4.1 Identification of lightning, elves, and sprites

Previously elves were identified only using video imagery or wide field-of-view photometry [Fukunishi et al., 1996b], allowing limited information on their characteristics and providing poor discrimination between elves and the optical flash due to scattered light from the parent lightning (Section 3.2.1). With wide field-of-view photometers, both of these phenomena appear as a $\sim$1 ms flash, while with video systems they may appear as broad luminosity in a single field or frame.

In this work a more detailed and unique signature of elves in a horizontal photometric array is deduced from the model predictions described in Section 2.5. In the following sections, the discovery of the signature in 1996 is discussed, and its features are contrasted with those of scattered lightning flashes. A comparison between signatures of elves and sprites in the Fly's Eye and other photometric arrays is deferred until Chapter 5.

4.1.1 Modeled optical signatures

Due to the competing factors of obscuration by the causative storm clouds and extinction at the horizon, elves and sprites have been most frequently observed from the ground at ranges of 300 to 800 km. At such ranges, the causative storm system is well below the observer's horizon, and lightning is not directly visible. Nevertheless, sprites, elves, and scattered light from lightning may be detected optically, and ELF/VLF impulsive electromagnetic emissions (sferics) from lightning and possibly sprites may be detected with radio receivers (Section 3.1). Simple geometric considerations shown in Figure 4.1 indicate that the radio emission (sferic) due to the lightning, propagating in the Earth-ionosphere waveguide essentially at the speed of light, reaches an observer before any optical flash originating in the mesosphere or ionosphere. Moreover, in the case of elves, light from the outer limb of an elve reaches the observer before emissions from the elve's inner portion, even though photons from the inner region are emitted first. An equivalent description of this interesting effect is that the edge of the luminous elve expands outward at a speed greater than that of light. For an observer with a limb view, this property causes ``temporal focusing'' of the front half (``front'') of the flash and ``temporal defocusing'' in the distant half (``back'') of the elve [Inan et al., 1996c].

Figure 4.1: Geometry for photometric observations of elves at 500 to 900 km range. Of the two VLF/optical paths shown, the one seen by the observer at a higher elevation angle is shorter. Light from this path arrives $\sim$150 $\mu$s after the radio sferic but before light from the longer, lower elevation path. The thunderstorm is beyond the observer's horizon, which is indicated by the straight dashed line. The EMP pulse due to lightning is very short compared with the propagation time across the radius of the elve, so that the luminous region is at any instant actually in the form of a thin annulus rather than a disc.

Figure 4.2: Modeled view of elves from the ground. Panels show 10 $\mu$s-long snapshots (every 100 $\mu$s) of the modeled luminosity in the event detailed in Figure 2.5 as seen from the ground 745 km away from the causative lightning. The color scale shows intensity of emission in the ${\ensuremath{{\rm N}_2(1{\rm P})}}$ band. The first snapshot begins 17 $\mu$s after the lightning sferic is received at the observer location.

Figure 4.3: Integrated model view of elves from the ground. The modeled flash of Figure 4.2 is shown integrated over 2 ms.

For an observer on the ground, the flash appears to drop in altitude and spread outward with time. While the EMP pulse, and therefore the optical flash emitted at any location, may last only tens of microseconds, light is emitted from different regions for $>$1 ms as the EMP propagates radially outward (see Figure 2.5 on page ).

Using the modeled optical emissions described in Section 2.5, a detailed prediction can be made for the optical form of an elve observed from any location. By taking into account the propagation delays and the contribution to each viewing direction at each point in time, the evolution shown in Figure 4.2 was predicted for the event described in Figure 2.5.

4.1.2 Observed photometric signatures

A telltale signature based on the rapid development of the flash shown in Figure 2.5 was documented using the ``Fly's Eye'' [Inan et al., 1997]. By aiming well above the ionospheric $D$ region overlying a strong CG, this array is used to identify unambiguously the optical emissions from elves. Based on the short ($\sim$150 $\mu$s) delay between reception of the sferic, and reception of the first photometric signature from the ionosphere, the optical emission can be located to be hundreds of km from the lightning (see discussion in Section 4.3). This timing constrains the physical mechanism to be one involving speed-of-light propagation only [Inan et al., 1997].

4.1.2.1 Observations on 24 July 1996

Observations carried out with the Fly's Eye on 22 July 1996 were particularly clear and were the first reported results [Inan et al., 1997] from the Fly's Eye instrument, and are described in this section.

On 22 July 1996, a large mesoscale convective system $\sim$650 km southeast of the Yucca Ridge Field Site (40.67$^\circ$N, 104.93$^\circ$W) produced many sprites and was observed at Yucca Ridge unimpeded by any intervening clouds. At this distance the ground under the storm was $\sim$35 km below the horizon from Yucca Ridge (see Figure 4.1) so that neither the cloud-to-ground nor intracloud flashes produced by the storm, nor their cloud-scattered light, were visible from Yucca Ridge. Figure 4.4 shows the first video frame of a sprite event observed coincident in time (within $\sim$30 ms) with a positive CG discharge of estimated peak-current +150 kA occurring at 669 km from Yucca Ridge at 07:17:38.767 UT, as recorded by the National Lightning Detection Network.

Figure 4.4: Video image showing a sprite and elve. (a) The video camera and approximate photometer array views of the sky during the 07:17:38 UT event on 22 July 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. (b) The observation geometry for the two cases reported here. The dashed lines show the viewing altitudes of P1 through P9 and P11.

At 07:17:38.769 UT the onset of an intense VLF radio atmospheric (`sferic' for short) was observed (Figure 4.5, top panel) followed in $\sim$150 $\mu$s (in agreement with the path length difference as discussed above) by a bright pulse in the center of the top row of photometers (Figure 4.5, middle panel). Photometer 11 detected the same event (Figure 4.5, lower panel) but also showed a less intense but longer lasting luminous event starting $\sim$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., 1996b] and filling part of the field-of-view of photometer 11 (Figure 4.4a), consistent with past observations [Fukunishi et al., 1996b].

Figure 4.5: Temporal resolution of sprites and elves. 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 in 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 in all nine photometers resolves the rapid lateral expansion as shown in Figure 4.6.

The left column shows the first 0.6 ms of optical signals following the sferic onset ($t=0$) for the event shown in Figures 4.4 and 4.5. The top-row photometers P1$-$P9, all pointed at 6.4$^\circ$ elevation, share time and luminosity scales in Figure 4.6, 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 $\sim$150 $\mu$s for pixel 5 to $\sim$220 $\mu$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 $\sim$25 km across for the top row of photometers. The luminosity lasts longer as observed by P11 due to its larger field-of-view, which includes the distant half (``back'') of the expanding ring (Figure 4.1).

Figure 4.6: Predicted and observed photometric signatures of elves. The two left columns show photometer recordings from two events on 22 July 1996. The responses recorded in the nine top-row photometers 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 ($\times$2) 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., 1996c].

A second event exhibiting similar lateral expansion is shown in the same format in the center column of Figure 4.6. This event was associated with a 120 kA +CG lightning discharge occurring at 666 km distance from Yucca Ridge at 07:10:14.100 UT on the same day.

The luminosity scales in Figure 4.6 range from 0 to the MegaRayleigh (MR) value inset in each column for P1$-$P9 and, separately, for P11. These values represent photon intensities assuming the incident radiation is at 700 nm, as described in Section 3.3. Based on the predicted spectral distribution [Taranenko et al., 1993b] 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 $\sim$350 nm), the signal is expected to come primarily (95%) from the short-wavelength portion of ( ${\rm N}_2(1{\rm P})$). The peak intensities for P4, and in the first event for P11, are uncertain due to saturation of the photomultipliers.

The features exhibited in Figures 4.5 and 4.6 are consistent with those expected to be produced by the electrodynamic interaction with the lower ionosphere of lightning EMP (Section 2.5). To illustrate, we calculate the light output in the ${\rm N}_2(1{\rm P})$ and ${\rm N}_2(2{\rm P})$ bands as would be measured by the Fly's Eye photometers pointed at 6.4$^\circ$ 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 4.6 (plotted with the same time scale as the data and with $t=0$ corresponding to the time of arrival of the sferic at Yucca Ridge) 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 ${\rm V\hbox{-}m}^{-1}$, empirically consistent with an NLDN-estimated peak current of 150 kA [Inan et al., 1996d]. 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 ${\rm N}_2(1{\rm P})$ and ${\rm N}_2(2{\rm P})$ bands, and the wavelength-dependent 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 4.6.

The calculated response of P11 shown in the solid line is for an elevation angle of $3.1^\circ$, which is $\sim$1.1$^\circ$ 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 elve (see Figure 4.1). The fact that such an initial peak is not observed in the 07:10:14 UT event data can well be 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$\pm5$ km and 92$\pm5$, respectively. As for atmospheric refraction, the bending in an ideal dry atmosphere can be $\sim$0.3$^\circ$ for the elevation angle of P11 and can greatly exceed this value for a disturbed atmosphere [Landolt and Hellwege, 1987, p. 229].

The close agreement between theory and experiment as illustrated in Figure 4.6 supports the predicted structure of elves as consisting of a rapidly expanding ring of luminosity in a narrow altitude range ($\sim$85-95 km). Both the high intensities ($>1$ MR) of the optical signals received by the top-row Fly's Eye photometers 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$\pm$30 and 164$\pm$30 $\mu$s delays respectively for the two events (07:17:38 and 07:10:14 UT) shown in Figure 4.6 between the onsets of the VLF sferic and the first optical pulse (P5) are in close accord with the calculated $\sim$150 $\mu$s delay.

Although the fields-of-view of the Fly's Eye photometers do not extend beyond a full range of $\sim$234 km (i.e., 117 km radius), the theoretical model (using a broader simulation region than used by Inan et al. [1996c]) indicates that, for the 44  ${\rm V\hbox{-}m}^{-1}$ 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 $\sim$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 (see Figure 2.3 on page ) with lateral extent $\sim500$ km [Boeck et al., 1992].

4.1.3 Discrimination of elves from Rayleigh-scattered lightning

The submillisecond development of elves in the Fly's Eye's horizontally-spaced photometers, described above, provides an unambiguous signature for the identification of laterally expanding (EMP-caused) flashes, and is used in this dissertation as a means to differentiate elves from the light which originates in the parent lightning. Light from a cloud-to-ground lightning return stroke can be Rayleigh-scattered in the lower atmosphere, but was shown with a Rayleigh-scattering model in Section 3.2.1 to produce neither an onset delay nor a horizontal dispersion greater than $\sim$20 $\mu$s.

Figure 4.7 shows examples of the signatures of elves and Rayleigh-scattered lightning recorded with the Fly's Eye. For the case of elves, the VLF/optical path lengths involved in photometric measurements at different azimuths result in a horizontal dispersion among the signal onsets in the photometers. In contrast, Rayleigh-scattered light from lightning appears in the Fly's Eye photometers with an onset that is simultaneous (to within one sample, $\sim$16 $\mu$s) amongst the different photometers, and with no more than one sample delay with respect to the associated sferic.

Figure 4.7: Photometric distinction between elves and lightning. Distinctive signatures of elves (with onset delay and dispersion) and scattered light (with neither) as seen in the Fly's Eye. The relative fields-of-view of the narrow (P1-P9) and broad (P11) photometers are shown in Figure 4.4.

In addition, Rayleigh-scattered light from the continuum emissions of lightning produces emissions in the blue almost as strong as those in the red (being governed only by atmospheric extinction). Not all strong lightning strokes produced a bright `sky flash,' presumably due to the variability in cloud geometry. Because sky flashes last 500 to 1000 $\mu$s, they can obscure any elves that are also present.

All events identified as `elves' herein exhibit (1) an appropriate onset delay following the parent lightning sferic ($\sim$120 to 160 $\mu$s is typical for elves 600 to 800 km away) recorded by the same data acquisition system, (2) fast lateral expansion [Inan et al., 1997], and (3) when recorded, much brighter red emissions than blue. These criteria are used to analyze elves recorded during August 1997 and described in Section 4.2.

4.1.4 Video signatures of elves

It should be noted that in especially rare (i.e., bright) cases elves are detectable in a 17 ms video field. In Figure 4.4a, after significant image enhancement, the broad luminosity due to an elve is visible. Figure 4.8 shows the video record of an even brighter event as seen from the ground. This event was due to an unusually impulsive negative cloud-to-ground stroke. Note the large ($\geqslant$250 km) spatial extent of the luminosity. Without the accompanying photometry, however, one could not unambiguously determine whether this was an elve or scattered light from lightning. Figure 4.9 shows another example of an elve bright enough to register in a video field.

Figure 4.8: Detection of blue emissions in an exceptionally bright elve. A 17 ms field from image-intensified video shows a broad flash, deduced to be an elve from the accompanying photometry. The photometers each saturated, but show the signature of an elve. The image shows altitudes overlying a strong $-$CG discharge reported by NLDN. P8 and P9 were pointed just to the right of the image. Dashed lines show the approximate extent of the elve and its central minimum. Some dark bands (foreground clouds) obscure part of the elve. The unusually short onset delay for an event 571 km away is indicative of the low vertical extent of this intense flash. The dotted curve is the absolute value of the sferic, showing variations in optical output and causative EMP with similar time scales. The dashed curve accompanying that from P8 shows the signal recorded by P2, a blue photometer which saturated at 20 kR.

As shown in Figure 4.3 on page , the optical signature of an elve caused by the EMP from a strictly vertical lightning current is expected to exhibit a central ``hole'' corresponding to the minimum in the radiation pattern of a vertical dipole. Such a central dimmed region may be perceptible in Figure 4.8, but it is ambiguous, given the existence of intervening cloud bands. In the extraordinary case shown in Figure 4.9 the shape of the elve as well as the central hole is remarkably evident.

Figure 4.9: Bright elve viewed from an aircraft. An image of an elve over Europe captured by M.J. Taylor and L.C. Gardner of the Space Dynamics Lab., Utah State University, during the 18 November 1999 Leonids meteor storm at 02:10:00 UT. The luminosity is similar in scale to that shown in Figure 2.3. From http://leonid.arc.nasa.gov/leonidnews.html

The vertical scale in Figure 4.8, as well as in subsequent video images for which the NLDN located an associated cloud-to-ground stroke, shows the azimuth of the parent lightning and indicates altitudes directly overlying it (see Section 3.5.3). Because the images are taken from the ground and are not in limb view, these altitudes do not necessarily correspond to the altitude of any horizontally-extended luminosity in the image. For a horizontal range uncertainty of $\sim$50 km, the corresponding uncertainty in the altitude scales is approximately 10 km.