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.

Using a new radio interferometric technique, we present observations of VHF lightning radiation source positions. The crossed base line interferometer provides the directions of arrival (azimuth and elevation) of VHF lightning radiation. Much flexibility exists in the choice of instrument parameters such as operating frequency and sampling interval. The present instrument operates at 34.3 MHz and provides an average position for every 2.5 μs of lightning radiation. Observations indicate that each 2.5 μs position should be attributed to a single discrete source in the sky. The positions of such sources define the temporal and spatial development of breakdown and rapid charge acceleration during the lightning flash. We have obtained VHF source positions correlated in time with electric field measurements. This allows us to identify the various processes occurring during a discharge and to compare the VHF phenomena with previously observed lightning phenomena. We have analyzed six flashes completely and portions of others. All six flashes contained ground strokes and two had intracloud discharges following the ground flash. We present data from three flashes that showed features common to the flashes we analyzed. The initial (preceding the first return stroke) and intracloud portions consisted of a large number of bursts of 20 μs average duration occurring on the average every 100 μs. The positions within each burst developed in a systematic, nearly linear, sequence with apparent speeds of ∼107 m/s. The relative positions from burst to burst drifted through the sky with speeds ∼105 m/s. In intracloud portions this drift was predominantly horizontal. In ground flashes the VHF sources drifted downward, in effect the leader for the first return stroke. They terminated at the return stroke.

We present a new technique for measuring the VHF radio centroid of nearby lightning flashes at 5‐μs intervals. Its ability to provide continuous positions during long ( true>˜ 100 μs) emissions is, we believe, new and reveals new information about the discharge process. The new technique solves many of the data‐handling problems in old techniques. We have built and demonstrated this technique in one angular coordinate of the lightning flash. We present data from five flashes showing complex positional and motional patterns. The breakdown phase consists of many impulses. The average speed from impulse to impulse lies in the range of 10–100 km s−1. During individual impulses, speeds measure from one to several tens of thousands of kilometers per second. At times of strong VLF bursts there is usually a similar VHF burst. Its speed is like the speeds of individual impulses. We identify VLF‐associated VHF burst sources with the main electrical current flow in lightning flashes. We identify the motion from one impulse to another in the breakdown phase as being caused by avalanching electrons accelerating along paths soon to become the discharge paths within thunderclouds. The high speeds in impulses represent the gross current flow in breakdown channels not yet large enough to create large VLF emissions or flashes.

By using a crossed base line interferometer, lightning VHF source positions correlated in time with electric field change measurements have been obtained. We present data in this paper showing azimuth and elevation pictures with high time resolution of the VHF (34.3 MHz) radiation for events near the time of return strokes. From their common characteristics we infer spatial and temporal features of these processes. No radiation was seen that could be attributed to the return stroke wave front itself. Radiation from stepped leaders continued up to and, in some cases, after initiation of the first return stroke. A strong burst of radiation often followed the first return stroke. Strong radiation began before subsequent return strokes, coinciding with the electric field change usually associated with the dart leader. The end of the radiation was independent of the return stroke. The source of this radiation was a well‐defined region within the cloud. A second burst often followed the return stroke located in the same region as that prior to the stroke. Sources from stroke to stroke showed horizontal displacement with little apparent change in elevation.

Lightning discharges radiate strongly at radio frequencies, and studies of the radiation provide valuable information on lightning breakdown processes. Interferometric techniques can be used to locate the numerous radiation events as a function of time during a discharge and to generate images of the developing lightning channels inside a storm, where they are obscured from view at optical wavelengths.