FORTE satellite observations of VHF radiation from lightning discharges (original) (raw)
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.
Radio signals high enough in frequency (>25 MHz) to reliably penetrate the ionosphere provide a means (complementary to optical) for satellitebased remote sensing of lightning. Seen from space, lighting radiofrequency signals must be exceptionally intense to compete with anthropogenic radio noise. This leads to the RF lightning observables seen from space being rather atypical of the electromagnetic signatures of lightning described in the literature on ground-based observations. We have used a two-year campaign of joint observations over the North America region to better understand the how FORTE "sees" storms (via their radiofrequency observables) that are also "seen" and characterized by the National Lightning Detection Network via their direct low-frequency (<100 kHz) radiation. We find that FORTE's radiofrequency events are likely to be associated with the same storms as detected by NLDN, but that beyond that, the information provided by space-based RF detection is dominated by intracloud processes, regardless of NLDN's dominant stroke type for the storm. Even in the case of close temporal coincidence between FORTE RF events and NLDN ground-return strokes, the RF emission tends to occur high in the cloud. There is an important exception to this rule, which is a spectacularly narrow and coherent RF emission during negative cloud-to-ground strokes over seawater.
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.
On-orbit direction finding of lightning radio frequency emissions recorded by the FORTE satellite
Radio Science, 2002
1] The recording of radio frequency signals from space potentially provides a means for global, near-real-time remote sensing of vigorous convective storms and a possible early warning system for convection-associated severe weather. In general, radio frequency signals arriving at a satellite with modest antenna gain do not directly reveal the ground location of those signals' source. We develop here a means of inferring the source location using repeated signal recordings from the same source storm, with the successive recordings taken along a significant segment of the satellite pass in view of the storm. The method is based on the ratio of received power on each of a pair of crossed dipole antennas. This method has a positional accuracy of 100-500 km. Moreover, the method has an intrinsic right-left (with respect to the subsatellite track) location ambiguity. A promising use of this technique in future applications will be as an aid in assigning lightning RF emission sources to meteorological features from other global remote-sensing products, for example satellite infrared imagery of clouds.
Polarization observations of lightning-produced VHF emissions by the FORTE satellite
Journal of Geophysical Research, 2002
1] Following an earlier polarization study for a well-defined man-made very high frequency (VHF) signal by using the two orthogonally oriented, linear polarization antennas aboard the FORTE satellite, we report in this paper similar polarization observations for lightning-produced radiation. A selected group of 313 transionospheric pulse pairs (TIPPs) that were geolocated by the National Lightning Detection Network (NLDN) has been analyzed. The TIPPs have been examined with high time resolution so that the magnetoionic modes can be resolved. Most of the TIPPs have been found highly polarized, with 40% of them far above the background polarization level. The polarization ellipticity and the orientation of the ellipse of the split modes have been examined as a function of the nadir and the azimuthal angles as referenced to satellite coordinates, and they are found in agreement with the predictions based on the antenna beam pattern. The original and the reflected pulses in a TIPP show nearly the same properties of polarization, except the latter appears less polarized. However, no recognizable polarization has been observed for the VHF signals accompanying more common discharge processes of initial ground strokes, dart leaders, and K streamers that usually produce continuous VHF radiation. Observations of a sequence of impulsive radiation bursts that is apparently associated with a normal negative cloud-to-ground flash indicate they are somewhat polarized, though not as much as the TIPPs. On the basis of the polarization observations, the possible breakdown mechanisms that are responsible for the VHF radiation have been discussed. For the highly polarized TIPP events, if they follow a cone-shaped discharge geometry, the half-angle of the cone is estimated to be less than 22°.
Satellite measurements of global lightning
Quarterly Journal of the Royal Meteorological Society, 1998
The satellite-borne NASA/MSFC Optical Transient Detector provides global distributions of lightning and lightning-stroke radiance. Measurements made during the first year of its operation show that lightning activity is particularly pronounced over the tropics, much greater over land than over the oceans, and exhibits great seasonal variability. The values of lightning-stroke radiance tend to be greater over the oceans, less when lightning activity is high, and greater in the northern hemisphere winter than summer.
Optical observations of terrestrial lightning by the FORTE satellite photodiode detector
Journal of Geophysical Research, 2001
We review data from observations of terrestrial lightning obtained by the FORTE satellite between September 1997 and January 2000. A silicon photodiode detector (PDD) records the intensity-time history of transient optical events occurring within its 80Њ circular field of view. This field of view corresponds to a circle on the Earth's surface having an approximate diameter of 1200 km. We describe the instrument, present examples of the data, explain how the data are screened for false triggers, and review, within the context of previous measurements, the general statistics of peak irradiance, pulse width, and energy associated with the data. We compare the FORTE data with National Lightning Detection Network (NLDN) reported cloud-to-ground (CG) strokes and find that the PDD detection efficiency for these CG strokes is ϳ6%. Moreover, we infer that FORTE preferentially detects the in-cloud portion of optical lightning signals. Events having inferred peak powers between 10 8 and 10 12 W and optical energy outputs between 10 3 and 10 9 J are observed. From a population of nearly 700,000 events we find that the median peak power and median detected optical energy at the source are estimated to be ϳ1 ϫ 10 9 W and 4.5 ϫ 10 5 J, respectively. These values of source peak power and energy are comparable to previous space-based measurements and consistent with aircraft-based and ground-based measurements. The observed median effective pulse width is about 590 microseconds. Further, the pulse widths for CG strokes, reported by NLDN, are inversely proportional to pulse peak power. This paper is not subject to U.S.
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-
Characteristics of impulsive VHF lightning signals observed by the FORTE satellite
Journal of Geophysical Research Atmospheres, 2002
We study very high frequency (VHF) and optical emissions from lightning, observed by the FORTE satellite, differentiating between impulsive (transionospheric pulse pairs (TIPPs)) and nonimpulsive events. TIPPs are seen to constitute 47% of the FORTE VHF data but only 32% of the optically coincident data. The median peak optical irradiance of the optical emission associated with TIPPs is 916 μW/m2 at FORTE and for non-TIPPs is 195 μW/m2. The median effective pulse width of the optical signal from TIPPs is 658 μs, and it is 548 μs for non-TIPPs. In the VHF, both event types have similar observed peak powers (0.086 mV2/m2 and 0.089 mV2/m2, for TIPPs and non-TIPPs, respectively). The optically coincident lightning (of either type) is weaker in peak VHF emission than is the lightning that lacks coincident optical signals, although for non-TIPPs, the stronger the VHF peak, the more likely the event is to have a coincident optical signal. For TIPPs, however, this is true only for events with peak E2 < 0.1 mV2/m2. Above that threshold, TIPPs are increasingly less likely to show coincident optical emission with increasing VHF peak E2. For both TIPPs and non-TIPPs, the peak current reported by the U.S. National Lightning Detection Network™ and peak VHF power reported by FORTE are statistically proportional. The nature of the proportionality appears to depend upon the polarity of the discharge but not upon the event type. We also find that only 11% of TIPPs are associated with negative-polarity discharges, compared to 75% of non-TIPPs. Finally, we find that TIPPs arise from events with altitudes of 6-15 km, although we see optical coincidence only for those TIPPs occurring above ˜10 km.
Chapter 13: Space- and Ground-Based Studies of Lightning Signatures
2009
This article provides a brief survey of the space-and ground-based studies of lightning performed by investigators at Los Alamos National Laboratory (LANL). The primary goal of these studies was to further understand unique lightning signatures known as Narrow Bipolar Events (NBEs). First, an overview is presented of the Fast On-orbit Recording of Transient Events (FORTE) satellite and of the ground-based Los Alamos Sferic Array (LASA). This is followed by a summary of the phenomenology, physics, and meteorological context of NBEs and NBE-related discharges. This article also discusses additional radio frequency and optical observations of lightning made by the FORTE satellite and concludes with an outlook on LANL's growing interest in the use of lightning observations in the study of severe weather and hurricane intensification.
Some Scientific Objectives of a Satellite-Borne Lightning Mapper
Bulletin of the American Meteorological Society, 1983
The Lightning Mapper Sensor is proposed as an instrument for use on a geosynchronous satellite in the late 1980s to monitor lightning activity continuously over broad areas of the earth. The system was suggested in response to a variety of needs and the resulting data will provide important research information for such fields of geoscience as magnetospheric and ionospheric physics, atmospheric electricity, atmospheric chemistry, and storm physics. The research applications of Lightning Mapper Sensor data and related research programs are explored and sensor requirements are discussed. trical circuit, atmospheric chemistry, storm physics, solartropospheric effects, and lightning science are discussed in the following sections, followed by a brief discussion of system requirements and related research activities. The report was assembled and edited by M. H. Davis, USRA. The coauthors were participants at the meeting or contributed material. We acknowledge the helpful comments of John Latham,
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.
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.
Physics of lightning: new model approaches and prospects of the satellite observations
Physics-Uspekhi, 2018
Contents 1. Question of lightning initiation and evolution and new observation possibilities 766 2. Satellites as unique instruments for detecting lightning discharge radiation 769 3. New approaches in the lightning discharge theory 772 3.1 Lightning initiation as a noise-induced kinetic transition; 3.2 Lightning discharge as a fractal dissipative structure; 3.3 Compact intracloud discharge model as an example of applying a new approach for describing discharge phenomena 4. Conclusions 776 References 778
Journal of Geophysical Research, 2003
1] Prior studies have noted a strongly nonlinear enhancement of lightning flash rates with increasing cloud height. Here we report a related observation, of a tendency for increasing intracloud-discharge radiofrequency-emission power for increased height of the electrified cloud. The FORTE satellite's radio-frequency-receiver payload has performed extensive recordings of electromagnetic emissions of lightning discharges. The most commonly occurring such emission arises from intracloud electrical breakdown and is usually recognizable by a pulse followed by a delayed echo from the ground reflection. We have used other systems of lightning monitors to provide source locations for an extended data set of FORTE intracloud-discharge signals. The interpulse separation within each pulse pair yields the discharge height above the reflective ground. The storm in which the pulse occurs usually provides many (at least 50) recorded events. From the pattern of these events' heights, we can usually infer a capping height, which serves as an upper bound on the lightning discharge heights for that storm. We find that there is a strong statistical increase of effective radiated power of intracloud discharges, for increasing capping height of the parent storm. Thus a future satellite-based lightning monitor that triggers on only the most intense radiofrequency emissions will be strongly selective for electrified storms with very deep vertical development. Such storms are also indicated in severe convective weather.
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.
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.