lasting luminous
event starting 7 ms after the sferic onset. This second
event is interpreted as the sprite itself, occurring over a
time scale of tens of ms [Pasko et al., 1996] and filling part of the
field of view of photometer 11 (Figure 1a), consistent with past
observations [Fukunishi et al., 1996].
 |
| FIGURE 2.
The relative timing of the VLF sferic (top row),
intensity in P5 (middle row) and that in P11 (bottom
row). The expanded sferic and P5 responses (right hand
panels) clearly show the delay between the onsets. P11 detects both
the fast event (i.e., elves) and the longer and dimmer sprites
starting several milliseconds later.
|
Although intense, the initial pulse observed on the top row of photometers
lasting for 1 ms is only visible in the video image as a diffuse glow
due to the
33 ms integration time of the video camera.
Simultaneous observation of the temporal signatures on all nine pixels
resolves the rapid lateral expansion as shown in Figure 3.
The left column shows the first 0.6 ms of optical signals following the
sferic onset () for the event shown in Figures 1 and 2. The top-row
pixels P1P9, all pointed at 6.4 elevation, share time and intensity
luminosity scales in Figure 3, while the less bright but
longer-lived signal from P11 is plotted with separate scaling. The increasing
delay of the flash onset with pixel
distance from the center, ranging from 150 micro-s for pixel 5 to
220 micro-s for P1 and P9, is clearly
apparent. The peak intensities of the pulses generally decrease with
lateral distance from the center. At a distance of 670 km, the fields of view
are
25 km across for the top row of pixels. The luminosity
lasts longer as observed by P11 due to its larger field of
view, which includes the distant half
of the expanding ring (Fig 1b).
A second event exhibiting similar lateral expansion is shown in the same format
in the center column of Figure 3. This event was associated with a 120 kA
+CG lightning discharge occurring at 666 km distance from YR at 07:10:14.100 UT
on the same day. Analysis of less than 5 of the data from the Summer 1996
campaign has revealed several
similar cases.
 |
| FIGURE 3.
The two left columns show photometer recordings from two events on
July 22, 1996. The responses recorded in the nine top-row pixels as well
as P11 are shown. The vertical axes range from 0 to the inset values (in MR)
in each column; P1-P9 share a common intensity scale. The
time axes for P1-P9 are also identical and show ms after the onset of the
associated VLF sferic. The time axes for P11 are compressed (x2) to
show the later part of the pulse. The righthand column shows the predicted
photometer responses using a two dimensional lightning EMP-ionosphere
interaction model [ Inan et al., 1996a].
|
The luminosity scales in Figure 3 range from 0 to the MegaRayleigh (MR)
value inset in each column for P1P9 and, separately, for
P11. These values represent photon intensities assuming the incident
radiation is at 700 nm. Based on the predicted spectral
distribution [Taranenko et al., 1993] of
lightning-EMP-produced optical emissions, and on the wavelength-dependence of
atmospheric Rayleigh scattering, and on the spectral response of the
photomultiplier tubes (which peaks at 350 nm), the signal is expected to
come primarily (95) from the lower 1st positive band of N. The peak
intensities for P4, and in the first event for P11, are uncertain due to
saturation of the photomultipliers.
Interpretation
The features exhibited in Figures 2 and 3 are consistent with
those expected to be produced by the electrodynamic interaction with the
lower ionosphere of lightning EMP, as modeled by Inan et al. [1996a]. To
illustrate, we use a modified version of the two-dimensional EMP-ionosphere
interaction model to calculate the light output in the N 1st positive and
N 2nd positive bands as would be measured by the Fly's Eye photometric
pixels pointed at 6.4 elevation and their individual azimuths, for a
source CG lightning discharge at 669 km range. These theoretical
predictions, shown in the righthand column of Figure 3 (plotted with the same
time scale as the data and with
corresponding to the time of arrival of the sferic at YR) are in good
agreement with the observations. The observed onset delay, the speed of lateral
expansion, the general form of the apparent vertical development as manifested
in P11, and the broadening of pulse widths and reduction of peak intensities
at wider angles are all represented in the model predictions. For the model
calculations, the intensity of the lightning flash was taken to correspond to
a peak electric field intensity at 100-km horizontal distance of 44 V/m,
empirically consistent with an NLDN-estimated peak current of 150 kA [
Inan et al., 1996b]. The width and shape of the shortest (P5) modeled
pulse reflects the current waveform of the modeled lightning. The actual
durations of the causative lightning flashes for the observed events were
not independently measured, and were thus not entered in the model.
The
spectral distribution of the N 1st positive and N 2nd positive
bands, and the heavily wavelength-dependent effects of Rayleigh scattering
and photometer response, were taken into account to predict voltage
levels in the photomultipliers. These were in turn expressed in
Rayleighs assuming a 700 nm source for direct comparison with the observations
in Figure 3.
The calculated response of P11 shown in the solid line is for an elevation
angle of
, which is
lower than the actual recorded elevation of this pixel. At
the recorded elevation, the computed response is very similar except for an
additional initial peak (shown as a dashed line), due to the front part of
the expanding ring of emission. The fact that such an initial peak is not
observed in the 07:10:14 UT event data can be well explained by a small
difference in the lower altitude limit of the luminosity, or by the
refractive bending of the light rays travelling nearly tangential to the
surface. Indeed, a slightly higher altitude for the latter event is
suggested by independent estimates based on timing and geometry; these
place the two light sources at 90 km and 92, respectively. As
for atmospheric refraction, the bending in an
ideal dry atmosphere can be 0.3 for
the elevation angle of P11 and can greatly exceed this value for a
disturbed atmosphere [Landolt-Bornstein, 1988, p. 229] .
Based on the photometer data alone, the apparent speed of lateral
expansion of each of the two luminous events in Figure 3 is
3.1 times the speed of light, in good agreement with the
original predictions [Inan et al., 1996a]. Both this fact and
the close agreement between theory and experiment as illustrated in
Figure 3, support the predicted structure of elves as consisting of a
rapidly expanding ring of luminosity in a narrow altitude range
(85-95 km). A time-integrated image of the optical luminosity
predicted by our model to be observed over the entire field of view of
the video camera is shown in Figure 4, together with the superimposed
fields of view of the Fly's Eye pixels. The black region in the
center is the center of the ring, corresponding to the minimum in the
radiated EMP intensity above the source CG discharge, while the lower
white region is the back part of the emission ring. The effect of
Rayleigh scattering is not reflected in the image; it would attenuate
slightly the lower limb with respect to the upper one. Comparing the
image of Figure 4 with that of Figure 1, we note that in this case the
central hole region is masked by the sprites.
 |
| FIGURE 4.
Calculated time-integrated spatial structure of the
lightning EMP-produced optical luminosity shown in the field of view of the
video camera. The Fly's Eye pixel views are superimposed.
|
Discussion
Both the high intensities ( MR) of the optical signals received by
the top-row Fly's Eye pixels and the fact that the luminosity is seen
in the top-row photometers before P11 indicate that the observed
signals are not due to Rayleigh scattering of light produced by the
parent lightning flash.
The 154+-30 and 164+-30 s delays
respectively for the two events (07:17:38 and 07:10:14 UT) shown in Figure 3
between the onsets of the VLF sferic and the first optical pulse (P5)
are in close accord with the calculated
s delay. Note that because the outer limb
of the luminous events moves considerably faster than the speed of light [
Inan et al., 1996a], the first signal detected by the photometers is the
outermost bright region along the line of sight.
Although the fields-of-view of the Fly's Eye pixels do not extend beyond a
full range of
234 km (i.e., 117 km radius), the theoretical model (using a
broader simulation region than used in [Inan et al., 1996a]) indicates
that, for the 44 V/m electric field intensity used here, the lateral
extent of the luminous region (defined as the region in which the emission rate
exceeds 1 of its peak value) is
600 km. The extension of luminosity over such a large region is
consistent with the video observations from the Space Shuttle of
lightning-associated airglow enhancements (which in retrospect are most
likely elves) with lateral extent km [Boeck
et al., 1992].
Summary and Conclusions
The remarkable agreement between data and theory as displayed in
Figure 3 provides strong evidence that the fast
lower-ionospheric optical flashes referred to as elves are manifestly distinct
from later arriving red sprite events and that they exhibit clearly
discernable outward expansion at apparent speeds greater than that of light.
All measured aspects of this phenomenon appear to be consistent with those
expected on the basis of heating by lightning-EMP as the root cause.
This work was supported by the Office of Naval Research
under AASERT grant N00014-95-1-1095 and by the National Science Foundation
under grant ATM-9412287. We thank Prof. J. Winckler for his counsel and
advice and W. Lyons and T. Nelson for hosting our
equipment at the YR observatory and for their organization of the Sprites 96
campaign. We also thank S.
Reising for his extensive help in the fielding of the experiment and Dr. V.
Pasko for his comments on the manuscript.
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