Research On Lightning Fine Location Method And Channel Electrical Parameter Inversion Method Based On LF/VLF Fast Electric Field

Lightning is a strong discharge phenomenon in the atmosphere.The discharge process is accompanied by the movement and change of current.The "photoelectric,thermal,magnetic and chemical effects" caused by lightning are fundamentally related to the current in lightning channels.However,the channel current of lightning can be measured only in a few cases(such as the return stroke process of artificially triggered lightning),and the current of most processes in the cloud cannot be known.It can be said that lightning channel current is a bridge and link to study various atmospheric physical processes and phenomena related to lightning.How to build this bridge has always been a difficult problem in lightning physics research.In this paper,some preliminary explorations and attempts have been made in the theory of lightning channel current inversion method.Combining the TOA-based radiation location technology with the multi-station lightning electric field change analysis method,a set of lightning location technology,discharge theoretical model and channel current inversion method based on the LF/VLF fast electric field change measurement of lightning are preliminarily established.The main contents of this paper are as follows:(1)Research and application of lightning fine location method based on LF/VLF fast electric field;(2)Derivation of universal electric field transmission equation and establishment of three-dimensional transmission line model;(3)Validation of inversion method system based on artificially triggered lightning return stroke current;and(4)Inversion and analysis of channel current of regular pulse bursts(RPBs).The following conclusions are obtained:1)In this paper,the common problems of LF/VLF lightning location system are analyzed,and the solutions are put forward.A low frequency lightning location method based on full pulse recognition is proposed.We introduce the Hilbert-Huang Transform(HHT)based on Empirical Mode Decomposition(EMD)algorithm and applied lowfrequency filtering and high-frequency noise reduction to the waveform of the electric field changes recorded in 1 ms segments.This algorithm greatly improved the accuracy of the peak time extraction and the number of pulses in the LF/VLF band,which enhanced the accuracy in positioning the pulse of the electric field change.Compared with the previous algorithm,the algorithm can significantly reduce the time error,giving a better positioning result.With a time error estimate of 100 ns and a limit of goodness-of-fit less than 5,the number of pulse locations is increased by nearly seven times.The goodness-of-fit distribution of the pulse location results had a normal distribution and the 95% confidence interval of goodness-of-fit was 0–4;the corresponding positioning space error was <60 m.The continuity of the lightning channel was significantly improved and the development characteristics and fine structure of the lightning channel were clearly distinguished.2)Observational data from the LFEDA and radar were used to study the precise location of a fatal accident caused by a lightning in Conghua,Guangzhou,China,2017.Based on a comprehensive analysis,the lightning was concluded to be a negative “bolt from the blue” ground flash with seven return strokes.The first return stroke was on June 3 at 16:36:49.219 with a peak current intensity of-30.9 kA.The LFEDA system mapped the development of the lightning channel.The horizontal distance from the edge of the cloud anvil area(0 dBz)to the return stroke point was about 0.3 km,the horizontal distance from the edge of the precipitation area(18 dBz)to the return stroke point was about 1.8 km,and the return stroke channel extended as far as 3 km out of the cloud.There was no cloud cover above the head of the victim.The electric field waveform of the return strokes detected by the LFEDA system showed multi-peak characteristics,and all seven return strokes were multiple-termination strokes.The most probable mechanism for the lightning injuries was the side flash.The LFEDA system based on full pulse recognition plays an important role in the traceability analysis of lightning disaster events,and objectively proves the excellent performance of the new location method.3)Based on Jefimenko equation,a universal electric field transfer equation is derived in this paper.The charge density term in Jefimenko equation is subtracted and converted into a simple form only related to current density.It can be used to calculate the time-varying electric field generated by an arbitrary form of current source.It has a completely consistent form with the electromagnetic field equation derived by Uman et al.(1975)according to Maxwell equation under the vertical channel condition.On this basis,the traditional one-dimensional transmission line model is extended to threedimensional space,so that the transmission line model has a broader application scenario.The three-dimensional transmission line model is used to simulate and analyze the electric field pulse waveform generated by a return stroke.It is shown that the electric field component generated by the horizontal motion component of the current in the lightning return stroke channel is very important when the lightning channel is inclined.This shows the limitation of the traditional one-dimensional transmission line model and the necessity of expanding the transmission line model to three-dimensional space.The derivation of the universal electric field transmission equation is the basis of expanding the dimension of the transmission line model.A modified transmission line model with Kumaraswamy distribution of current in channel is introduced(MTLK),and the value of K distribution function is analyzed.The substitution of MTLK model for modified transmission line model with linear attenuation of channel current(MTLL)and modified transmission line model with exponential attenuation of channel current(MTLK)is demonstrated.It is considered that MTLK model with K distribution has more extensive physical significance and can replace MTLL and MTLE models.In practical application,the two parameters of K distribution function should be limited to(1,2)and(2,10),respectively.A modified transmission line model,3D-MTLK model,is proposed,in which the current in a threedimensional linear channel conforms to K distribution.4)A channel current inversion method based on MTLK model is constructed by using Particle Swarm Optimization(PSO)algorithm,which is optimized iteratively.The channel current is obtained by inversion of the measured electric field with a timewidth of 5?s in a triggered lightning process.The inversion results and the convergence curve of PSO process prove that the three-dimensional extension of the transmission line model is closer to the actual situation of the return stroke channel.The proposed three-dimensional transmission line model(3D-MTLK)and channel current inversion method can accurately invert the channel current in the return stroke process to a certain extent(non-folded and curved channels).5)The method system of electric field signal analysis,radiation source location and channel current inversion for lightning discharge events in clouds is established by using the lightning electric field waveform detected by LFEDA system.The waveform characteristics,pulse positioning and multi-station electric field inversion of a kind of electric field waveform called RPBs,which is ubiquitous in cloud and ground lightning discharge,are studied.The electric field waveform characteristics,channel development characteristics and channel current characteristics of an RPBs case are obtained.Compared with the current pulses such as Initial Breakdown Pulses(IBPs)and Narrow Bipolar Pulses(NBPs)of lightning,the current characteristics of these pulses are mainly characterized by smaller time and space scales,relatively smaller development speed and peak current intensity similar to IBPs.Through analysis,it is found that the signal of RPBs in LF/VLF band electric field detection equipment mainly appears between 80 and 220 kHz band,and the corresponding pulse signal width is about 4.5 to 12.5?s.The RPBs detected by the LF/VLF band detection system are pulse sequences superimposed on the ultra-low frequency electrostatic field waveforms between tens of kHz and hundreds of kHz in succession.The channel current of 63 groups of electric field pulse signals in RPBs waveform is inverted,and the channel characteristics and channel current characteristics in cloud which generate these electric field pulse signals are obtained.Statistical results show that the length of the sectional channel corresponding to a single electric field pulse is about 43 m at the minimum and 633 m at the longest,mainly distributed below 250 m,and the average length is 201.9 m.The development channel between the pulses is a cascade pattern of overlapping progressive forward development,and there are time intervals between the cascades.The average propagation speed of the 63 current pulses in the cloud channel is 1.22 10 8)/,and the distribution range is between 0.92~1.49 10 8)/.The standard deviation is 1.67 10 8)/,which is relatively small,indicating that the propagation speed is relatively stable.The peak current varies greatly from-20 kA to 160 kA,with an average value of 88.8kA.The rise time of these current pulses ranges from 1.54 to 2.79?s with an average of 1.93?s,while the decrease time ranges from 1.16 to 4.23?s with an average of 2.26?s,while the initial current duration ranges from 2.91 to 6.33?s.Generally speaking,the morphological characteristics of the initial current are quasi-symmetrical with a slightly larger falling time than rising time.The decay rate of pulse current in these channels is comparatively consistent.