Summary

In this chapter, relatively close sprite-producing discharges at $<$100 km range have been analyzed. The 3-dimensional development of the flashes was observed at VHF using time-of-arrival LDAR system. The flash development was compared with slow antenna measurements of the electric field, and with NEXRAD radar reflectivity data. The plan locations of the sprites were estimated from video observations and compared to the flash location and dimensions up to the time of the sprite event. A non-sprite-producing flash was also analyzed. The key results are as follows:

• A comparison of charge moment changes during a 4 ms interval after the RS for both sprite-producing and non-sprite-producing discharges indicates that the charge moment threshold for sprite initiation is probably between 200-400 C$\cdot$km. This is consistent with ELF-based estimates (Cummer and Inan, 1997; Huang et al., 1999; Cummer and Stanley, 1999).

• As was already observed by Berger (1972), Krehbiel (1981), and Brook et al. (1982), the $+$CGs sustained large currents of $>\!$10 kA magnitude much longer than the $-$CGs. The sprite-producing discharges had average downward ($-$z direction) currents of $\simeq$28-37 kA within the initial 1 ms post-return stroke interval and $\simeq$13-16 kA within the initial 4 ms. The range of charge transfers to ground (-z direction) for these time intervals was $\simeq$28-37 C and $\simeq$52-64 C, respectively. The source of the general discrepancy between $+$CG and $-$CG sustained currents may simply be that all of the $+$CGs were associated with horizontally extensive discharges while the $-$CGs were not.

• The positive charge lowered to ground by the sprite-producing discharges was located at an average altitude of 7-8 km within the stratiform region of the MCS. This is in agreement with previous multi-station electric measurements of $+$CG charge removal altitudes within the stratiform region of an MCS (Krehbiel, 1981), but contradicts numerous predictions that the sprite-producing CGs would discharge a portion of the positive charge layer at or just below the 0$^{\circ}$ C isotherm at $\simeq$4.5 km altitude.

• The electric field and VHF source pattern are consistent with the interpretation that +CGs within the stratiform region of an MCS are produced after horizontally extensive channels become ``cutoff'' (nonconducting), as was proposed by Krehbiel (1981).

• The sprites occurred primarily above the periphery of the most recent ($\sim$200-300 ms) portion of the discharge. This most recent discharge section was almost entirely located between the +CG and the negative leader advancement front. This provides further evidence for the channel cutoff hypothesis.

• Surprisingly, the plan locations of most sprites were correlated with the periphery of the above mentioned discharge section. The sprites might, in essence, provide a crude outline of the horizontal extent of charge removal.

• In two of the sprite-producing flashes, the +CGs were followed by one or more -CG strokes which occurred underneath a region of rapid charge removal. I speculate that these -CG flashes were separate discharges initiated from the top of one or more tall structures due to the influence of the +CG's charge removal.