Effects of 200 km propagation on Florida lightning return stroke electric fields (original) (raw)

Currents in Florida lightning return strokes

Journal of Geophysical Research, 1973

The transmission line return stroke model is used to derive individual current versus time wave forms from measured electric fields. A total of 98 strokes in 21 flashes at distances less than or equal to 10 km and 63 strokes in 18 flashes at distances between 1(} and 32 km are analyzed for current wave shape and magnitude. For the closer strokes, return stroke, velocity is also determined from the electric field wave forms. Values are in the range 0.8-2.4 X l0 s m/sec. Eighteen detailed current and field wave forms and a statistical distribution of peak currents are presented. The maximum value of peak current is near 100 ka; the maximum value of rate of change of current averaged from 0 to peak is near 200 ka/•sec. Return stroke channel currents are found to exhibit a sharp initial peak which currents measured in tall structures at the base of the lightning channel apparently do not exhibit. A complete discussion of errors is given.

University of Florida lightning research at the Kennedy Space Center

1987

A variety of basic and applied research programs are being conducted at the Kennedy Space Center. As an example of this research we describe the University of Florida program to characterize the electric and magnetic fields of lightning and the coupling of those fields to utility power lines. Specifically, we consider in detail the measurements of horizontal and vertical electric fields made during the previous three summers at KSC and the simultaneous measurements of the voltages on a 500 m test line made during the past two summers at KSC. Theory to support these measurements is also presented.

The close lightning electromagnetic environment: Dart-leader electric field change versus distance

Journal of Geophysical Research, 2001

Net electric field changes due to dart leaders in triggered lightning from the fields were measured at 2-10 stations with distances from the lightning cham•el ranging from 10 to 621 m, while in 1993 the fields were measured at three distances, 30, 50, and 110 m, in Florida and at two distances, about 10 and 20 m, in Alabama. With a few exceptions, the 1997-1999 data indicate that the distance dependence of the leader electric field change is close to an inverse proportionality (r-l), in contrast to the 1993 data in which a somewhat weaker distance dependence was observed. The typically observed r-1 dependence is consistent with a uniform distribution of leader charge along the bottom kilometer or so of the channel.

The spatial variations of lightning during small Florida thunderstorms

1991

Networks of field mills (FMs) and lightning direction finders (LDFs) have been used to locate lightning over the NASA Kennedy Space Center (KSC) on three storm days. Over 90 percent of all cloud-to-ground (CG) flashes that were detected by the LDFs in the study area were also detected by the FM network. 27 percent of the FM lightning events were correlated with CG flashes detected by the LDFs. About 17 percent of the FM CG events could be fitted to either a monopole or a dipole charge model. These projected FM charge locations are compared to LDF locations, i.e. the ground strike points. We find that 95% of the LDF points are within 12 km of the FM charge, 75% are within 8 km, and 50% are within 4 km. For a storm on July 22, 1988, there was a systematic 5.6 km shift between the FM charge centers and the LDF strike points that might have been caused by the meteorological structure of the storm.

Parameters of triggered-lightning flashes in Florida and Alabama

Journal of Geophysical Research, 1993

Channel base currents from triggered lightning were measured at the NASA Kennedy Space Center, Florida, during summer 1990 and at Fort McClellan, Alabama, during summer 1991. Additionally, 16-mm cinematic records with 3-or 5-ms resolution were obtained for all flashes, and streak camera records were obtained for three of the Florida flashes. The 17 flashes analyzed here contained 69 strokes, all lowering negative charge from cloud to ground. Statistics on interstroke interval, no-current interstroke interval, total stroke duration, total stroke charge, total stroke action integral ($ i 2dt), return stroke current wave front characteristics, time to half peak value, and return stroke peak current are presented. Return stroke current pulses, characterized by rise times of the order of a few microseconds or less and peak values in the range of 4 to 38 kA, were found not to occur until after any preceding current at the bottom of the lightning channel fell below the noise level of less than 2 A. Current pulses associated with M components, characterized by slower rise times (typically tens to hundreds of microseconds) and peak values generally smaller than those of the return stroke pulses, occurred during established channel current flow of some tens to some hundreds of amperes. A relatively strong positive correlation was found between return stroke current average rate of rise and current peak. There was essentially no correlation between return stroke current peak and 10-90% rise time or between return stroke peak and the width of the current waveform at half of its peak value.

The temporal structure of HF and VHF radiations during Florida lightning return strokes

Geophysical Research Letters, 1977

Measurements have been made of the HF and VHF radiation at 3, 30, 139, and 295 MHz produced during lightning return strokes, in correlation with wideband electric field measurements covering from about 300 Hz to 2 MHz. Intense pulses of RF are observed during the first return stroke in a discharge to ground, but with a 10-30 •sec time delay between the beginning of the stroke and the occurrence of significant RF radiation. The delay occurs at all frequencies and is short enough to suggest that the main channel and/or branches are the main initial sources of RF during the first stroke. During a subsequent return stroke, the RF starts an average of 265 •sec prior to the onset of the stroke. At 139 and 295 MHz, the subsequent stroke RF lasts only about 115 •sec; however, at 3 MHz, the radiation frequently persists up to and during the stroke.

Electric field intensity of the lightning return stroke

Journal of Geophysical Research, 1973

Institute o] Atmospheric Physics, University o] Arizona, Tucson, Arizona 857•1 From an examination of about 1000 electric field wave forms produced by lightning return strokes in 16 storms at distances between 20 and 100 km from an observation site at the Kennedy Space Center, Florida, a typical return stroke current wave form is derived. For this current wave form, the electric field intensity at distances between 0.5 and 100 km is computed for three values of return stroke velocity. The resultant curves for close lightning • Now at

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.

On remote measurements of lightning return stroke peak currents

Atmospheric Research, 2014

Return-stroke peak current is one of the most important measures of lightning intensity needed in different areas of atmospheric electricity research. It can be estimated from the corresponding electric or magnetic radiation field peak. Electric fields of 89 strokes in lightning flashes triggered using the rocket-and-wire technique at Camp Blanding (CB), Florida, were recorded at the Lightning Observatory in Gainesville, about 45 km from the lightning channel. Lightning return-stroke peak currents were estimated from the measured electric field peaks using the empirical formula of and the field-to-current conversion equation based on the transmission line model . These estimates, along with peak currents reported by the U.S. National Lightning Detection Network (NLDN), were compared with the ground-truth data, currents directly measured at the lightning channel base. The empirical formula, based on data for 28 triggered-lightning strokes acquired at the Kennedy Space Center (KSC), tends to overestimate peak currents, whereas the NLDNreported peak currents are on average underestimates. The field-to-current conversion equation based on the transmission line model gives the best match with directly measured peak currents for return-stroke speeds between c/2 and 2c/3 (1.5 and 2 × 10 8 m/s, respectively). Possible reasons for the discrepancy in the peak current estimates from the empirical formula and the ground-truth data include an error in the field calibration factor, difference in the typical return-stroke speeds at CB and at the KSC (considered here to be the most likely reason), and limited sample sizes, particularly for the KSC data. A new empirical formula, I = −0.66-0.028rE, based on data for 89 strokes in lightning flashes triggered at CB, is derived.

Electric fields produced by Florida thunderstorms

Journal of Geophysical Research, 1978

Electric fields produced by air mass thunderstorms have been recorded at 25 field mill sites at the NASA Kennedy Space Center (KSC) during the summers of 1975 and 1976. Time-and area-averaged fields produced by individual storms are typically-0.8 to-2.1 kV m-• during periods of intense lightning activity and usually 2-4 times larger (-2.3 to-4.3 kV m-•) in the final, less active storm periods. The total number of lightning discharges generated by individual storms ranged from 8 to 1987. The average flashing rate per storm was 0.3-9.3 discharges per minute. Large storms tend to evolve through an initial, an active, and a final phase of electrical activity. The average point discharge computed during the active period is 3-4 times less than that computed during the final phase. In 1975, about 71% of all lightning discharges occurred during active storm periods, which in turn represented only about 27% of the total storm durations. In general, the structures of the electrostatic fields produced by lightning discharges in Florida were found to be similar to those in other geographical locations. During active storm periods, 42-52% of all lightning discharges were to ground,land during a final storm period only about 20% were to ground. The fraction of discharges to ground which contained continuing currents ranged from 29 to 46%; the mean duration of 239 ground flashes was about 420 ms. The average density of lightning flashes to ground d•uring 1974 and 1975 at KSC was about 6 km-•' month-• during the months of June and July. ' detect the electrified Clouds and lightning discharges which represent hazards to various ground and launch operations, KSC has constructed and operated an extensive network of instruments to measure the vertical electric field of the atmosphe•re. Recently, KSC has also provided facilities and support for a number of other coordinated measurements through the Thunderstorm Research International Program [Pierce, 1976]. Since the characteristics of the electric fields produced by thunderstorms and lightning at KSC are of great practical as well as scientific importance, it is appropriate to present examp!es of these fields under a variety of storm conditions and to survey their overall behavior. Most thunderstorms which occur in central Florida during the summer are air mass storms initiated by local heating and. sea breeze convergence [Byers and Rodebush, 1948]. The synoptic conditions which tend to produce storms over the KSC complex and Merritt Island have been analyzed in detail by N•umann [1971 ]. M0st storms ar.e produced under conditions of moderate westssOuthWesterly flow through the 500-mbar level. With these winds, convective'cells tend to,develop west of the Indian River in the early afternoon and later merge to form !a•rge organized systems which advance east-northeastwar d over KSC. The electrical activity produced by these lat. ge storms may affe ct KSC for periods of 3 hours or more. Under light wind conditions, 'one or more isolated cells may develop directly over KSC with little or no hOriZOntal movement nt. As we shall see, the durations of these small storms are usually less than I hour. In this paper we show examples of the electric fields produced by large and small storms at KSC, present time averages of these fields, estimate the magnitude O f point discharge currents, examine the time development of lightning activity in large storms, and, finally, examine the detailed structure of the changes in electrostatic field produced by Florida lightning.