01:55:47 UT Sprite-producer

The fourth sprite-producing discharge occurred at 01:55:47 UT. The video sequence for this event is shown in Figure 3.18. This sprite cluster was not as bright as the previous one, but the heads of the sprites still saturated the camera. A few small sprites are also visible on each side of the bright sprite cluster. The first video frame began at 1:55:47.953 UT, $\sim\,$3 ms before the occurrence of the +CG stroke.

Figure 3.18: The temporal development at 17 ms intervals of the 01:55:47 UT sprite cluster (top), as seen through a wide angle lens. The LDAR data are plotted on top of NEXRAD reflectivity (bottom) and follows a rainbow-sequence from blue to red with respect to time. The reflectivity is contoured at 10 dBZ intervals starting with 0 dBZ.

Figure 3.18 shows that the flash originated in a convective cell immediately to the southwest of the convective cell which initiated the 01:47:52 UT flash. Thus, it appears that all of the sprite-producing discharges up to and including this discharge originated in a different convective cell. In spite of this, all of the flashes propagated into the same charge region within the stratiform region.

The temporal development of the flash is detailed in Figures 3.19 and 3.20. The initial increase in the electric field in interval t1 was due to the vertical development of the IC flash. The magnitude of this increase was less than that shown for previous sprite-producing flashes due to the greater range.

The negative leaders propagated outward through the stratiform region for $\sim$20 km during interval t1, but the electric field did not change significantly since the motion was primarily tangential relative to the slow antenna. The negative leaders continued their northward progression during interval t2 and the electric field decreased due to a net outward motion of negative charge away from the slow antenna. The average negative leader speed during interval t2 was $\sim$1.1$\times$10$^5$ m/s.

The electric field stabilized during interval t3 and even increased towards the end of the interval. Figure 3.20c indicates that the negative leaders were still progressing outwards throughout the interval, so one would expect the electric field to have decreased. It is possible that a vertical moment due to ``VHF quiet'' positive leaders propagating at lower altitude is responsible for the discrepancy.

Figure 3.20: The spatial development of the 01:55:47 UT sprite-producing discharge as a function of time. The VHF sources ($\Diamond$) are rainbow-colored from blue to red according to elapsed time within the respective time interval. VHF sources from previous intervals ($\cdot$) are purple. The +CG (+), LDAR-indicated sprite-producing discharge center (*), sprite region (dashed circle), and individual sprites (x) are shown in interval t4.

A 85.5 kA +CG occurred at 01:55:47.9564 UT, the start of interval t4. The sprite cluster location was determined in the same way as the 1:51:02 UT sprite cluster (see Section 3.8). The sprite cluster was well correlated with the most recent negative leader activity as well as with the LDAR-indicated charge center derived from that activity. As with all previous sprite events, the +CG was displaced from the center of sprite activity in the direction opposite of the negative leader advancement. Several of the individual sprites were near or just beyond the periphery of the discharge, though one sprite was almost directly over the +CG.

The electric field waveform of the +CG is shown in Figure 3.21 along with the log-RF at 274 MHz. The most dramatic increase in electric field occurred within the initial $\sim$300 $\mu$s after the return stroke onset. Continuing current was readily evident up to about 2.5 ms after the return stroke, at which point the electric field increased more gradually.

This +CG is unusual in several respects compared to the others. First, there was a field change resembling a return stroke $\simeq$180 $\mu$s after the +CG. The delay was too short and the slant angle to the ionosphere was too great for this to have been an ionospheric reflection. The cause of this field change is unknown. Second, the +CG was preceeded for a full millisecond by stepped leader pulses, as evidenced by the log-RF bursts and noisy electric field prior to the +CG. The pulses were not located by LDAR. Finally, the log-RF within the initial millisecond was stronger than that of the other sprite-producing +CGs, possibly indicating that there was unusually vigorous negative leader activity. This shows that there can be significant differences between sprite-producing +CGs, the interpretation and effects of which are not yet understood.

The post-sprite development of the flash (Figure 3.20e-f) was similar to that of previous sprite-producing flashes. The slow electric field change at the start of interval t6 (see Figure 3.19) was due to negative breakdown which first propagated towards the slow antenna and then away, as evidenced by the blue data points in Figure 3.20f. Comparison of Figures 3.18 and 3.3 shows that the forward (towards) motion of the negative breakdown was directed into the convective cell which initiated the 01:42:57 UT sprite-producing flash.

Figure 3.22: The density of VHF sources in 500 m height bins for the 01:55:47 UT flash. The peak at 7-8 km altitude corresponds to the average altitude of charge removal for the sprite-producing +CG.

The height distribution of VHF sources for the 01:55:47 UT flash is shown in Figure 3.22. The density peak at 7-8 km altitude corresponded to the average charge removal height of the +CG. Assuming an average height of $\sim$7.5 km altitude and the LDAR-indicated charge moment change in Table 3.1, the charge transferred to ground (-Z direction) would have been 37 C at 1 ms and 54 C at 4 ms. The average current to ground for these intervals would have been 37 kA and 14 kA respectively.