Discussion

Several attempts were made in 1997 to detect daytime sprite signatures. In spite of this, no such signatures were detected. The fortuitous observation in 1998 of the three daytime sprite events presented in this chapter are the only such events detected to date. As was noted earlier, daytime sprite events are expected to be rare since the charge moment threshold for sprite initiation is very much higher during the day than at night.

Observations of nighttime sprites indicate that their bases are typically located at $\sim$50 km altitude, but can be as low as $\sim$31 km or lower Lyons (1996); Sentman et al. (1995); Winckler (1995). It will be shown in Chapter 5 that the tendril region of sprites is composed of downward-developing positive streamers. Theoretical models of positive streamer development under the influence of a 350 C$\cdot$km (+CG) parent discharge predict a terminal altitude of $\sim$48 km (Raizer et al., 1998). Increasing the charge moment change to 1000 C$\cdot$km results in a lower terminal altitude of $\sim$40 km Pasko et al. (2000). These altitudes are in rough agreement with those observed. The terminal altitude of streamers will, of course, be even lower under the influence of the much larger daytime charge moment changes.

Assuming that the minimum altitude drops at least another 10 km when the charge moment change is increased from 1000 C$\cdot$km to 3900 C$\cdot$km (corresponding to the weakest event), then the vertical extent of the sprite will be at least 32 km for an ionosphere height of 62 km. Thus, the maximum charge transfers within the 2800, 1200, and 910 C$\cdot$km sprite events would be 88, 38, and 28 C respectively. These charge transfers are much larger than the 5-6 C previously estimated for a couple of nighttime sprite ELF signatures, but are comparable to the $\sim$42 C estimated for an unusual long-duration nighttime sprite ELF signature (Cummer et al., 1998).