Current moment in sprite-producing lightning

Studies of sprite-producing lightning have revealed much of what we currently know about the mechanisms responsible for this phenomenon. With a combination of new and previous results, we summarize the currently known quantitative characteristics of this special class of lightning. This information has come primarily from the quantitative analysis of the electromagnetic fields produced by distant lightning. The long range of this technique has made it especially powerful, but there are important limitations on what can be measured that are related to the bandwidth of the measurement system. The lightning charge moment change required to initiate sprites varies across a relatively wide range, from approximately 100 -2000 C km. Note that this is not the total charge moment change in sprite producing lightning, which is by definition greater than the initiation threshold. This range is in very good agreement with the predictions of streamer-based sprite modeling. We also summarize the strong evidence, from a variety of sources, in favor of sprite currents as the origin of ELF pulses seen in a significant fraction of sprite events. The largest events show sprite current moment amplitudes of similar to1000 kA km and sprite charge moment changes of at least 1200 C km, and perhaps significantly more. Lastly, we show that delayed sprites are generated from very strong continuing currents (20-60 kA km) following a +CG return stroke. In the few cases analyzed, this current generates a charge moment change from 2000 to 6000 C km at the time of sprite initiation, which appears to be consistent with theoretical predictions of larger charge moment changes required for delayed sprite initiation. This current can be detected and analyzed with distant, sensitive ULF magnetic field measurements. Although much is known about sprite-producing lightning, there remain some fundamental yet unanswered questions about the lightning-sprite relationship. We expect that at least some of these will be answered in the relatively near future. (C) 2003 Elsevier Science Ltd. All rights reserved.