Lightning electromagnetic radiation field spectra in the interval from 0.2 to 20 MHz (original) (raw)
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Submicrosecond fields radiated during the onset of first return strokes in cloud-to-ground lightning
Journal of Geophysical Research, 1996
An experiment to measure the electric field E and dE/dr signatures that are radiated by the first return stroke in cloud-to-ground lightning was conducted on the eastern tip of Cape Canaveral, Florida, during the summer of 1984. At this site, there was minimal distortion in the fields due to ground wave propagation when the lightning struck within a few tens of kilometers to the east over the Atlantic Ocean. Biases that are introduced by a finite threshold in the triggered recording system were kept to a minimum by triggering this system on the output of a wideband RF receiver tuned to 5 MHz. Values of the peak dE/dr during the initial onset of 63 first strokes were found to be normally distributed with a mean and standard deviation of 39 _+ 11 V m-• /xs -• after they were normalized to a range of 100 km using an inverse distance relation. Values of the full width at half maximum (FWHM) of the initial half-cycle of dE/dr in 61 first strokes had a mean and standard deviation of 100 +_ 20 ns and were approximately Gaussian. When these results are interpreted using the simple transmission line model, after correcting for the effects of propagation over 35 km of seawater, the average value of the maximum current derivative, (dI/dt)p, and its standard deviation are inferred to be 115 + 32 kA/•s -•, with a systematic uncertainty of about 30%. The FWHM after correction for propagation is about 75 +_ 15 ns.
Atmospheric Research, 2005
We have analyzed the fine-structure of 131 electric field (E) waveforms that were radiated during the onset of first return strokes in cloud-to-ocean lightning. The dE/dt waveforms were recorded using an 8-bit waveform digitizer sampling at 100 MHz, and the E waveforms were sampled at 10 MHz using a 10-bit digitizer. 49 (or 37%) of the dE/dt waveforms contain one or more large pulses within ±1 µs of the largest (or dominant) peak in dE/dt, i.e. within an interval from -1 µs to +1µs, where t = 0 µs is the time of the dominant peak, and 37 (or 28%) have one or more large pulses in the interval from 4 µs before to 1 µs before the dominant peak, i.e. -4 µs to -1 µs, and only the dominant peak within ± 1 µs .
Distinctive Features of Radiation Pulses in the Very First Moment of Lightning Events
Journal of Atmospheric and Solar-Terrestrial Physics, 2014
This paper investigates the existence of distinctive features between 4 different types of lightning discharges, namely negative cloud to ground discharge (-CG), positive cloud to ground discharge (+CG), cloud discharge (IC) and isolated breakdown discharge (IB). A total of 110 very fine structure waveforms of 44 -CG, 16 +CG, 39 IC, and 11 IB discharges have been selected from a collection of 885 waveforms measured using fast electric field broadband antenna system. The measurements were carried out in Uppsala, Sweden from May to August 2010. We found that there are significant distinctions within the first 1 ms among different types of lightning discharges (-CG, +CG, IC, and IB). For example, the pulses in -CG discharges are more frequent than other discharges; the pulses in +CG discharges have the highest intensity and the IC discharge pulses tend to have shorter duration.
Radiation field pulses associated with the initiation of positive cloud to ground lightning flashes
Journal of atmospheric and solar-terrestrial physics, 2004
Seventy one electric ÿeld pulse trains that occurred during millisecond-scale time intervals before positive cloud to ground lightning ashes were analysed. These pulses are bipolar in nature and somewhat similar in pulse characteristics to the breakdown pulses preceding negative cloud to ground lightning. However, in the case of these positive ashes, the pulse characteristics of the pulse trains are conÿned in a much wider range of values than those of the pulse trains associated with negative return strokes. The leading edge of the pulses of the most commonly observed pulse trains that precede positive return strokes are relatively smooth, thus, di erent from their counterparts associated with negative ashes, in which case a few narrow pulses are superimposed on the rising edge of the bipolar pulses. Considering the initial polarity of pulses, four types of bipolar pulse trains preceding positive return strokes were identiÿed. For each type of pulse trains, statistics of pulse characteristics were given. In contrast, in the case of negative ground ashes, the bipolar pulse trains were almost always composed of pulses of the same polarity as that of the succeeding return stroke. The possible causes of the observation of several types of pulse trains and the signiÿcantly diversiÿed pulse characteristics of the breakdown pulse trains of positive ashes were discussed. The frequency spectrum of the electric ÿelds of the most common type of pulse trains was compared with the spectrum of the breakdown pulses of negative ashes and those of negative return strokes. This spectrum of the preliminary breakdown pulse trains of positive ground ashes is comparable with that of the preliminary breakdown pulse trains of negative ground ashes.
Characteristics of lightning vhf radiation near the time of return strokes
Journal of Geophysical Research, 1984
By using a crossed base line interferometer, lightning VHF source positions correlated in time with electric field change measurements have been obtained. We present data in this paper showing azimuth and elevation pictures with high time resolution of the VHF (34.3 MHz) radiation for events near the time of return strokes. From their common characteristics we infer spatial and temporal features of these processes. No radiation was seen that could be attributed to the return stroke wave front itself. Radiation from stepped leaders continued up to and, in some cases, after initiation of the first return stroke. A strong burst of radiation often followed the first return stroke. Strong radiation began before subsequent return strokes, coinciding with the electric field change usually associated with the dart leader. The end of the radiation was independent of the return stroke. The source of this radiation was a well-defined region within the cloud. A second burst often followed the return stroke located in the same region as that prior to the stroke. Sources from stroke to stroke showed horizontal displacement with little apparent change in elevation. breakdown within the cloud and was not associated with dart leaders or return strokes in the channel to ground. Takagi [1969] grouped radiation occurring near the time of return strokes into three categories. In the first type, radiation began 100-1000 •s before the return stroke. The end of the radiation was not correlated with the return stroke and often radiation continued past the time of the return stroke without any noticeable alteration at the time of the return stroke. In the second type, radiation appeared 5-500 •s after the return stroke had started. This type had two subgroups. The first group began 10-30 •s after the return stroke and was interpreted as coming from the return stroke progressing upward. The second group started more than 50 •s after the Copyright 1984 by the American Geophysical Union. Paper number 3D1710. 0148-0227 / 84/003D-1710505 ß 00 return stroke, continued for 40 to 300 •s and was associated with processes inside the cloud. The third type of radiation was a combination of the first two with a quiet period between. Takagi made no distinction between first or subsequent return strokes in his papers. LeVine and Krider [1977] also presented observations of temporal relationships between VHF radiation and electric field changes near the time of return strokes. They found strong radiation beginning ~30 •s after the start of first return stroke. Because this interval was
The lightning radiation field spectra of cloud flashes in the interval from 20 kHz to 20 MHz
IEEE Transactions on Electromagnetic Compatibility, 48 1 (2006) 234-239
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
Journal of Geophysical Research, 2008
1] In this study, we identify and examine electric field pulse trains that are characteristic of preliminary breakdown in negative cloud-to-ground discharges but are not followed by return stroke waveforms. We assume that such trains are manifestations of the initiation of downward negative stepped leaders that fail to propagate all the way to the ground and refer to these events as ''attempted first cloud-to-ground leaders,'' although some of them were followed by full-fledged cloud discharges. We examined a total of 2475 electric field records of lightning events acquired in Gainesville, Florida, in 2006, and waveforms in 33 of them were found to satisfy criteria set for attempted cloud-to-ground leaders. In addition to pronounced bipolar pulses with positive (atmospheric electricity sign convention) initial half cycle, negative unipolar and negative (initial half cycle) bipolar pulses were sometimes seen toward the end of the train. We also observed that at the beginning and at the end of the pulse train, there were narrower pulses, often having durations in the range of 1-2 ms, which are more than an order of magnitude shorter than for ''classical'' preliminary breakdown pulses. The arithmetic mean of total pulse train durations is 2.7 ms, and the weighted arithmetic means of individual pulse durations and interpulse intervals are 17 and 73 ms, respectively. Some of the attempted cloud-to-ground leaders, which should belong to the cloud discharge category, can be misclassified as negative cloud-to-ground discharges by lightning locating systems such as the U.S. National Lightning Detection Network.
The fine structure of positive lightning return-stroke radiation fields
IEEE Transactions on Electromagnetic Compatibility, 2004
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 Fanö 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 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 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 s. The full width at half maximum of the radiation field derivative had a mean of 170 ns.
Microsecond-scale electric field pulses in cloud lightning discharges
J. Geophys. Res
From wideband electric field records acquired using a 12-bit digitizing system with a 500-ns sampling interval, microsecond-scale pulses in different stages of cloud flashes in Florida and New Mexico are analyzed. Pulse occurrence statistics and waveshape characteristics are presented. The larger pulses tend to occur early in the flash, confirming the results of Bils et al. (1988) and in contrast with the three-stage representation of cloud-discharge electric fields suggested by Kitagawa and Brook (1960). Possible explanations for the discrepancy are discussed. The tendency for the larger pulses to occur early in the cloud flash suggests that they are related to the initial in-cloud channel formation processes and contradicts the common view found in the atmospheric radio-noise literature that the main sources of VLF/LF electromagnetic radiation in cloud flashes are the K processes which occur in the final, or J type, part of the cloud discharge. Introduction It is common, following the work of Kitagawa and Brook [1960, Figure 6] to portray the electric field signature of cloud lightning discharges as composed of three portions which are labeled initial, very active, and final. These three portions were identified by Kitagawa and Brook [1960] in records from their two field-measuring systems: one, a "slow antenna," having a 4-s decay time constant and the other, a "fast antenna," having a submillisecond (300 I•s or 70 I•s)decay time constant and a significantly higher gain than the "slow antenna." The initial portion of the cloud flash was reported to have a duration of typically 100 to 200 ms [Kitagawa and Brook, 1960, Figure 11] and to be characterized by microsecondscale pulses (identified in the fast-antenna records)of relatively small amplitude and by relatively small electrostatic field change (identified in the slow-antenna records). Kitagawa and Brook [1960] observed the electrostatic field change during the very active portion to be the largest of the flash and the pulse amplitudes to become much larger than during the initial portion. The final, or J type, portion was reported by to be similar to the field changes between strokes and after the last stroke of the cloud-to-ground discharge and is characterized by relatively small steplike K field changes. Note that the K changes appear (similar to the steplike field changes during the very active stage) not as steps but rather as pulses ff they are recorded with a measuring system having a submillisecond decay time constant