When does the lightning attachment process actually begin?

Abstract

High-speed video and electric field records of 43 first and 7 new-ground-termination subsequent strokes in negative lightning flashes, obtained at the Lightning Observatory in Gainesville, Florida, were examined. Eighteen (36%) of these strokes exhibited faintly luminous formations (FLFs) below the downward leader tip just prior to the return stroke. All the 18 FLFs were connected to the strike object on the ground, with 14 of them being also in contact with the downward leader tip. For 11 located events showing FLFs in contact with the downward leader tip, the FLF 2-D length ranged from 51 to 200 m and the time interval between the end of exposure of the FLF frame and the return stroke onset ranged from 42 to 106 µs. We inferred that the FLFs mostly consisted of relatively low conductivity streamers, based on the following observations and estimates. (a) The rates at which the 11 FLFs were replaced by channels capable of guiding return stroke waves ranged from 0.7 × 106 m/s to 2.2 × 106 m/s, which is comparable to leader speeds in virgin air (in the same records). (b) The overwhelming majority of downward leaders continued stepping while being connected to ground via FLFs. (c) The average electric field along FLFs is 1 to 2 orders of magnitude higher than expected for hot, leader-like channels. Since in all cases the FLF determined the strike point, we conclude that its inferred initiation up to a few hundreds of microseconds before the return stroke onset signified the beginning of the lightning attachment process.