FORTE satellite observations of VHF radiation from lightning discharges (original) (raw)
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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.
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.
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-
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.
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.