Observations of lightning phenomena using radio interferometry

A radio interferometer system is described which utilizes multiple baselines to determine the direction of lightning radiation sources with an angular resolution of a few degrees and with microsecond time resolution. An interactive graphics analysis procedure is used to remove fringe ambiguities from the data and to reveal the structure and development of lightning discharges inside the storm. Radiation source directions and electric field waveforms have been analyzed for different types of breakdown events for two lightning flashes. These include the initial breakdown and K type events of in-cloud activity, the leaders of initial and subsequent strokes to ground, and activity during and following return strokes. Radiation during the initial breakdown of one flash was found to consist of intermittent, localized bursts of radiation that were slow moving. Source motion within a given burst was unresolved by the interferometer but was detected from burst to burst, with negative charge being transported in the direction of the breakdown progression. Radiation during initial leaders to ground was similar but more intense and continuous and had a characteristic intensity waveform. Radiation from in-cloud K type events is essentially the same as for dart leaders; in both cases it is produced at the leading edge of a fast-moving negative streamer that propagates along a well-defined, often extensive, path. K type events are sometimes terminated by a fast field change that appears analogous to the field change of a return stroke. Dart leaders are sometimes observed to die out before reaching ground; these are termed “attempted leaders” and, except for their greater extent, are no different than K type events. Several modes of breakdown during and after return strokes have been documented and analyzed. One mode corresponds to the launching of a positive streamer away from the upper end of the leader channel, apparently as the return stroke reaches the leader start point. In another mode, the quenching of the dart leader radiation upon reaching ground reveals concurrent breakdown in the vicinity of the source region for the leader. In both instances the breakdown appears to establish channel extensions or branches that are followed by later activity of the flash. Finally, a new type of breakdown event has been identified whose electric field change and source development resemble those of an initial negative leader but which progresses horizontally through the storm. An example is shown which spawned a dart leader to ground.