Stepped-to-dart leaders preceding lightning return strokes

摘要

Using time-correlated high-speed video and electric field change data, three cases are described in which dart leaders toward ground are immediately preceded by stepped leaders that find and use previously used channels. These "stepped-to-dart leaders" occur in three natural negative ground flashes. Prior to the stepped-to-dart connection, the leaders have characteristics of stepped leaders, including average two-dimensional speeds of 1.6-2.7 × 10~5 m s~(-1) (visible from 5.5, 3.4, and 0.9 km altitude). After the connection, they behave as dart (or dart-stepped) leaders, with larger amplitude E-change pulses and faster average speeds of 3.4-7.8 × 10~6 m s~(-1). Connection altitudes are 3.32, 1.57, and 0.75 km. Immediately after the connection, there is a brief lighting in a short part of the prior return stroke channel. Luminosity travels up the stepped leader path after the connection, while the dart leader proceeds toward ground. In two cases, all the strokes subsequent to the stepped-to-dart stroke follow the visible portion of its channel. The other case has two subsequent strokes which do not reuse any portion of the stepped-to-dart channel. For the other 12 strokes in these flashes, stepped leader average speeds range from 1.7 to 3.0 × 10~5 m s~(-1), and dart leader average speeds are 0.82 to 16.67 × 10~6 m s~(-1). Overall, the return stroke waveforms give reasonable indication of the type of leader that preceded the stroke. Stepped-to-dart leaders are apparently rare in optical data, occurring in about 1% of subsequent strokes and 2.5% of flashes. Key Points Video data show stepped leaders can connect to prior channels and dart to groundLeader character changes from stepped to dart within 0.04 ms after connectionAfter the leader connection, luminosity also travels up the stepped leader path