Mahendra Fernando

Propagation of lightning generated transient electromagnetic fields over finitely conducting ground

Abstract This paper elucidates the propagation effects on lightning generated electric fields. The mathematics of the problem is described and a simple procedure that can be used to predict the propagation effect is outlined together with its experimental confirmation. This procedure is used to analyse the propagation effects, over distances less than about 300 km, on the radiation fields of negative first return strokes, positive first return strokes and subsequent return strokes in triggered lightning flashes. From the results, empirical equations that can be used to correct for the propagation effects are extracted. The results show that the attenuation due to propagation effects of the initial peak amplitudes may differ in negative and positive return strokes and that the data from triggered lightning flashes should be applied with caution in correcting for the propagation effects of natural lightning flashes.

The lightning radiation field spectra of cloud flashes in the interval from 20 kHz to 20 MHz

Electric radiation fields produced by lightning cloud flashes have been analyzed using Fourier methods to obtain amplitude spectra for frequencies in the range of 20 kHz to 20 MHz. Time to thunder measurements were used to extract the distances to the cloud flashes from the measurement site. The spectra were generated by analyzing the first 10 ms time window of cloud flashes with 10 ns sampling resolution, and they show f-1 frequency dependence up to 2 MHz followed by f-2 dependence and higher for frequencies above 2 MHz. The results agree very well with previous measurements carried out for individual pulses produced by various lightning processes. By utilizing digital filters, it has been shown that measurements taken with narrowband filters agree with the results produced under wideband measurements

The fine structure of positive lightning return-stroke radiation fields

The electric fields generated by lightning flashes striking the North Sea were measured, with a time resolution better than a few tens of nanoseconds, at Fano island in Denmark. The measuring station was located a few tens of meters away from the high water mark at the west coast of the island. This particular location made it possible to capture, with minimal propagation effects, the electromagnetic fields from lightning flashes striking the North Sea. The waveforms were recorded by a measuring system that could provide a time resolution of about 10 ns. The data recorded had the following features. The initial rising part of the positive return-stroke fields contains a slow front followed by a fast transition. The duration of the slow front of the positive return-stroke fields had an average of 8.3 /spl mu/s and its amplitude, measured as a fraction of the initial peak, had an average of 0.61. The 10%-90% rise time of the fast rising portion of the positive return-stroke fields was about 0.26 /spl mu/s, on average. The average peak value of the measured radiation fields normalized to 100 km was 15.7 V/m. The mean of the peak value of the time derivative of the radiation fields was 25 V/m//spl mu/s. The full width at half maximum of the radiation field derivative had a mean of 170 ns.

Comparative study on preliminary breakdown pulse trains observed in Malaysia and Florida

In this paper, the preliminary breakdown pulse (PBP) trains preceding the negative first return strokes recorded using a broad band antenna system in Johor Bahru, Malaysia and Florida, United States were analyzed. In lightning flashes observed in Malaysia, the ratio between the amplitude of the largest preliminary breakdown pulse (PBP) and the peak return stroke electric field (RS) was 27.8% and the corresponding value in Florida was 29.4%. The time interval between the beginning of the preliminary breakdown sequence and the initiation of the return stroke was 57.6 ms in Malaysia and 22 ms in Florida. The results of the study support the hypothesis that the PBP/RS ratio increases with increasing latitude.

Interaction of multiple connecting leaders issued from a grounded structure simulated using a self consistent leader inception and propagation model (slim)

A self consistent lightning connecting leader inception and propagation model is utilized to study the effect of multiple connecting leaders on the attachment of lightning flashes to grounded structures. In the present study the results obtained in the case of a grounded structure equipped with two lightning rods were presented. As the stepped leader approaches the grounded structure, connecting leaders are issued from both lightning conductors but depending on the location of the stepped leader only one of them succeed in completing the attachment process. Results show that the presence of a small advantage for the growth of a connecting leader from one conductor may drastically reduce the ability of other conductors to launch successful connecting leaders. Results also show that the effect of the competition between multiple leaders is to delay the attachment process.

Time-frequency analysis of narrow bipolar pulses observed in Sri Lanka

Narrow Bipolar Pulses (NBP) of a single thunderstorm from a coastal location in Sri Lanka were subjected to S-transformation in order to study their time-frequency information. This study is an extension to a similar study carried out by the authors where properties of NBPs were analyzed using the wavelet transformation. Keeping in line with the previous study, 18 events pertaining to equal number of positive and negative NBPs with the narrowest temporal widths were studied. The data was obtained from the southern coastal area of Sri Lanka (Matara -5.95° N, 8.53° E) from a highly active thunderstorm, which occurred during the month of May in 2013. The waveforms were recorded with a 10 ns resolution within a 100 ms time window. The spanning (width) and the ratio of peak power amongst the initial and overshoot pulses were measured and compared for each polarity. The negative NBPs had an average spanning of 134-371 kHz. The overshoot of the same had an average range of 127-255 kHz. The positive NBPs had an average spanning of 103-245 kHz. The overshoot of the same had an average range of 102-195 kHz. The ratio of peak power of NBPs to overshoot had an average of 1.08 for both positive and negative pulses. The spectrogram reveals the initial and overshoots pulses to be relatively equal in power intensities.

High Speed Optical Observations of Cloud Flashes

Luminosity variations of natural cloud lightning channels were studied using high speed video recordings in Sri Lanka in the tropic. Five cloud flashes were selected for the study, which were recorded with 5000 fps with 512X512 pixel resolution. Luminosity variation along channels and along selected channel segment, duration of cloud activities was studied. It was found that the average duration of selected flashes is 266.44 ms. No particular pattern was observed for the luminosity variation along channel segments. However the luminosity variation across the channel followed a Gaussian distribution pattern. Pattern of the luminosity variation along channels were compared with existing models to understand the development process of cloud flashes. Results show that it is hard to collaborate the observation to a single process for cloud discharges.

Electric field signatures of narrow negative bipolar pulse activities from lightning observed in Sri Lanka

Isolated Narrow Bipolar Pulses (NBP) have been observed in the past in sub tropical and tropical regions. This study presents detail electric field characteristics of NBP pulses observed in Sri Lanka in the tropic. NBP analyzed in this work were recorded at Matara (5.95 °N, 8.53 °E), southern coast of Sri Lanka from four highly active thunderstorm days in May 2013. The waveforms were recorded with a 10 ns resolution within a 100 ms time window. In contrary to previous observations at the same geographical region, both positive and negative NBP pulses were observed in this study. Parameters related to Narrow Negative Bipolar Pulses (NNBP) are presented in this study since majority of the observations were in that category. The parameters measured for NNBPs were rise time (Tr), zero crossing time (Tz), the duration of slow front (Ts), the full width at half maximum (FWHM) and the ratio of amplitude of overshoot to the corresponding peak amplitude (Os/Pa). The corresponding average values for these parameters were found to be 0.49 μs, 2.83 μs, 0.18 μs, 1.18 μs and 0.17 respectively. The above values conforms to a much narrower bipolar pulses when compared to values reported in earlier studies.

Features of K-Changes Observed in Sri Lanka in the Tropics

General characteristics of K-changes, including their duration and probability of occurrence associated with ground flashes in Sri Lanka in the tropics, together with their fine structure, are presented. In 98 ground flashes where the small step changes associated with K-changes are clearly visible, there were about two K-changes per flash on average. The mean K-change time duration observed in this study is 0.38 ms. In 53 of the ground flashes, there were 120 consecutive K-changes. In these cases, the geometric mean of the time interval between K-changes was 12 ms. Analysis of the fine structure of the K-changes reveals the K-changes are always associated with either a chaotic pulse train or a combination of chaotic and regular pulse trains. The results suggest that the small step-like static electric fields identified in the literature as K-changes are the step-like static fields associated with the processes that generate chaotic or a combination of chaotic and regular pulse trains. Thus, at larger distances where the static fields are negligible, K-changes may appear as a chaotic pulse train or a combination of chaotic and regular pulse trains.

New circuit for the measurement of lightning generated electric fields

In many studies of lightning flash characterization, the lightning generated electric field measuring system uses a circuit based on the buffer-amplifier LH0033. This circuit was proposed 35 years ago and there is no doubt of its excellent performance to measure the lightning electric field intensity. However, LH0033 is no longer manufactured, making it necessary to find a replacement. This paper proposes an improvement of the electric field measuring system, which consists of a new electronic circuit that uses the buffer-amplifier BUF602. This circuit presents a similar performance of the classic circuit, but has a lower energy consumption and allows its implementation in surface mount technology, which reduces the space required for the circuit. The field tests were performed in Bogotá, Colombia during a complete thunderstorm season and their results show that new circuit is a good alternative for measuring electric field signals with a bandwidth up to 11 MHz.