Archive for category Negative Leaders
A recent result published in the science journal Nature caught my attention. It described “needle-like” structures on positive leader channels that pulse in trail of the positive leader tip.
Two discussions on this recent publication can be found at the links below.
By Earle Williams and Joan Montanyà
By University of Groningen
The article itself is available at the link below, but must be purchased or obtained through an institution.
Back in 2008 and 2010, we captured upward positive leaders propagating from towers and noticed some pulsing structures that developed in trail of the positive leader tip. I called these “thorns” as they reminded me of thorns on a rose bush stem.
These “thorns” appeared to form at the location where the positive leader unsuccessfully attempted to branch as seen by the splitting of the corona brush shown in the video below.
These thorns started pulsing at regular intervals and in one case developed into a negative leader that propagated away from the positive leader channel as seen in the video below.
We published a brief description of our observations in a conference paper, but did not pursue investigating them further as this was outside of our research focus at the time. The recent findings published in Nature shed further insight into the how these “needles” (or “thorns”) develop and possibly relate to channel decay, current cutoff and redevelopment as well as new branch development; a challenging but important research topic at present. With the increased number of investigators utilizing high-speed video cameras, I suspect more video examples of this behavior will soon surface, and along with correlated electric field sensing instrumentation data, may allow for better understanding of this interesting behavior.
On the evening of 16 July 2012, a weak cluster of storms moved north over Rapid City, South Dakota. A single visible rainshaft formed on the leading edge of the approaching development. At the time of the rainshaft formation, there was no lightning activity along the leading edge. However, lightning flashes were visible to the distant south in the more active trailing portion of the storms. At 04:20:35, (17 July 2012) UT two digital still cameras captured a ground flash near the rainshaft. This was the first visible flash along the leading edge. One camera, a Canon 5D2 Mark III, captured the image using a 16 mm lens set at f/2.8 using ISO 800 and an exposure time of 11 sec. This camera was capturing continuous 11 sec exposures for a timelapse sequence. A second camera, a Canon 7D, captured the image using a 20 mm lens set at f/8 using ISO 100 and an exposure time of 30 sec.
The captured images, which show the entire flash due to the long exposure times, showed a unique feature that I have not seen previously with any flash images that I have captured. The visible channels below cloud base show that there was a main vertical channel that connected with ground and a branch that propagated somewhat horizontally to the left and did not connect with ground. This second branch appeared to propagate toward the rainshaft and upon entering the rain, spread out vertically in both directions while branching extensively. The change in propagation direction and increase in branching appears isolated to inside the rainshaft, and is not apparent on any other channel segments.
An analysis of National Lightning Detection Network (NLDN) data revealed the NLDN recorded a corresponding 6.8 kA estimated peak current, negative cloud to ground stroke (-CG) 8 km southwest of the cameras. This location correlated in both time and direction, and all other preceding NLDN-indicated flash activity was south of the area by 20 km.
I believe that this image provides evidence that a negative leader branch propagated into a positively charged rainshaft that served as a positive potential well favorable for negative leader propagation (Coleman et al., 2003 and Coleman et al., 2008).
Coleman, L. M., T. C. Marshall, M. Stolzenburg, T. Hamlin, P. R. Krehbiel, W. Rison, and R. J. Thomas (2003), Effects of charge and electrostatic potential on lightning propagation, J. Geophys. Res., 108(D9), 4298, doi:10.1029/2002JD002718.
Coleman, L. M., M. Stolzenburg, T. C. Marshall, and M. Stanley (2008), Horizontal lightning propagation, preliminary breakdown, and electric potential in New Mexico thunderstorms, J. Geophys. Res., 113, D09208, doi:10.1029/2007JD009459.