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.