Summary

As discussed in this chapter and in the papers by Reising et al. (1999); Pasko et al. (1998b); Cummer and Stanley (1999); Cummer et al. (1998), sprites can emit ELF radiation. The ELF sprite emissions are separate and distinct from the ELF slow tail which often follows sprite-producing CGs. This ``sprite signature'' was used to detect the presence of daytime sprites. An ELF propagation model was applied to the observed electric field waveform in order to determine the charge moment change of the daytime sprite-producing discharges and sprites. The key results of this study are summarized below:

• The slow field changes attributed to sprites by New Mexico Tech Researchers were found to be well correlated with spatially-integrated sprite light output (Reising et al., 1999; Brook et al., 1997; Cummer et al., 1998). This proved that sprites are the source of the slow field changes which are temporally distinct from the ELF slow tail of a parent discharge. This ``sprite signature'' can be used to detect the presence of sprites when video is not available.

• Three different sprite events were detected during the daytime on August 14, 1998, by virtue of their ELF signature. All three slow field changes had unusually large amplitudes. The onset of the sprite ELF signatures was delayed by 11.0-13.2 ms from +CGs which had unusually large slow tails. All of the parent CGs occurred beneath the stratiform region of a large MCS in southern Texas.

• The charge moment changes of the parent discharges, in chronological order, were 6100, 4300, and 3900 C$\cdot$km at the onset of the sprite ELF signatures. A charge moment change of 6100 C$\cdot$km may have been sufficient for conventional breakdown at $\simeq$54 km altitude, assuming an experimentally measured ion conductivity profile of Holzworth et al. (1985). However, the other parent discharges would have been unable to initiate conventional breakdown unless the conductivity of the atmosphere had somehow been altered by the first sprite event.

• The daytime sprites themselves produced unusually large charge moment changes of $\simeq$2800 C$\cdot$km, $\simeq$1200 C$\cdot$km, and $\simeq$910 C$\cdot$km. These charge moments are larger than the largest nighttime sprite charge moment change published to date ($\sim$840 C$\cdot$km by Cummer and Stanley (1999) and shown in Section 5.3.3). This suggests that daytime sprites dissipate much more stored electrical energy on average than their nighttime counterparts, consistent with the much larger charge moment threshold for daytime sprite initiation.