Summary

High-speed video observations of sprites were analyzed in this chapter. The approximate initiation altitude, development sequence, and average velocities were presented for several sprite clusters. The charge moment change of the parent discharge was also correlated with the initiation and development of some sprite clusters. The key results of this analysis are summarized as follows:

• The initiation altitude of all sprites was 76$\pm$6 km MSL with most sprites initiating at 76-79 km MSL. The initiation altitude is roughly consistent with conventional breakdown predictions. The initiation altitudes were based on fixing the terminal altitude of the brightest carrot sprites at 90 km MSL and the moderate-brightness columniform sprites at 87 km MSL.

• Sprites were initiated when the charge moment change of the parent discharge exceeded $\simeq\,$300 C$\cdot$km. This is consistent with previous ELF-based estimates (Cummer and Inan, 1997; Huang et al., 1999) and with the 200-400 C$\cdot$km estimate based on static-field measurements presented in Chapter 5. Using an E/N breakdown threshold of 100 Td (compared to 123 Td used in this study), Fernsler and Rowland (1996) predicted that 300 C$\cdot$km would be sufficient to initiate conventional breakdown at 80 km MSL altitude. The result that initiation heights appear to be below 80 km MSL, compared with the compact origin of most sprites, suggests that significant conductivity inhomogeneities may locally intensify the electric field to produce breakdown at lower altitudes.

• Most sprites developed bidirectionally upward and downward from the point of initiation. The initial development characteristics are consistent with experimental observations (Loeb and Meek, 1940) and theoretical predictions (Dhali and Williams, 1985) of bidirectional streamer development in long air gaps. In sprites produced by +CGs, the upward component would be a negative streamer while the downward component would be a positive streamer.

• Parent discharges having unusually large and rapid charge moment changes produced sprite halos, consistent with the theoretical predictions of Pasko et al. (1997b). Angel sprites were observed to initiate from enhanced luminous regions in the sprite halos.

• The average velocity of the inferred positive streamers which propagated downward could exceed $1{\times}10^{7}$ m/s, consistent with the predictions of Raizer et al. (1998). The average velocities of the positive streamers of angel sprites were as fast as $3.4{\times}10^{7}$ m/s, which is significantly faster than that of other types of sprites.

• During some sprites, apparent negative streamers were spawned from regions through which the positive streamers propagated previously. The negative streamers propagated and branched upwards, transforming the sprite from a columniform shape (with tendrils) to an upward-V, or ``carrot'', shape.