1.2 Thunderstorms and cloud-to-ground lightning

The existence of liquid water in Earth's troposphere is primary among the special characteristics on this planet that are thought to have led to the development of complex life. One remarkable and still imperfectly understood [e.g., Wettlaufer and Dash, 2000] terrestrial process resulting from water's presence is the convective charge separation which causes thunderstorms. About 40,000 thunderstorms per day worldwide produce $\sim$100 cloud-to-ground lightning flashes per second [Chalmers, 1967]. While these discharges have obvious impacts in the troposphere and to humanity, they also play a significant role in the global electric circuit. While lightning currents may flow with either an upward or downward electrical sense, $\sim$90% of cloud-to-ground lightning has an upward current (i.e., negative charge moves from the cloud to the surface of the Earth), resulting in a steady net negative potential of the Earth with respect to the ionosphere and a steady fair-weather downward electric field at the surface of the Earth of $\sim$100  ${\rm V\hbox{-}m}^{-1}$ [Feynman et al., 1989, p. 9-3].

The return stroke of a cloud-to-ground discharge [Uman, 1987] lasts $\sim$100 $\mu$s and generates the largest currents, brightest optical emissions, and peak radiated power of a lightning flash. Such currents are typically $\sim$30 kA but may surpass 200 kA, and the radiated electromagnetic pulse may reach electric field amplitudes of 50  ${\rm V\hbox{-}m}^{-1}$ at a horizontal range of 100 km from the lightning [Uman, 1987, p. 110].

The largest amounts of charge transfer between cloud and ground occur on slower timescales. Following a return stroke, especially those of positive (downward current) cloud-to-ground flashes, ``continuing current'' flows on time scales of several to hundreds of milliseconds and may transfer more than 300 C of charge to ground [Uman, 1987, p. 200]. Large continuing currents in lightning are associated with the production of sprites [Reising et al., 1996], as are storms with especially large reservoirs of charge, i.e., with large horizontal and vertical extent [Lyons, 1996]. Sprites are often recorded in association with mesoscale convective systems well over 100 km in horizontal extent [Lyons, 1996].

Sprites and elves have also for many years been reported in association primarily with large peak current positive cloud-to-ground flashes [e.g., Lyons et al., 1998]. This particular association later proved to be an issue of misinterpretation (see Sections 4.2 and 5.1) in the case of elves, but sprites are still thought to occur most often in a downward electric field following one or more positive cloud-to-ground strokes. Detailed measurements of small samples of lightning suggest that nearly all positive strokes are followed by significant continuing current and that this current is an order of magnitude greater than that following negative strokes [Uman, 1987, p. 210]. This distinction may well be the sole reason for the predominant association of sprites with positive (rather than negative) discharges. The median charge transfer for positive cloud to ground discharges was found to be 80 C, with 5% of flashes lowering more than 350 C [Uman, 1987, p. 210].

The National Lightning Detection Network [NLDN; Cummins et al., 1998b] analyzes radio pulses, or atmospherics (abbreviated to ``sferics''), from cloud-to-ground lightning in and near the continental United States. The processed data specify the location to within $\sim$300 m, the time of occurrence to within $\sim$1 $\mu$s, and the peak current of each recorded stroke. These data not only facilitate real-time sprite hunting in the United States; they also are invaluable for providing lightning locations which help in the interpretation of video, photometric, and radio recordings of upper atmospheric discharges. However, the NLDN is only 80 to 90% efficient in detecting cloud-to-ground lightning, and the accuracy of reported peak currents has only been ascertained for values below 60 kA [Cummins et al., 1998b; Cummins et al., 1998a].