RF and X-ray source locations during the lightning attachment process

Using an eight-station array of electric field derivative (dE/dt) sensors and colocated NaI X-ray detectors, we have obtained 3-D RF source locations during the leaders and attachment processes of three first strokes initiated by stepped leaders in natural cloud-to-ground lightning and one stroke initiated by a dart-stepped leader in a rocket-and-wire triggered flash. Stepped leader and dart-stepped leader dE/dt pulses are tracked from a few hundred meters to a few tens of meters above ground, after which pulses of different characteristics than the step pulses are observed to occur at lower altitudes. These postleader pulses include: (1) the "leader burst,'' a group of pulses in the dE/dt waveform occurring just prior to the slow front in the corresponding return-stroke electric field waveform; (2) dE/dt pulses occurring during the slow front; and (3) the fast-transition or dominant dE/dt pulse that is usually associated with the rapid transition to peak in the return-stroke electric field waveform. Additionally, the timing coincidence between X rays and dE/dt pulses on colocated measurements is used to examine the X-ray production by the postleader processes. Leader bursts (LBs) are the largest X-ray producers of the three postleader processes and exhibit propagation speeds that exceed the preceding stepped leader speeds by more than an order of magnitude. Slow-front (SF) and fast-transition pulses appear to originate from similar physical processes, probably the multiple connections of upward and downward leaders. However, more X-rays are coincident with slow-front pulses than with fast-transition pulses.