NEW INSIGHTS INTO LIGHTNING PROCESSES GAINED FROM TRIGGERED-LIGHTNINGEXPERIMENTS IN FLORIDA AND ALABAMA

Analyses of electric and magnetic fields measured at distances from te ns to hundreds of meters from the ground strike point of triggered lig htning at Camp Blanding, Florida, and at 10 and 20 m at Fort McClellan , Alabama, in conjunction with currents measured at the lightning chan nel base and with optical observations, allow us to make new inference s on several aspects of the lightning discharge and additionally to ve rify the recently published ''two-wave'' mechanism of the lightning M component. At very close ranges (a few tens of meters or less) the tim e rate of change of the final portion of the dart leader electric fiel d can be comparable to that of the return stroke. The variation of the close dart leader electric field change with distance is somewhat slo wer than the inverse proportionality predicted by the uniformly charge d leader model, perhaps because of a decrease of leader charge density with decreasing height associated with an incomplete development of t he corona sheath at the bottom of the channel. There is a positive lin ear correlation between the leader electric field change at close rang e and the succeeding return stroke current peak at the channel base. T he formation of each step of a dart-stepped leader is associated with a charge of a few millicoulombs and a current of a few kiloamperes. In an altitude-triggered lightning the downward negative leader of the b idirectional leader system and the resulting return stroke serve to pr ovide a relatively low-impedance connection between the upward moving positive leader tip and the ground, the processes that follow likely b eing similar to those in classical triggered lightning. Lightning appe ars to be able to reduce, via breakdown processes in the soil and on t he ground surface, the grounding impedance which it initially encounte rs at the strike point, so at the time of channel-base current peak th e reduced grounding impedance is always much lower than the equivalent impedance of the channel. At close rangers the measured M-component m agnetic fields have waveshapes that are similar to those of the channe l-base currents, whereas the measured M-component electric fields have waveforms that appear to be the time derivatives of the channel-base current waveforms, in further confirmation of the ''two-wave'' M-compo nent mechanism.