03:40:37 UT, 1000 frames/second

A 42.5 kA peak-current +CG occurred at 3:40:37.832 UT. Figure 5.7 shows the development of a cluster of columniform sprites which were produced by the +CG. The apparent initiation of a couple of sprites in frame $a$ was delayed by $\simeq\,$2-3 ms after the +CG. Based on the height above the +CG (shown in 10 km increments), the sprite $2$ luminosity in frame $a$ was between 77.3 and 78.2 km MSL altitude. Sprite $2$ appeared to reach an upper terminal altitude of about 88 km in frame $d$ based on the height above the +CG. Assuming an upper terminal altitude of $\simeq\,$87 km (Wescott et al., 1998) would shift the apparent initiation altitude down by only $\simeq\,$1 km to 76-77 km.

Figure 5.7: Initial 4 ms of the development of a cluster of columniform sprites. A couple of sprites initiated in the first frame and extended bidirectionally upward and downward in the second frame. The horizontal streaks at left are artifacts at the left edge of the camera sensor.

Sprite $1$ reached an apparent upper terminal altitude of about 94 km MSL in frame $d$ based on the height above the +CG. Assuming an upper terminal altitude of $\simeq\,$87 km MSL, the initial luminosity of sprite $1$ in frame $a$ was between 78.1 and 79.0 km MSL altitude. This is about 2 km higher than the initial luminosity of sprite $2$ (for the same terminal altitude). However, the heights based on an upper terminal altitude of 87 km for the sprites are in better agreement than the heights obtained relative to the +CG, which differ by about 7 km. An initiation height of 76-79 km MSL was found to be typical of most sprites in this study.

Sprite $2$ developed downward to 69.5 km MSL altitude in frame $b$ at an average velocity of $0.72{\times}10^{7}$ m/s. It developed further downward to a lower terminal altitude of 62.8 km MSL in frame $c$. The minimum downward velocity in frame $c$ was $0.62{\times}10^{7}$ m/s. The downward velocities of the other sprites in the cluster were very similar. These velocities are less than predicted by Raizer et al. (1998) and were also less than the average velocities shown for the 3:03:59 UT sprite cluster, which appeared to initiate at a lower altitude of about 73 km MSL (Section 5.2.3).