The lightning radiation field spectra of cloud flashes in the interval from 20 kHz to 20 MHz (original) (raw)
Related papers
Lightning electromagnetic radiation field spectra in the interval from 0.2 to 20 MHz
Journal of Geophysical Research, 1990
Average energy spectral densities are presented for the fast transitions in most of the components that produce large radiation field impulses from cloud-to-ground lightning: first and subsequent return strokes; stepped, dart-stepped, and "chaotic" leaders; and "characteristic" cloud pulses. A disagreement in the previous literature about the spectral energy radiated by return strokes at high frequencies is noted and explained. We show that our spectral amplitudes are not seriously distorted by propagation over less than 35 km of seawater, although as much as 45 km of such propagation does appear to produce significant attenuation above about 10 MHz. First and subsequent return strokes produce identical spectra between 0.2 and 20 MHz. The spectra of stepped and dart-stepped leader steps are nearly identical and are very similar to that of characteristic pulses. The spectra of leader steps also match return stroke spectra above 2-3 MHz after the former are increased by about 7 dB. The shapes of individual spectra do not depend on their amplitude, so the shapes of the average spectra are probably not distorted by the trigger thresholds used in the data acquisition. Return strokes are the strongest sources of radiation from cloud-to-ground lightning in the 0.2-to 20-MHz frequency range, although certain intracloud processes are stronger radiators above about 8 MHz. invoking a small-scale breakdown mechanism.
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.
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
The first electric field pulse of cloud and cloud-to-ground lightning discharges
Journal of Atmospheric and Solar-terrestrial Physics, 2010
In this study, the first electric field pulse of cloud and cloud-to-ground discharges were analyzed and compared with other pulses of cloud discharges. Thirty eight cloud discharges and 101 cloud-to-ground discharges have been studied in this analysis. Pulses in cloud discharges were classified as 'small', 'medium' and 'large', depending upon the value of their relative amplitude with respect to that of the average amplitude of the five largest pulses in the flash. We found that parameters, such as pulse duration, rise time, zero crossing time and full-width at half-maximum (FWHMs) of the first pulse of cloud and cloud-to-ground discharges are similar to small pulses that appear in the later stage of cloud discharges. Hence, we suggest that the mechanism of the first pulse of cloud and cloud-to-ground discharges and the mechanism of pulses at the later stage of cloud discharges could be the same.
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.
Journal of Geophysical Research, 1982
The characteristics of some radiation field waveforms of lightning from frontal thunderstorms in Sweden are presented. The waveforms are distinctly different from previously published signatures from intracloud discharges. In general, they are similar to the radiation fields produced by return strokes in negative ground flashes except for the initial polarity, but several important differences are found in the detailed characteristics. The zero-to-peak rise times of these waveforms are found to be in the range 5-25 /as. The waveforms begin with an initial portion or front which rises slowly for 3-20 /as to about half of the field peak amplitude. The observed mean values of 13 /as and 9 /as of zero-to-peak rise time and front duration, respectively, of these waveforms are about twice the corresponding values observed for negative return strokes. The mean radiation field peak value, normalized to 100 km, for these waveforms is 2 times that for negative return strokes. Some waveforms were preceded by small-amplitude pulses which are assumed to be produced by a leader process. The mean separation in time of these pulses is about 26 /as, which may be compared with 14 /as observed for negative return strokes. Another important feature is the presence of 'slow tails' in some of these waveforms, indicating the presence of long-lasting currents in their sources. It is suggested that the sources of the observed waveforms are return strokes bringing down positive charge to earth.
Intense electromagnetic radiation from cloud lightning discharges
2009
We examined wideband electric fields, electric and magnetic field derivatives, and narrowband VHF (36 MHz) radiation bursts produced by 157 Compact Intracloud Discharges (CIDs). These poorly understood lightning events appear to be the strongest natural producers of HF-VHF radiation. All the events transported negative charge upwards (or lowered positive charge), and 149 of them were correctly identified by NLDN as cloud discharges. NLDNreported distances were 5 to 132 km. Different types of wideband electric field waveforms, ranging from essentially radiation to essentially induction and electrostatic, were observed. About 24% of CIDs occurred prior to, during, or following cloud-to-ground or "normal" cloud lightning, with 72% occurring in isolation. About 6% appeared to be associated with cloud-to-ground lightning. In three cases, two CIDs occurred within less than 200 ms of each other (the first documented "multiple" CIDs), with a total of 4% of CIDs occurring in pairs. For a subset of 48 CIDs, the geometric mean of radiation source height was estimated to be 16 km. For the same 48 CIDs, the geometric mean electric field peak normalized to 100 km and zero elevation angle was as high as 20 V/m.
FORTE observations of simultaneous VHF and optical emissions from lightning: Basic phenomenology
Journal of Geophysical Research, 2000
Preliminary observations of simultaneous VHF and optical emissions from lightning as seen by the Fast on-Orbit Recording of Transient Events (FORTE) spacecraft are presented. VHF/optical waveform pairs are routinely collected both as individual lightning events and as sequences of events associated with cloud-to-ground (CG) and intracloud (IC) flashes. CG pulses can be distinguished from IC pulses on the basis of the properties of the VHF and optical waveforms but mostly on the basis of the associated VHF spectrograms. The VHF spectrograms are very similar to previous ground-based HF and VHF observations of lightning and show signatures associated with return strokes, stepped and dart leaders, attachment processes, and intracloud activity. For a typical IC flash, the FORTE-detected VHF is generally characterized by impulsive broadband bursts of emission, and the associated optical emissions are often highly structured. For a typical initial return stroke, the FORTE-detected VHF is generated by the stepped leader, the attachment process, and the actual return stroke. For a typical subsequent return stroke, the FORTE-detected VHF is mainly generated by dart leader processes. The detected optical signal in both return stroke cases is primarily produced by the in-cloud portion of the discharge and lags the arrival of the corresponding VHF emissions at the satellite by a mean value of 243 s. This delay is composed of a transit time delay (mean of 105 s) as the return stroke current propagates from the attachment point up into the region of in-cloud activity plus an additional delay due to the scattering of light during its traversal through the clouds. The broadening of the light pulse during its propagation through the clouds is measured and used to infer a mean of this scattering delay of about 138 s (41 km additional path length) for CG light. This value for the mean scattering delay is consistent with the Thomason and Krider [1982] model for light propagation through clouds.
Coincident radio frequency and optical emissions from lightning, observed with the FORTE satellite
Journal of Geophysical Research: Atmospheres, 2001
We present long optical and radio frequency (RF) time series of lightning events observed with the FORTE satellite in January 2000. Each record contains multiple RF and optical impulses. We use the RF signatures to identify the general type of discharge for each impulse according to the discrimination techniques described by and reviewed herein. We see a large number of paired, impulsive events in the RF which allow us to study the heights within clouds of several events. We also see that the rate of RF/optical coincidence depends on the type of discharge: nearly 100% of VHF signals from first negative return strokes have an associated optical signal, whereas a mere 50% of impulsive intracloud events appear to have an optical counterpart. While the RF signals from ground strokes clearly coincide with simple optical signals in almost all cases, the intracloud lightning often shows nearly continuous, complicated RF and optical emissions which do not cleanly correlate with one another. The RF and optical pulses do not show a well-defined relationship of intensities, for any lightning type. The observed delay between the RF and optical pulses we interpret as mainly an effect of the scattering experienced by the light as it traverses the cloud. For intracloud lightning, we find no evidence of an intrinsic delay at the source between the onset of the RF and optical signals. Impulsive in-cloud RF events are seen to occur on average every 0.9 ms during a flash. This paper is not subject to U.S.