Effects of propagation on the rise times and the initial peaks of radiation fields from return strokes (original) (raw)
Related papers
Journal of Geophysical Research, 1988
The "transmission-line" model of return-stroke radiation, proposed by Uman and and invoked frequently thereafter to deduce peak currents from remote fields or to estimate propagation velocities from measured fields and currents, has never received a thorough experimental test. During the summer of 1985 at the Kennedy Space Center in Florida, we were able to measure peak currents (with a coaxial shunt), two-dimensional average propagation speeds (with a high-speed streak camera), and electric field waveforms (at 5.15-km range) for a number of subsequent return strokes in rocket-triggered lightning flashes. Because of the temporal ambiguity on the streak-camera films, it has not been possible to identify individual velocity measurements with particular strokes for which current and field data are available. Three multistroke flashes, however, each yielded a tight cluster of velocity measurements and a group of peak field to peak current ratios, though not necessarily for the same strokes. A further six flashes provided more current and field measurements for which no velocity information was obtained, and velocity measurements only are available for still other flashes. It is shown that these data indicate reasonable agreement between the propagation speeds measured with the streak camera and those deduced from the transmission-line model. The previously observed difference between current and radiation-field waveforms suggests a modification of the model, involving two wave fronts traveling upward and downward away from a junction point a short distance above the ground, which substantially improves the agreement between measured and inferred propagation speeds.
Effect of the propagation path on lightning-induced transient fields
Radio Science, 1981
Propagation over the earth's surface of electromagnetic transient waveforms radiated by lightning return strokes is examined. Radio propagation theory is applied to the particular case of the return stroke current modeled as a series of connected straight line segments. A known current pulse propagates along the line segments as in the discharge of a precharged transmission line. The effects of an imperfectly conducting ground and an anisotropic ionosphere also are considered by using an idealized waveguide model. Signals above about 10 kHz show significant attenuation due to the imperfectly conducting earth, as have been observed previously. There is qualitative agreement between the model predictions and observed transient data, but lack of detail of the earth model limits the applicability of the results.
On the electromagnetic fields, Poynting vector, and peak power radiated by lightning return strokes
Journal of Geophysical Research, 1992
The initial radiation fields, Poynting vector, and total electromagnetic power that a vertical return stroke radiates into the upper half space have been computed when the speed of the stroke, v, is a significant fraction of the speed of light, c, assuming that at large distances and early times the source is an infinitesimal dipole. The initial current is also assumed to satisfy the transmission-line model with a constant v and to be perpendicular to an infinite, perfectly conducting ground. The effect of a large vis to increase the radiation fields by a factor of (1-/32 cos 2 0)-1 , where/3 = v/c and 0is measured from the vertical, and the Poynting vector by a factor of (1-/32 cos 2 0)-2. This increase is just a few percent or less at small/3, but when/3-0.67, the fields are about 80% larger at small 0 and 50% larger at 0 = 30 ø, and the power that is radiated is increased by 26%. When/3 = 0.5 and the peak current is 30 kA, typical values for negative first strokes, the peak power that is radiated into the upper hemisphere is 1.0 x 10 lø W. 1.
The peak electromagnetic power radiated by lightning return strokes
Journal of Geophysical Research, 1983
The peak field that is radiated by a distant lightning return stroke almost always occurs within a few microseconds after the onset of the stroke, and therefore this peak is produced at a time when most of the stroke current is still close to the ground. Estimates of the peak electromagnetic (EM) power radiated by return strokes have been made by integrating the Poynting vector of measured fields over an imaginary hemispherical surface that is centered on the lightning source, assuming that ground losses are negligible. Values of the peak EM power from first and subsequent strokes have means and standard deviations of 2 _+ 2 x 10 •ø and 3 _+ 4 x 10 9 W, respectively. The average EM power that is radiated by subsequent strokes, at the time of the field peak, is about 2 orders of magnitude larger than the optical power that is radiated by these strokes in the wavelength interval from 0.4 to 1.1 #m; hence an upper limit to the radiative efficiency of a subsequent stroke is of the order of 1% or less at this time. From the above values of the radiated power, we also infer that at the time of the initial current peak, (1) the total voltage drop on the high-current portion of a return stroke must be at least 2 to 7 x l0 s V in first strokes and 2 to 4 x l0 s V in subsequent strokes, (2) the total resistance of the high-current channel must be at least 6 to 20 fl in first strokes and 9 to 35 fl in subsequent strokes, and (3) the energy that is required to form the propagating tip of the channel must be at least 102 to 103 J/m.
Journal of Geophysical Research: Atmospheres, 2008
1] In this paper, the developed formulation, which we shall call the ''reference'' one, is used to assess the validity of the most popular simplified approach for the calculation of the lightning electromagnetic field over a conducting earth, namely, the Cooray-Rubinstein (CR) approximation. This formula provides a simple method to evaluate the radial component of the electric field which is the component most affected by the finite ground conductivity and which plays an important role within the Agrawal et al. (1980) field-to-transmission line-coupling model. Several configurations are examined, with different values for the ground conductivity and different field observation points. A thorough analysis of all the simulated field components is carried out and comparisons are also made with the ''ideal'' field, namely, the field that would be present under the assumption of perfectly conducting ground. It is shown that for channel base current typical of subsequent strokes and for very low conductivities, the CR formula exhibits some deviations from the reference one but it still represents a conservative estimation of the radial field component, since it behaves as un upper bound for the exact curve. The developed algorithm is characterized by fast performances in terms of CPU time, lending itself to be used for several applications, including a coupling code for lightning induced overvoltages calculations.
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.
Journal of Geophysical Research, 2004
1] Return stroke current pulses can propagate at speeds approaching the speed of light c. Such a fast-moving pulse is expected to radiate differently than conventional dipole emitters. In this study, we revisit the theoretical analysis for the high-speed effect on the radiation beam pattern. Instead of starting with specific return stroke models, as has been done before by other investigators, we start the analysis with a general moving current pulse. Through a simple differential transformation between the retarded time and stationary time/space, the so-called F factor (1 À v cos q/c) À1 can be readily obtained. This factor is found to be fundamental and is explicitly associated with the radiation beam pattern but is not limited only to the lossless transmission line (TL) return stroke model. It is demonstrated that different beam pattern factors could be derived from this fundamental factor under different return stroke model assumptions.
Propagation of lightning generated transient electromagnetic fields over finitely conducting ground
Journal of Atmospheric and Solar-Terrestrial Physics, 2000
This paper elucidates the propagation eects on lightning generated electric ®elds. The mathematics of the problem is described and a simple procedure that can be used to predict the propagation eect is outlined together with its experimental con®rmation. This procedure is used to analyse the propagation eects, over distances less than about 300 km, on the radiation ®elds of negative ®rst return strokes, positive ®rst return strokes and subsequent return strokes in triggered lightning¯ashes. From the results, empirical equations that can be used to correct for the propagation eects are extracted. The results show that the attenuation due to propagation eects of the initial peak amplitudes may dier in negative and positive return strokes and that the data from triggered lightning¯ashes should be applied with caution in correcting for the propagation eects of natural lightninḡ ashes. 7
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.
2014
In this paper detailed numerical results are presented for the estimation of the electric field generated by the first return stroke, in order to reproduce the main characteristics of field waveforms measured at distances beyond 50 km. The effect of parameters such as the lightning channel geometry, distance from the source, return-stroke current speed, its attenuation along the channel is discussed by comparing numerical and experimental results.