4.2 Correlation with positive and negative lightning strokes

In this and the following two sections, we consider only events in the period 03:00 to 10:00 UT on 27 August 1997, when a large mesoscale convective system was very active 700 to 850 km southwest of the Langmuir Laboratory for Atmospheric Research (107.19$^\circ$ W $\times$ 33.98$^\circ$ N $\times$ 3200 m) in New Mexico. Instruments deployed at Langmuir included a broadband (300 Hz to 20 kHz) VLF receiver to record sferics, an intensified video system, and the Fly's Eye photometric array. Signals from the sferic receiver and photometers were recorded in data snapshots of $\sim$2 seconds duration by a pre-trigger buffering system (Section 3.4, with timing synchronized to within 16 $\mu$s of GPS time through a parallel New Mexico Tech data stream.

On this night the triggering was done manually, based on observations with the bore-sighted intensified video system. Due to the much longer ($\sim$5 to 150 ms) duration of sprites as compared with elves ($<$1 ms), sprites are better suited for detection at a video frame rate than are elves, and many elves are not detectable above the night sky background with our video imaging system. Consequently our manual triggering method was biased towards events associated with sprites, and thus towards large positive cloud-to-ground lightning discharges [Boccippio et al., 1995]. On the other hand, manual triggering was not exclusively selective of events with sprites, since occasionally bright Rayleigh-scattered light from the parent flash and brighter than usual elves were also evident in video. Photometric signatures of sprites were never confused with the onset of elves, in part because sprites begin at least $\sim$500 to 2000 $\mu$s after any closely associated sferic.

Figure 4.10: Correlation between elves and lightning polarity. Histogram of elves detected versus the strength of their causative VLF sferic, sorted by CG polarity.

The identification of elves in this study does not rely either on recordings from the 30 frame-per-second video which was bore-sighted with the photometer array, nor on the microsecond timing provided by the NLDN. During the period 03:00 to 10:00 UT on August 27, at least 39 flashes were identified as elves, based on the criteria described above on page  (Section 4.1.3). Figure 4.10 shows a histogram of these events sorted by the peak-to-peak intensity of their associated sferics. While most of the +CG events in Figure 4.10 were associated with sprites, it is remarkable that a considerable fraction (31%) of the events were associated with $-$CG flashes, in spite of the fact that the manual triggering method used on this day was highly biased towards sprite-associated discharges and towards the very brightest of elves.

The fact that $-$CG flashes also produce elves is consistent with our theoretical understanding of collisional heating by the lightning electromagnetic pulse, a process which is independent of the polarity of the field. During the period 03:00 to 10:00 UT, 90% of the CG flashes recorded by NLDN from the Mexican MCS and with peak current greater than 25 kA were $-$CG. Based on the occurrence rate of highly energetic $-$CG and +CG discharges the above result indicates that EMP-induced heating and ionization of the lower ionosphere (as manifested by elves) above nighttime thunderstorm systems may well be much more prevalent than sprites.

To further assess the prevalence of elves, we surveyed all the NLDN flashes with peak current over 38 kA that were within the Fly's Eye field-of-view and which occurred during one of the 261 recorded data samples, each lasting about 2 seconds. Of the 86 NLDN events in this set, the photometric records for 13 events were dominated by the Rayleigh-scattered light due to the parent lightning flash. Of the remaining 73 flashes, 52% (38) exhibited the telltale signature of elves. Above 45 kA in the NLDN record, this fraction was 73% (37); above 57 kA, all (34) of the flashes had accompanying elves.

Figure 4.11: Theoretical and observed correlation between peak brightness and causative lightning intensity. Squares and crosses show $-$CG and +CG events, respectively. A circle indicates that sprites were associated (within 100 ms) with the event. The dotted line shows the theoretical calculated ${\rm N}_2(1{\rm P})$ emissions multiplied by 0.075 to account for atmospheric and filter transmittance and incomplete coverage by the photometers.

These statistics are necessarily affected by our manual triggering method. Nevertheless, Figure 4.11 shows a good correlation between the peak VLF fields produced by lightning and the maximum optical intensity seen by any of the Fly's Eye photometers, even though the photometers were not necessarily looking at the same part of the flash in different events. The scale on the top of Figure 4.11 shows NLDN peak current values based on a linear fit to the good correlation between VLF peak and NLDN peak which was found for all but a few outliers among these events. This fit is shown in Figure 4.12. The model of Section 2.4 was run for cloud-to-ground lightning 10 km high and reaching its peak current in 30 $\mu$s. The maximum ${\rm N}_2(1{\rm P})$ optical intensity as would be seen from the ground at 745 km distance for a range of peak current values was calculated and is plotted as a dotted line in Figure 4.11. The shape of this curve agrees well with previous calculations which did not take into account the viewing geometry [Taranenko et al., 1993b]. A threshold in the VLF peak is evident; this results from a combination of the instrument background signal level and the highly nonlinear dependence of ${\rm N}_2(1{\rm P})$ optical output on the instantaneous field strength.

Figure 4.12: Correlation between VLF intensity and NLDN reported peak current. VLF sferic peak-to-peak intensity (measured in equivalent wave magnetic field) is shown versus NLDN peak current for the same elves events shown in Figure 4.11.

All elves events as identified by the Fly's Eye have been found in direct association with the sferic signature of a cloud-to-ground, rather than intracloud, discharge. Furthermore, the timing always indicates that the elve is caused by the CG rather than by any associated sprite (as suggested by Taranenko et al. [1997] and Roussel-Dupre et al. [1998]).