Editing Lightning (section)

==== Lightning leaders ====
[[File: Lightning formation.gif|thumb|A downward leader travels towards earth, branching as it goes.]]
[[File:Leaderlightnig.gif|thumbnail|Lightning strike caused by the connection of two leaders, positive shown in blue and negative in red]]

In a process not well understood, a bidirectional channel of [[ionized]] air, called a "[[leader (spark)|leader]]", is initiated between oppositely-charged regions in a thundercloud. Leaders are electrically conductive channels of ionized gas that propagate through, or are otherwise attracted to, regions with a charge opposite of that of the leader tip. The negative end of the bidirectional leader fills a positive charge region, also called a well, inside the cloud while the positive end fills a negative charge well. Leaders often split, forming branches in a tree-like pattern.<ref>Ultraslow-motion video of stepped leader propagation: [http://www.ztresearch.com/ ztresearch.com] {{webarchive|url=https://web.archive.org/web/20100413125231/http://www.ztresearch.com/ |date=April 13, 2010 }}</ref> In addition, negative and some positive leaders travel in a discontinuous fashion, in a process called "stepping". The resulting jerky movement of the leaders can be readily observed in slow-motion videos of lightning flashes.

It is possible for one end of the leader to fill the oppositely-charged well entirely while the other end is still active. When this happens, the leader end which filled the well may propagate outside of the thundercloud and result in either a cloud-to-air flash or a cloud-to-ground flash. In a typical cloud-to-ground flash, a bidirectional leader initiates between the main negative and lower positive charge regions in a thundercloud. The weaker positive charge region is filled quickly by the negative leader which then propagates toward the inductively-charged ground.

The positively and negatively charged leaders proceed in opposite directions, positive upwards within the cloud and [[Electric charge|negative]] towards the earth. Both ionic channels proceed, in their respective directions, in a number of successive spurts. Each leader "pools" ions at the leading tips, shooting out one or more new leaders, momentarily pooling again to concentrate charged ions, then shooting out another leader. The negative leader continues to propagate and split as it heads downward, often speeding up as it gets closer to the Earth's surface.

About 90% of ionic channel lengths between "pools" are approximately {{convert|45|m|ft|abbr=on}} in length.<ref>Goulde, R.H. (1977) "The lightning conductor", pp. 545–576 in ''Lightning Protection'', R.H. Golde, Ed., ''Lightning, Vol. 2'', Academic Press.</ref> The establishment of the ionic channel takes a comparatively long amount of time (hundreds of [[millisecond]]s) in comparison to the resulting discharge, which occurs within a few dozen microseconds. The [[electric current]] needed to establish the channel, measured in the tens or hundreds of [[ampere]]s, is dwarfed by subsequent currents during the actual discharge.

Initiation of the lightning leader is not well understood. The electric field strength within the thundercloud is not typically large enough to initiate this process by itself.<ref>{{cite journal|doi=10.1007/s11214-008-9338-z|title=Charge Structure and Dynamics in Thunderstorms|date=2008|last1=Stolzenburg|first1=Maribeth|last2=Marshall|first2=Thomas C.|journal=Space Science Reviews|volume=137|issue=1–4|page=355|bibcode = 2008SSRv..137..355S |s2cid=119997418}}</ref> Many hypotheses have been proposed. One hypothesis postulates that showers of relativistic electrons are created by [[cosmic rays]] and are then accelerated to higher velocities via a process called [[runaway breakdown]]. As these relativistic electrons collide and ionize neutral air molecules, they initiate leader formation. Another hypothesis involves locally enhanced electric fields being formed near elongated water droplets or ice crystals.<ref>{{cite journal|doi=10.1029/2007JD009036|title=A brief review of the problem of lightning initiation and a hypothesis of initial lightning leader formation|date=2008|last1=Petersen|first1=Danyal|last2=Bailey|first2=Matthew|last3=Beasley|first3=William H.|last4=Hallett|first4=John|journal=Journal of Geophysical Research|volume=113|issue=D17|pages=D17205|bibcode = 2008JGRD..11317205P }}</ref> [[Percolation theory]], especially for the case of biased percolation,<ref>{{cite journal|doi=10.1103/PhysRevE.81.011102|pmid=20365318|title=Biased percolation on scale-free networks|date=2010|last1=Hooyberghs|first1=Hans|last2=Van Schaeybroeck|first2=Bert|last3=Moreira|first3=André A.|last4=Andrade|first4=José S.|last5=Herrmann|first5=Hans J.|last6=Indekeu|first6=Joseph O.|journal=Physical Review E|volume=81|issue=1|page=011102|bibcode = 2010PhRvE..81a1102H |arxiv = 0908.3786 |s2cid=7872437}}</ref> {{clarify| what does 'biased percolation' mean?|date= July 2013}} describes random connectivity phenomena, which produce an evolution of connected structures similar to that of lightning strikes. A streamer avalanche model<ref>{{Cite journal|last1=Griffiths|first1=R. F.|last2=Phelps|first2=C. T.|date=1976|title=A model for lightning initiation arising from positive corona streamer development|journal=Journal of Geophysical Research|volume=81|issue=21|pages=3671–3676|doi=10.1029/JC081i021p03671|bibcode=1976JGR....81.3671G}}</ref> has recently been favored by observational data taken by LOFAR during storms.<ref>{{Cite journal|last1=Sterpka|first1=Christopher|last2=Dwyer|first2=J|last3=Liu|first3=N|last4=Hare|first4=B M|last5=Scholten|first5=O|last6=Buitink|first6=S|last7=Ter Veen|first7=S|last8=Nelles|first8=A|date=November 24, 2021|title=The Spontaneous Nature of Lightning Initiation Revealed|journal=Ess Open Archive ePrints |volume=105 |issue=23 |pages=GL095511 |doi=10.1002/essoar.10508882.1|bibcode=2021GeoRL..4895511S |s2cid=244646368|url=https://bib-pubdb1.desy.de/record/474239 |hdl=2066/242824|hdl-access=free}}</ref><ref>{{Cite web|last=Lewton|first=Thomas|date=December 20, 2021|title=Detailed Footage Finally Reveals What Triggers Lightning|url=https://www.quantamagazine.org/radio-telescope-reveals-how-lightning-begins-20211220/|access-date=December 21, 2021|website=Quanta Magazine}}</ref>