Propagation of positive,negative,and recoil leaders in upward lightning flashes

Leaderpropagationisafundamentalissueinlightningphysics.Thepropagationcharacteristicsofpositiveleadersandnegativeleadersaresummarizedandcomparedbasedondatafromhigh-speedcameraandelectromagneticfieldinrocket-triggeredlightningandtower-initiatedlightningdischarges;availablechannelbasecurrentdatarecordedinrocket-triggeredlightningarealsoused.Thenegativeleaderspropagateinasteppedfashionaccompaniedbymanybranches.Thestemsaheadofthenegativeleadertipdeterminethemanneranddirectionoftheleaderpropagation,andeventhebranchingandwindingofthelightningchannel.Theimpulsivecurrent,electromagneticfield,andrelatedopticalimagessuggestthatthepositiveleadermaydevelopinastep-likefashionatitsinitialstageoftriggeredlightning.However,thesteppingprocessesofthepositiveleaderareobviouslydifferentfromthoseofthenegativeleader.Tower-initiatedlightningrevealedthatthemostconspicuouscharacteristicsofthestepwisepositiveleaderinvolvetheintermittentbrush-likecoronazoneinfrontoftheleadertipandtheluminosityenhancementofthechannelbehindthetip.Inrockettriggeredlightningflashes,thechargetransferredduringanindividualstepforthenegativeleaderwasnearlyanordergreaterthanforthepositivecounterpart.Thesuccessivestreamersaheadoftheleadertipareessentialforbothnegativeandpositiveleaderpropagation,andthestemscouldbeformedfromoneormorestreamersinthepreviousnegativestreamerzonewiththemainleaderchanneldim.High-resolutionobservationoftowerlightningalsorevealedanewtypeofbidirectionalrecoilleader,withpolaritycontrarytothetraditionalone,traversinginnegativechannelsassociatedwithtower-initiatedandrocket-triggeredlightning.