PROPERTIES OF M-COMPONENTS FROM CURRENTS MEASURED AT TRIGGERED LIGHTNING CHANNEL BASE

Channel base currents from triggered lightning were measured at the NA SA Kennedy Space Center, Florida, during summer 1990 and at Fort McCle llan, Alabama, during summer 1991. An analysis of the return stroke da ta and overall continuing current data has been published by Fisher et al. [1993]. Here an analysis is given of the impulsive processes, cal led M components, that occur during the continuing current following r eturn strokes. The 14 flashes analyzed contain 37 leader-return stroke sequences and 158 M components, both processes lowering negative char ge from cloud to ground. Statistics are presented for the following M current pulse parameters: magnitude, rise time, duration, half-peak wi dth, preceding continuing current level, M interval, elapsed time sinc e the return stroke, and charge transferred by the M current pulse. A typical M component in triggered lightning is characterized by a more or less symmetrical current pulse having an amplitude of 100-200 A (2 orders of magnitude lower than that for a typical return stroke [Fishe r et al., 1993]), a 10-90% rise time of 300-500 mu s (3 orders of magn itude larger than that for a typical return stroke [Fisher et al., 199 3]), and a charge transfer to ground of the order of 0.1 to 0.2 C (1 o rder of magnitude smaller than that for a typical subsequent return st roke pulse [Berger et al., 1975]). About one third of M components tra nsferred charge greater than the minimum charge reported by Berger et al. [1975] for subsequent leader-return stroke sequences. No correlati on was found between either the M charge or the magnitude of the M com ponent current (the two are moderately correlated) and any other param eter considered. M current pulses occurring soon after the return stro ke tend to have shorter rise times, shorter durations, and shorter M i ntervals than those which occur later. M current pulses were observed to be superimposed on continuing currents greater than 30 A or so, wit h one exception out of 140 cases, wherein the continuing current level was measured to be about 20 A. The first M component virtually always (one exception out of 34 cases) occurred within 4 ms of the return st roke. This relatively short separation time between return stroke and the first M component, coupled with the observation of Fisher et al. [ 1993] that continuing currents lasting longer than 10 ms never occur w ithout M current pulses, implies that the M component is a necessary f eature of the continuing current mode of charge transfer to ground.