Response of the systems for lightning generated high frequency wave

Abstract

Responseof the Systems for Lightning Generated High Frequency Wave Velauthampillai Jeyanthiran HFradiationsassociated with lightning generated by tropical and temperate thunderstorm werestudiedto elaborate the existing knowledge of interaction of the HF radiation due to lightningwith structures.The study concentrateson the temporal behavior of the 3,5 and 10 MHzHFradiationgeneratedby both ground and cloud flashesin tropics(Sri Lanka) and 10 MHzHFradiationgeneratedby cloud flashesin temperateregion (Sweden). TheHF radiation was observed with the onset of preliminary breakdown (PB) activity and return stroke together with corresponding broad band electric field changes for ground flashespertaining to tropical thunderstorms. The PB activity has been found to radiate the HFradiations intermittently and found to be stronger in the return stroke stage compared to thoseatthe PB stage. The mean duration of PB activitywas found to be 3.5 msfor both HF andbroadband fields. The mean time betWeenRS to predominant PB pulse was 6.5 ms. Thetropical cloud flashes were observed to begin with a large electrostatic field change with submicro-second scale electric field pulses embedded in it. The corresponding HF radiation isfoundto begin with the onset of electric field change. It was observed that the amplitude of the high frequency radiations to be maximum at the initial stage with a gradual decay. This clearlysupports the two stage model of cloud flashes in which the initial stage is.the very active stage. Theanalysisof HF radiation at 10 MHz correspondingto cloud flashes pertinent to the temperatethunderstormsindicatesthat the HF radiations at 10 MHz are initiated.with the initiationofthe cloud flasheswithoutany significantdelay. Furthermore,the temperatecloud flashesalsohavebeen foundto radiatetheHF radiationintermittentlythroughoutthe flash.In themajorityof temperatecloud flashesthe amplitudeof HF radiationswas foundhigher at the beginning. Ananalysis of the amplitudes of the HF radiation and corresponding broadband electric field pulsesreveals weak correlations with the' amplitudes of the corresponding broadband fields whilea strong correlation was found among the amplitudes of the HF radiation with each other. TheHF response for a smalJ structure has also been computed using transmission line theoryby applying the broadband electric field on the structures. It is observed that the signatureof the HF radiation and the response of the structure to the broadband electric fieldshowpeaks locatedat the same instantof time. This indicatesthat the inducedvoltages showcharacteristicsimilarto the 3MHz- 10MHzradiationassociatedwiththe lightning flash.Thus, these radiations can be used as a vehicle to identify the sections of the broadbandelectric fields that are important in the study of the interaction of lightning electromagneticfieldswith structures. Thetrendof lightningactivity over Sri Lanka is of interestto the lightningcommunityand hasalso been analysedin this study.An analysis of seasonal lightning flash density shows thatthe first inter monsoon has the maximumdensity. The mean monthly lightning flash countsshowsthat the most of the lightningactivityoccur from March to May with a peak in April.Thediurnalvariationof maximumflashrate is observedto peak at 1630LT