2002), LF/VLF and VHF lightning fast-stepped leader observations (original) (raw)
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LF/VLF and VHF lightning fast-stepped leader observations
Journal of Geophysical Research, 2002
1] This paper reports multiplatform observations of leader radiation preceding initial cloud-to-ground lightning return strokes by <6 ms. Specifically, we present multistation ground-based LF/VLF recordings of events with large-amplitude leader activity (comparable in amplitude to the return stroke amplitude). The events were selected for their coincidence with VHF observations by the FORTE satellite. Some FORTE VHF leader step observations have obvious direct and ground reflection components. For these steps, which temporally correspond to specific features in the ground LF/VLF field change records, we calculate a source height based on event satellite geometry. We determine source heights between 4.0 and 5.5 km. For FORTE records with multiple reflected events we calculate vertical leader propagation velocities on the order of 10 6 m/s. The determined vertical leader propagation speeds are an order of magnitude greater than those reported as typical values for stepped leader velocities associated with initial return strokes.
Journal of Geophysical Research, 2000
This work compares simultaneous observations of lightning from two complementary systems. FORTE is a low-Earth-orbit satellite carrying radiowave and optical instruments for the study of lightning. The radio receivers aboard FORTE observe very high frequency (VHF) emissions from the air-breakdown process preceding (and sometimes accompanying) a lightning current. The National Lightning Detection Network (NLDN) is a ground-based array of sensors in the contiguous United States observing the low-frequency (LF) and very low frequency (VLF) radiation from vertical currents. Prior to the launch of FORTE in 1997, essentially no work had been done on the statistical correlations between (1) ground-based LF/VLF and (2) spaced-based VHF remote sensing of lightning. During a 6-month campaign in April-September 1998, FORTE took most of its triggered VHF data over and near the contiguous United States, and NLDN data were specially postprocessed in a loosened-criterion mode providing enhanced detection range beyond the coastline and borders of the array itself. The time history of reported events from the two systems was compared, and event pairs (each pair containing one event from FORTE, the other from NLDN) which were candidate correlations (closer than 200 ms from each other) were scrutinized to determine whether the members of a pair actually came from the same discharge process. We have found that there is a statistically significant correlation, for a subset of FORTE events. This correlation is most likely to occur for intracloud and less likely to occur for cloud-to-ground discharges. The correlated VHF and NLDN events tend to occur within +30 Us of each other, after correction for the propagation of the VHF signal to FORTE from the NLDN-geolocated stroke location. Most correlations outside of _+30 Us turn out to be merely a statistical accident. The NLDN-furnished geolocation allows the correlated FORTE-detected VHF pulses to be better interpreted. In particular, we can deduce, from the lag of the VHF groundreflection echo, the height of the VHF emission region in the storm. These workers also studied a very complex and varied interrelationship between the lowfrequency/very low frequency (LF/VLF) and VHF discharge signatures during the development and decay of the lightning flash. Furthermore, VHF pulses emitted by the storm have been found to be either "major," i.e., in a set of pulses grouped in time according to flashes and associated with LF/VLF signatures, or "minor," occurring higher in altitude and less ob-
Journal of Geophysical Research, 1999
Using a high-speed digital optical system, we determined the propagation characteristics of two leader/return-stroke sequences in the bottom 400 rn of the channel of two lightning flashes triggered at Camp Blanding, Florida. One sequence involved a dart leader and the other a dart-stepped leader. The time resolution of the measuring system was 100 ns, and the spatial resolution was about 30 m. The leaders exhibit an increasing speed in propagating downward over the bottom some hundreds of meters, while the return strokes show a decreasing speed when propagating upward over the same distance. Twelve dartstepped leader luminosity pulses observed in the bottom 200 m of the channel have been analyzed in detail. The luminosity pulses associated with steps have a 10-90% risetime ranging from 0.3 to 0.8 •s with a mean value of 0.5 •ts and a half-peak width ranging from 0.9 to 1.9 •ts with a mean of 1.3 •ts. The interpulse interval ranges from 1.7 to 7.2 •ts with a mean value of 4.6 •ts. The step luminosity pulses apparently originate in the process of step formation, which is unresolved with our limited spatial resolution of 30 m, and propagate upward over distances from several tens of meters to more than 200 m, beyond which they are undetectable. This finding represents the first experimental evidence that the luminosity pulses associated with the steps of a downward moving leader propagate upward. The upward propagation speeds of the step luminosity pulses range from 1.9x 107 to 1.0x 10 • m/s with a mean value of 6.7x 107 m/s. In particular, the last seven pronounced light pulses immediately prior to the return stroke pulse exhibit more or less similar upward speeds, near 8x 107 m/s, very close to the return-stroke speed over the same portion of the channel. On the basis of this result, we infer that the propagation speed of a pulse traveling along the leaderconditioned channel is primarily determined by the channel characteristics rather than the pulse magnitude. An inspection of four selected step luminosity pulses shows that the pulse peak decreases significantly as the pulse propagates in the upward direction, to about 10% of the original value within the first 50 m. The return-stroke speeds within the bottom 60 rn or so of the channel are 1.3x 108 and 1.5x 108 m/s for the two events analyzed, with a potential error of less than 20%.
Lightning leader characteristics in the Thunderstorm Research International Program (TRIP)
Journal of Geophysical Research, 1982
We have used high speed streaking photographic techniques to time-resolve the luminous components of cloud-to-ground lightning flashes. All recordings were made during our participation in the Thunderstorm Research International Program (TRIP), conducted at the Kennedy Space Center, Florida, during the summers of 1977 and 1978, and at the Langmuir Laboratory, near Socorro, New Mexico, during the summer of 1979. Twenty one dart leaders, four dart-stepped leaders and three stepped leaders were recorded, the majority under daylight conditions. The mean two-dimensional propagation speed of the dart leaders, evaluated over channel lengths less than or equal to 0.8 km above ground, is 11 x 10 6 m/s, with a range of 2.9 to 23 x 10 6 m/s. Several of the dart leaders reveal a decrease in propagation speed as ground is approached. However, four of the dart leaders in two separate flashes show an increase in speed near the ground, an observation not previously reported in the literature. In two multistroke flashes, we examine the variation of dart leader propagation speed along the channel and find very similar behavior for different strokes in the same flash. The speed variations that we observe may be predominantly caused by geometrical variations of the channel. The dart leader propagation speeds reported in this study are compared with the earlier works of Schonland, McEachron, and Kitagawa and Brook. Agreement among the studies is good, with a common range of observed dart leader propagation speed of 2 to 23 x 106 m/s. The major discrepancy among these studies is the observation, by Schonland, of a distribution of dart leader propagation speeds positively skewed toward the lower limit of reported values. Eleven of the dart leaders are analyzed at upper and lower levels along the visible channel to give 22 dart 'lengths.' They range from 7 to 75 m with a mean of 34 m. For these 22 determinations, we calculate a correlation coefficient of 0.85 between the dart length and the dart leader propagation speed. The correlation of greater dart length with higher propagation speed is consistent with the slower decay of channel luminosity due to the greater initial input of energy to the channel by the faster and, presumably, more energetic dart leader. Four dart-stepped leaders are examined in detail with regard to variation of propagation speed, step length, stepping interval, and luminous intensity during propagation between the cloud base and ground. Significant differences in the tendencies of these parameters are found within these four leaders. For example, one dart-stepped leader recording shows a decreasing propagation speed and an increasing step interval near ground, whereas another shows the opposite behavior. In the best event recorded, several of the individual steps reveal a photographic film density structure, with the lower portion of the step exhibiting a distinct, bright tip that fans out into a symmetrically diffuse image in the upper portion of the step. Our analysis indicates that this spread in the upper portion of the step image is not the result of streaking photography distortion but, rather, represents the luminous structure of the step. We estimate that the step image is recorded in less than 1/xs. Consequently, with a measured step length of---20 m, the luminous pulse must propagate along the step at a speed of at least 2 x 10 7 m/s. The mean propagation speed for three stepped leaders is found to be 1.1 x 106 m/s. All three stepped leaders are very faint, and were recorded only in the last 100-200 m above ground. Two stepped leaders and one dart-stepped leader do not propagate completely to ground before initiation of the return stroke. Apparently, these leaders are met by an upward propagating discharge at heights above ground of 20, 30, and 20 m, respectively. Other stepped and dart-stepped leader cases are indeterminate because an obstacle or the horizon prevent the recording of the leaders near the ground. Connecting discharges are not observed for any of the dart leader events with a resolution of 10 m at 5 km, implying that upward discharges initiated by the approach of dart leader do not occur or are substantially less than a few tens of meters in length. Dart leaders, apparently, propagate completely to ground.
Journal of Geophysical Research, 2002
1] The FORTE satellite's radiofreqency receiver/recorder system has been used to study extremely narrow ($100 ns width) radiofrequency pulses accompanying the initiation of negative cloud-to-ground strokes. These pulses have been observed from the ground for over two decades. The FORTE space-based observations are substantially consistent with the prior ground-based results, at least in regard to pulsewidth, distance-scaled pulse amplitude, and the pulses' basic association with negative cloud-to-ground strokes, relative to either positive cloud-to-ground or intracloud strokes. New results from the FORTE observations include (1) information on the radiation pattern (versus elevation angle), (2) the tendency of the underlying fast-pulse radiation process to occur preferentially in marine, rather than dry-land, locations, and (3) the high degree of linear polarization of the radiated signal. These three new results were not accessible to ground-based measurements, which do not sample elevation angles other than zero, whose signal distortion with overland propagation paths (at zero elevation angle) tends to confuse the issue of the (land versus sea) location affinity of the pulse source itself, and whose received signal is a fortiori linearly polarized because of the adjacent ground plane. The narrow radiofrequency pulse that accompanies negative cloud-to-ground strokes provides a useful identifier, in the satellite's radiofrequency datastream, of the occurrence of this particular kind of stroke. Additionally, the observations reported here indicate that the radiating element is a single, vertical current stalk, rather than a collection of randomly oriented, mutually incoherent dipoles, such as are believed to be responsible for most very high frequency signals from lightning.
Radio Frequency Observations of Lightning Discharges by the Forte Satellite
2002
FORTE-observed VHF signatures for different lightning discharges are presented. For in-cloud discharges, a pulse pair is typically recorded and is named a "transionospheric pulse pair" (TIPP). Many intense TIPPs are coherent and polarized, whereas initial and dart leaders do not show a recognizable degree of polarization. TIPPs are optically weaker than cloud-to-ground (CG) strokes, and stronger VHF TIPPs are optically darker. About 10% of CG strokes, mostly over seawater, produce extremely narrow, powerful VHF pulses at the very beginning of the return strokes. These narrow pulses are found to form an upward beam pattern.
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.
Journal of Geophysical Research: Space Physics, 2009
TLEs are optically observed from the U.S. Langmuir Laboratory, while ELF/VLF waveform data are simultaneously recorded on board the Centre National d'Etudes Spatiales microsatellite DEMETER and on the ground at Langmuir. Analyses of ELF/VLF measurements associated with sprite events observed on 28 July 2005 and 3 August 2005 are presented. Conditions to trace back the wave emissions from the satellite to the source region of the parent lightning discharge are discussed. The main results concern: (1) the identification from a low Earth orbit satellite of the 0+ whistler signatures of the TLE causative lightning; (2) the identification of the propagation characteristics of proton whistlers triggered by the 0+ whistlers of the causative lightning, and the potential use of those characteristics; (3) recognition of the difficulty to observe sprite-produced ELF bursts in the presence of proton-whistlers; (4) the use of geographical displays of the average power received by the DEMETER electric field antennas over the U.S. Navy transmitter North West Cape (NWC) located in Western Australia to evaluate VLF transmission cones which explain the presence (28 July events) or the absence (3 August events) of propagation links between sferics observed at ground and 0+ whistlers observed on DEMETER; and (5) owing to electron-collisions, an optimum transfer of energy from the atmosphere to the ionosphere for waves with k vectors antiparallel, or quasi-antiparallel, to Earth's magnetic field direction.
1985
The speed and current during the final stages of the lightning stepped leader were determined from features of the electric field waveform. The average speed of the leader tip was found below the height where dE/dt was zero for close lightning. This height was in the range 0.6-2.0 km for flashes at the distances considered here, 3.6-5.7 km. For 10 stepped leaders observed near Tampa, Florida, the mean final one-dimensional speed was 4.4 x 105 m/s, the standard deviation was 3.8 x 105 m/s, and the range was 1.0-14 x 105 m/s. These speeds are consistent with other stepped leader speeds reported in the literature that were found using optical techniques. The current was estimated when the leader was near ground from dE/dt during the last hundred microseconds before the return stroke. For 62 stepped leaders observed at distances of 3.6-14 km, the mean final leader current was 1.3 kA, the standard deviation was 1.0 kA, the geometric mean was 1.0 kA, and the range was 110 A to 5.0 kA. Errors resulting from uncertainties in distance, charge center height, calibration, ground channel inclination, and channel branching were found to be about 40-50% rms for both speed and current. 1.
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 characteris-