Radio interferometric observations of cloud‐to‐ground lightning phenomena in Florida

Abstract

Radio frequency observations of cloud‐to‐ground lightning in Florida have been analyzed to document a number of features of the lightning. The observations have been made using an interferometer system which determined the direction to the lightning radiation as a function of time during close lightning discharges. Observations are presented for about 50 radiating events during five multiple‐stroke, normal‐polarity flashes to ground. The results confirm and extend results of a similar study of New Mexico lightning by Rhodes et al. (1994). Dart leaders, in‐cloud “K” events, and attempted leaders are found to be the same phenomena, namely, a negative‐polarity streamer that propagates horizontally and/or vertically to ground at estimated speeds of about 107 ms−1 down to 106 ms−1. The step or impulsive fast electric field change produced by some K events is found to be caused by positive breakdown back at the starting point of the negative streamer, after the streamer has progressed over some or all of its distance. This breakdown appears to initiate a “forward” stroke along the streamer channel which can renew the breakdown at the front end of the channel. K breakdown which happens to occur during a continuing current discharge to ground produces an M event (channel brightening) when it connects with the conducting channel to ground. The step fast field change of the M events is found to be produced at the time of connection inside the cloud; no fast field change is detected when this breakdown reaches ground that would be associated with a return stroke. M events can also be initiated by fast (107 ms−1) positive streamers which often propagate away from the leader source region within a few milliseconds of the arrival of a return stroke back in the source region. These streamers often generate an even faster negative recoil event back along their extent which propagates down the channel to ground, as the M event. In all cases the current increase or channel brightening produced by K and M events is predicted to occur in a forward direction, that is, toward or down the channel to ground, in the same direction as the initiating negative streamer. Positive breakdown is detected only immediately following return strokes or during some K streamers. In both cases the electric stresses are expected to be very large, suggesting that positive breakdown which produces a conducting channel is difficult to initiate. Results for one flash suggest that negative charge was left at low altitude by the initial stroke to ground; this forced the subsequent leaders to develop a new path to ground, first as series of attempted leaders and then as a dart‐stepped leader.

Compact Intracloud Discharges

“…It has been previously been observed that detectable HF and VHF radiation from thunderstorms is produced primarily by breakdown processes and not by charge flow along already conducting channels [Rhodes et al, 1994;Shao et al, 1995]. This result is exemplified by the absence of detectable RF radiation during the initial parts of return strokes initiated by dart leaders and during long intervals of continuing current discharges to ground.…”

“…Based on their results and the fact that the RF radiation from CIDS is very powerful (20 dB brighter than that produced by other lightning breakdown processes), it might be concluded that the strongly radiating part of the streamer results from negative breakdown progressing in an upward direction and transporting negative charge upward. On the other hand, it was also concluded by Shao et al [1995] that positive breakdown that produces a conducting channel is difficult to initiate. As will be shown shortly, electric field strengths just prior to CIDS are comparable to or greater than the highest electric fields that have been measured within thunderstorms.…”

“…Whether positive-polarity breakdown produces powerful or even detectable RF radiation has been a subject of much debate [J4a.zw, 1989]. However, the observations of Shao et al [1995] show that positive breakdown events are observed that radiate strongly at radio frequencies. Propagation speeds of positivepolarity streamers were estimated by Mao et al [1995] to fall in the range from 1 x 107 to 6 x 107 m/s.…”

3D imaging of lightning channel and leader progression velocity

“…At each R in the diagram, a sharp change toward positive polarity was observed for several microseconds during the buildup of electric field strength; since the positive change prevails, this example presents a negative cloud-to-ground discharge with a multiplicity of 6. Regarding UHF radiation, a strong emission caused by return stroke is observed as well as a preceding emission that is related to the leader progression, which is a feature of negative cloud-to-ground strokes [3,12]. Figure 4 shows a 3D image of the lightning channel in the time region corresponding to the first leader progression marked L1 in Fig.…”

VHF broadband digital interferometer

“…When the phase difference goes through more than one cycle in a high-frequency component, there is the possibility of a fringe ambiguity [4][5][6]. We remove the ambiguity using the feature that the phase difference should be linearly dependent on frequency.…”