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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-
Lightning-Initiation Locations as a Remote Sensing Tool of Large Thunderstorm Electric Field Vectors
Journal of Atmospheric and Oceanic Technology, 2005
The lightning data that are recorded with a three-dimensional lightning mapping array (LMA) are compared with data from an electric field change sensor (in this case a flat-plate antenna operated both as a “slow” and a “fast” antenna). The goal of these comparisons is to quantify any time difference that may exist between the initial responses of the two instruments to a lightning flash. The data consist of 136 flashes from two New Mexico thunderstorms. It is found that the initial radiation source detected by the LMA usually precedes the initial response of both the slow and fast antennas. In a small number of cases, the flat-plate antenna response precedes the initial LMA source, but by no more than 2 ms. The observations of such a close time coincidence suggest that the first LMA radiation source of each flash was located at or very near the flash-initiation point. Thus, the first LMA radiation source and the initial sequence of sources from a lightning flash can be used as remot...
A GPS-based three-dimensional lightning mapping system: Initial observations in central New Mexico
Geophysical Research Letters, 1999
A GPS-based system has been developed that accurately locates the sources of VHF radiation from lightning discharges in three spatial dimensions and time. The observations are found to reflect the basic charge structure of electrified storms. Observations have also been obtained of a distinct type of energetic discharge referred to as positive bipolar breakdown, recently identified as the source of trans-ionospheric pulse pairs (TIPPs) observed by satellites from space. The bipolar breakdown has been confirmed to occur between the main negative and upper positive charge regions of a storm and found to be the initial event of otherwise normal intracloud discharges. The latter is contrary to previous findings that the breakdown appeared to be temporally isolated from other lightning in a storm. Peak VHF radiation from the energetic discharges is observed to be typically 30 dB stronger than that from other lightning processes and to correspond to source power in excess of 100 kW over a 6 MHz bandwidth centered at 63 MHz.
A distinct class of isolated intracloud lightning discharges and their associated radio emissions
Journal of Geophysical Research, 1999
Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short-duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 gs. The HF emissions associated with NPBPs were broadband noise-like radiation bursts with a mean duration of 2.8 gs and amplitudes 10 times larger than emissions from typical intracloud and cloud-to-ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes. that different lightning processes produce characteristically different emissions. The emissions may differ in their amplitudes, spectra, temporal evolution, or other respects. Three previously reported classes of events that have been characterized by their distinct radio emissions are narrow positive bipolar pulses [Le Vine, 1980; Willett et al., 1989; Medelius et al., 1991], transionospheric pulse pairs [Holden et al., 1995; Massey and Holden, 1995; Massey et al., 1998], and subionospheric pulse pairs [Smith, 1995; Smith and Holden, 1996]. A comparison presented in this paper provides evidence that all three types of emissions are produced by the same sources within thunderstorms. First evidence of this association was presented
Journal of Geophysical Research, 2003
1] The FORTE satellite's radio frequency receiver payload has made millions of recordings of lightning discharges. The most commonly occurring such radio emission arises from intracloud (IC) electrical breakdown and is usually recognizable by a pulse followed by a delayed echo from the ground reflection. We show that these IC pulses have two polar opposite types that together account for much of the pulse population. One type is a very bright pulse characterized by extended width (>2 ms), deep random fading within the pulse, and lack of prior pulses within a flash to which it belongs. The other type of IC pulse is two orders of magnitude less intense and is characterized by narrow width (<0.1 ms), a simple pulse shape evidencing no random fading, linear polarization, and occurrence in close association with other such pulses within the same flash. We develop the characteristics of these two pulse types by extensive statistical analysis of FORTE data. We relate the two pulse types to prior observations by other instruments.
Radio Emission Reveals Inner Meter-Scale Structure of Negative Lightning Leader Steps
Physical Review Letters, 2020
We use the Low Frequency Array (LOFAR) to probe the dynamics of the stepping process of negatively charged plasma channels (negative leaders) in a lightning discharge. We observe that at each step of a leader, multiple pulses of vhf (30-80 MHz) radiation are emitted in short-duration bursts (< 10 μs). This is evidence for streamer formation during corona flashes that occur with each leader step, which has not been observed before in natural lightning and it could help explain x-ray emission from lightning leaders, as x rays from laboratory leaders tend to be associated with corona flashes. Surprisingly, we find that the stepping length is very similar to what was observed near the ground, however with a stepping time that is considerably larger, which as yet is not understood. These results will help to improve lightning propagation models, and eventually lightning protection models.
A Distinct Class of High Peak‐Current Lightning Pulses Over Mountainous Terrain in Thunderstorms
Geophysical Research Letters
Analysis of the radio emissions from lightning discharges continues to provide new insight into lightning and atmospheric electricity processes, especially classes of energetic discharges that have particularly strong radio emissions. Cloud-to-ground (CG) return strokes with large peak-current or large charge moment changes (Lyons et al., 1998) produce transient luminous events between cloud tops and the ionosphere. Narrow bipolar events (NBEs) (Le Vine, 1980; Smith et al., 1999; Willett et al., 1989) demonstrate the existence of powerful very high frequency (VHF) radio frequency emissions inside thunderclouds and also serve as the initiation process for some lightning flashes (
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.
The Los Alamos Sferic Array: A research tool for lightning investigations
Journal of Geophysical Research, 2002
Since 1998 the Los Alamos Sferic Array (LASA) has recorded electric field change signals from lightning in support of radio frequency (RF) and optical observations by the Fast On-orbit Recording of Transient Events (FORTE) satellite. By ''sferic'' (a colloquial abbreviation for ''atmospheric''), we refer to a remote measurement of the transient electric field produced by a lightning flash. LASA consisted of five stations in New Mexico in 1998 and was expanded to 11 stations in New Mexico, Texas, Florida, and Nebraska in 1999. During the 2 years of operation described in this paper, the remote stations acquired triggered 8-or 16-ms duration, 12-bit waveforms and GPS-based sferic time tags 24 hours per day year-round. Source locations were determined daily using differential time of arrival techniques, and the waveforms from all geolocated events were transferred to Los Alamos National Laboratory (LANL), where they have been archived for further analysis, including event classification and characterization. We evaluated LASA location accuracy by comparing temporally coincident (occurring within 100 ms) LASA and National Lightning Detection Network (NLDN) event locations. Approximately one half of the locations agreed to within 2 km, with better agreement for events that occurred within the confines of LASA subarrays in New Mexico and Florida. Of the $900,000 events located by the sferic array in 1998 and 1999, nearly 13,000 produced distinctive narrow bipolar field change pulses resembling those previously identified as intracloud discharges.