Fast On-orbit Recording Of Transient Events Research Articles

Making a Superbolt: Reconciling Observations of the Optically Brightest Lightning on Earth From Different Satellites

AbstractWe previously documented geographic distributions of the optically brightest lightning on Earth—known as “superbolts”—using two space‐based instruments: the photodiode detector (PDD) on the Fast On‐orbit Recording of Transient Events (FORTE) satellite and the Geostationary Lightning Mapper (GLM) on NOAA's Geostationary Operational Environmental Satellites. In this study, we further examine the superbolts identified by the PDD and GLM to reconcile the differences between their geographic distributions. We find that both the physical extent of the parent flash and the development speed of its leaders are important for making a superbolt. The oceanic PDD superbolts tend to occur early in flashes that rapidly expand laterally into long horizontal “megaflashes.” The top GLM superbolts occur over land at later times in particularly large megaflashes. These land‐based flashes grow more slowly until they extend over multiple hundreds of kilometers. The FORTE PDD missed these delayed superbolts due to limitations in its triggering. Coincident Tropical Rainfall Measuring Mission measurements show that the warm season megaflash superbolts detected by Lightning Imaging Sensor/GLM and wintertime oceanic superbolts observed by the PDD occur in otherwise similar thunderstorm environments. Both are marked by: low storm heights (<10 km), widespread precipitation near the surface, small infrared brightness temperature gradients, and low flash rates. We suggest that the vertically compact, stratiform nature of these clouds provides favorable conditions for superbolt production.

FORTE Measurements of Global Lightning Altitudes

AbstractWhile multiple lightning detection systems provide geographical locations of lightning events across the globe, robust lightning altitude measurements on a global scale have proven elusive. Space‐based platforms have an advantageous viewing geometry for making these measurements, but prior studies with the Fast On‐orbit Recording of Transient Events (FORTE) satellite were limited to a few thousand events. In this study, we apply the same technique for calculating source altitude from the previous efforts to a large catalog of hundreds of thousands of global FORTE in‐cloud lightning events that were coincident with flashes geolocated by its lightning imager between 1997 and 2003. We use this new data set to document global variations in lightning altitude. As in previous studies, we find that FORTE primarily resolves sources from the upper (positive) charge layer at ∼11 km altitude in normal thunderstorms. However, sources are also recorded from other charge layers in the storm and from leaders developing between layers. In particular, we note a pronounced increase in source altitude in the first 20 ms of FORTE flashes from the negative leader developing upward into the upper positive charge layer. Regions known for wintertime and/or stratiform lightning have increased contributions from low‐altitude sources, while tropical regions particularly around Panama and the Maritime Continent have the greatest concentrations of high‐altitude sources.

FORTE Measurements of Global Optical Lightning Waveforms and Implications for Optical Lightning Detection

AbstractLightning processes generate a diverse collection of optical pulses from current traversing the lightning channels. These signals are then broadened spatially and temporally via scattering in the clouds. The resulting waveforms measured from space with instruments like the photodiode detector (PDD) on the Fast On‐orbit Recording of Transient Events (FORTE) satellite have a variety of shapes. In this study, we use coincident optical and Radio‐Frequency measurements to document the properties of optical PDD waveforms associated with different types of lightning, estimate delays from scattering in the clouds, and comment on how pulse shape impacts optical lightning detection. We find that the attributes of optical pulses recorded by the PDD are consistent with prior studies, but vary globally and with event amplitude. The most powerful lightning tends to be single‐peaked with faster rise times (median: ∼100 µs) and shorter effective widths (median: ∼400 µs) than normal lightning. Particularly dim events, meanwhile, include cases of broad optical waveforms with sustained optical emission throughout the PDD record, which the pixelated FORTE Lightning Location System (LLS) instrument has difficulty detecting. We propose that this is due to the optical signal being divided between individual LLS pixels that are each, individually, not bright enough to trigger. We use PDD waveforms and Monte Carlo radiative transfer modeling to demonstrate that increasing the temporal and spatial resolution of a pixelated lightning imager will make it more difficult to detect these broad/dim pulses as their energy becomes divided between additional pixels/integration frames.

Combined Optical and Radio‐Frequency Perspectives on the Time Evolution of Lightning Measured by the FORTE Satellite

AbstractWe use a cluster feature data set for the Fast On‐orbit Recording of Transient Events (FORTE) satellite that combines detections from its pixelated Lightning Locating System (LLS), photodiode detector (PDD) and Radio‐Frequency (RF) instrumentation to generate statistics describing the frequency and timing of lightning events detected by each instrument during lightning flashes. Coincident observations from the same vantage point allow us to directly compare flash details that can be resolved by the wide Field of View (FOV) instruments relative to the pixelated LLS—whose design is based on NASA's Lightning Imaging Sensor. We find that both the PDD and RF system typically generate more detections than the lightning imager (mean: 1.5 PDD events per LLS group, 2 RF events per LLS group) from pulses that are either not sufficiently bright in the optical band (in the case of RF) or that lack the optical energy density (in the case of the PDD) required to trigger one of the pixels on the LLS imaging array. This includes additional activity before the first LLS group or after the final LLS group. These FORTE results demonstrate that certain lightning processes would be better resolved by wide‐FOV optical and RF instruments than lightning imagers. Current/future space‐based missions that use/plan to use similar instruments will improve our understanding of flash evolution by resolving details missed by lightning imagers.

Combined Optical and Radio‐Frequency Measurements of a Lightning Megaflash by the FORTE Satellite

AbstractThe optical and VHF instrumentation on the fast on‐orbit recording of transient events (FORTE) satellite is used to document the combined phenomenology evolution of a lightning “megaflash”—Mesoscale lightning that propagates laterally over exceptional distances. We identify a FORTE flash whose maximum extent was 82 km and inferred length over multiple distinct branches exceeded 100 km. This flash lasted 1.2 s and produced 250 optical and 591 radio frequency events. We find that the channel development mapped by FORTE's pixelated lightning imager (LLS) occurred at a typical speed of 2.6 × 105 m s−1 and was accompanied by sustained periods of VHF emission that could individually exceed 100 ms in duration. The impulsive IC events generated by the flash indicate that this development occurred at altitudes between 3 and 8 km. Four +CG strokes were identified in the VHF waveform data that are responsible for two of the three highly radiant LLS groups (two of the +CGs were not as optically bright as the others). These strokes occurred at different locations throughout the flash footprint with the most distant strokes separated by approximately 50 km. These space‐based observations match previous observations of megaflashes as well as ground‐based measurements of negative leader development during “spider” lightning, suggesting that FORTE is sensing the same phenomena.

Combined Optical and Radio‐Frequency Perspectives on a Hybrid Cloud‐To‐Ground Lightning Flash Observed by the FORTE Satellite

AbstractWe use the coincident optical and radio‐frequency (RF) measurements taken by the Fast On‐orbit Recording of Transient Events (FORTE) satellite to shed light on common optical signatures recorded by NASA and NOAA lightning imagers during Cloud‐to‐Ground (CG) lightning. We build flash cluster data for FORTE using the same techniques as the NASA/NOAA instruments to document the optical/RF evolution of an oceanic hybrid ‐CG flash. The flash began with strong Very High Frequency (VHF)‐band emission from a Narrow Bipolar Event (NBE) that initiated a period of normal bilevel intracloud (IC) activity in two vertical layers (8 and 12 km) that lasted for 490 ms. VHF waveforms show step leader activity ahead of the return stroke. All impulsive VHF sources after the stroke come from the lower (8 km) layer. K‐changes are noted following the return stroke, but no subsequent strokes are detected. The optical flash began 136 ms after the NBE RF pulse. Twenty‐two of the 33 optical groups were dim and occurred during the in‐cloud phase of the flash. This activity included both isolated pulses and sustained periods of illumination over tens of milliseconds. Initial cloud pulses accounted for 23% of the total optical radiance from the flash. Illumination during the return stroke contributed 58% of the total radiance, and the K‐changes and cloud pulses after the stroke supplied the remaining 19%. These results highlight the benefit of having RF alongside optical lightning measurements for clarifying signatures in the optical data and providing information on their physical origins.

Classification of satellite-based radio frequency transient recordings using sparse approximations over learned dictionaries

Ongoing research at Los Alamos National Laboratory studies the Earth’s radio frequency (RF) background utilizing satellite-based RF observations of terrestrial lightning. Such impulsive events occur in the presence of additive noise and structured clutter and appear as broadband nonlinear chirps at a receiver on-orbit due to ionospheric dispersion. The Fast On-orbit Recording of Transient Events (FORTE) satellite provided a rich RF lightning database. Application of modern pattern recognition techniques to this database may further lightning research and potentially improve event discrimination capabilities for future satellite payloads. We extend two established dictionary learning algorithms, K-SVD and Hebbian, for use in classification of satellite RF data. Both algorithms allow us to learn features without relying on analytical constraints or additional knowledge about the expected signal characteristics. We use a pursuit search over the learned dictionaries to generate sparse classification features and discuss performance in terms of event classification using a nearest subspace classifier. We show a use of the two dictionary types in a mixed implementation to showcase algorithm distinctions in extracting discriminative information. We use principal component analysis to analyze and compare the learned dictionary spaces to the real data space, and we discuss some aspects of computational complexity and implementation.

The directional dependence of apertures, limits and sensitivity of the lunar Cherenkov technique to a UHE neutrino flux

We use computer simulations to obtain the directional-dependence of the lunar Cherenkov technique for ultra-high energy (UHE) neutrino detection. We calculate the instantaneous effective area of past lunar Cherenkov experiments at Parkes, Goldstone (Goldstone Lunar Ultra-high energy neutrino Experiment, GLUE), and Kalyazin, as a function of neutrino arrival direction, finding that the potential sensitivity to a point source of UHE neutrinos for these experiments was as much as thirty times that to an isotropic flux, depending on the beam-pointing position and incident neutrino energy. Convolving our results with the known lunar positions during the Parkes and Goldstone experiments allows us to calculate an exposure map, and hence the directional-dependence of the combined limit imposed by these experiments. In the 10 21–10 23 eV range, we find parts of the sky where the GLUE limit likely still dominates, and areas where none of the limits from either Parkes, GLUE, or experiments such as the Antarctic Impulsive Transient Antenna (ANITA) balloon experiment or FORTE (Fast On-orbit Recording of Transient Events) satellite experiment are likely to be significant. Hence a large anisotropic flux of UHE neutrinos from these regions is not yet excluded. We also determine the directional dependence of the aperture of future planned experiments with the Australia Telescope Compact Array (ATCA), Australian SKA Pathfinder (ASKAP) and the Square Kilometre Array (SKA) to a UHE neutrino flux, and calculate the potential annual exposure to astronomical objects as a function of angular distance from the lunar trajectory through celestial coordinates. We find that the potential exposure of all experiments at 10 20 eV and below, integrated over a calendar year, is flat out to ∼25° from the lunar trajectory and then drops off rapidly. The region of greater sensitivity includes much of the Supergalactic Plane, including M87 and Cen A, as well as the Galactic Centre. At higher energies this high-sensitivity region becomes broader, and we find that the potential exposure of the SKA at 10 21 eV and above is almost uniform over celestial coordinates.

Cluster-Based Nearest-Neighbour Classifier and Its Application on the Lightning Classification

The problem addressed in this paper concerns the prototype generation for a cluster-based nearest-neighbour classifier. It considers, to classify a test pattern, the lines that link the patterns of the training set and a set of prototypes. An efficient method based on clustering is here used for finding subgroups of similar patterns with centroid being used as prototype. A learning method is used for iteratively adjusting both position and local-metric of the prototypes. Finally, we show that a simple adaptive distance measure improves the performance of our nearest-neighbour-based classifier. The performance improvement with respect to other nearest-neighbour-based classifiers is validated by testing our method on a lightning classification task using data acquired from the Fast On-orbit Recording of Transient Events (FORTE) satellite, moreover the performance improvement is validated through experiments with several benchmark datasets. The performance of the proposed methods are also validated using the Wilcoxon Signed-Rank test.

Local time variation in land/ocean lightning flash density as measured by the World Wide Lightning Location Network

We study local time variation in high peak current lightning over land versus over ocean by using lightning locations from the World Wide Lightning Location Network (WWLLN). Optical lightning data from the photodiode detector on the Fast On‐Orbit Recording of Transient Events (FORTE) satellite are used to determine the relative detection efficiency of the WWLLN for lightning events by region, as well as over land versus over ocean. We find that the peak lightning flash density varies for the different continents by up to 5 hours in local time. Because the WWLLN measures lightning strokes with large peak currents, the variation in local time of WWLLN‐detected strokes suggests a similar variation in local time of transient luminous events (e.g., elves) and their effects on the lower ionosphere.

Total electron content (TEC) variability at Los Alamos, New Mexico: A comparative study: 2. Comparisons with other TEC sources

In this study, the Fast On‐Orbit Recording of Transient Events (FORTE) derived TEC variabilities on a diurnal cycle, seasonal cycle, 11‐year solar cycle, and 27‐day solar cycle are compared with the TEC estimates from the Los Alamos ionospheric transfer function implemented with the International Reference Ionosphere model, the GPS‐derived TEC maps from NOAA, the GPS measurements made at Los Alamos, and the ionosonde critical frequency data at the closest station, Boulder, Colorado. The results show good agreement on average (monthly, annual, or multiyear means) in TEC variability at Los Alamos between the various data sources with relative RMS errors of about 5–10%. The results also show RMS errors larger than 30% for point‐to‐point comparisons, with the most significant errors found during high solar activity years, during summer seasons, and during strong geomagnetic storm conditions. This comparative study suggests that the FORTE‐derived TECs combined with other TEC sources can help to better understand the TEC variability at Los Alamos in providing more accurate time‐dependent site TECs than those derived from a single source or extrapolated from global model predictions.

A demonstration of the capabilities of multisatellite observations of oceanic lightning

We have examined lightning flashes in five nighttime, oceanic thunderstorms, which were jointly observed by Tropical Rainfall Measuring Mission (TRMM) and Fast On‐Orbit Recording of Transient Events (FORTE). The multiplicity of instruments on board these satellites presents a multiphenomenological snapshot view of oceanic nighttime convection. Data are available for five oceanic storms with a total of 40 flashes. The independent optical imagers on each satellite establish the flash locations. The relative fraction of Lightning Imaging Sensor (LIS)–detected optical pulses that were also observed by the FORTE/photodiode detector varied from 0% for LIS range less than 104 J sr−1 m−2 μm−1 to 100% for LIS range greater than 106 μJ−1 sr−1 m−2 μm−1. The FORTE/VHF data sometimes allow estimation of the VHF source emissions heights and identification of individual discharge processes as positive or negative, in‐cloud or cloud‐to‐ground. These observations reinforce the concepts that the VHF pulses are produced by a breakdown process, while the optical pulses are the result of current flow.

A global radio frequency noise survey as observed by the FORTE satellite at 800 km altitude

We present a global radio frequency noise survey observed from the Fast on‐Orbit Recording of Transient Events (FORTE) satellite at 800 km altitude. This is a survey of squared amplitudes (R2) in 44 frequency subbands spaced by 0.5 MHz centered at 38 MHz (“low band”) and 44 subbands spaced by 0.5 MHz centered at 130 MHz (“high band”). We define 13 geographic regions and analyze signal‐free regions of event data, generated from November 1997 to December 1999, that we assume are representative of the noise of interest (discussion provided). Because this noise distribution is nonstationary, we use cross validation to sample the distribution across time. Summary statistics presented for R2 include the mean, median, percentiles, and fractions exceeding thresholds in each of the 44 subbands for the low‐ and high‐frequency bands in each of the geographic regions. Fractions exceeding thresholds are particularly relevant to multiband signal detection. In the low band the R2 versus frequency plot for each of the 13 regions has peaks in the mean and median at approximately 35 MHz; many regions have peaks at approximately 28 and 45 MHz. The high‐band spectral plots are generally slightly smoother with less dramatic peaks than in the low band. The values of 10 × log10(R2) range from approximately −110 to −70 for both the low band and high band and are generally slightly larger in the low band. The continental United States (CONUS) region low band is notably higher, with values ranging from −105 to −62. Several future satellite projects involving triggered radio frequency broadband signal recordings can benefit from a realistic trigger‐noise survey such as done here.

FORTE satellite constraints on ultrahigh energy cosmic particle fluxes

The FORTE (Fast On-orbit Recording of Transient Events) satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (rf) range of 30 to 300 MHz with a dual polarization antenna. We investigate the possible rf signature of ultrahigh energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in ice. We calculate the sensitivity of the FORTE satellite to ultrahigh energy neutrino (UHE $\ensuremath{\nu})$ fluxes at different energies beyond the Greisen-Zatsepin-Kuzmin cutoff. Some constraints on supersymmetry model parameters are also estimated due to the limits that FORTE sets on the UHE neutralino flux. The FORTE database consists of over 4 million recorded events to date, including in principle some events associated with UHE $\ensuremath{\nu}.$ We search for candidate FORTE events in the period from September 1997 to December 1999. The candidate production mechanism is via coherent VHF radiation from a UHE $\ensuremath{\nu}$ shower in the Greenland ice sheet. We demonstrate a high efficiency for selection against lightning and anthropogenic backgrounds. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the corresponding particle fluxes assuming this event represents our background level.

FORTE observations of optical emissions from lightning: Optical properties and discrimination capability

Lightning type identification of temporally coincident optical‐VHF time series event pairs collected by the Fast On‐orbit Recording of Transient Events (FORTE) satellite's photodiode detector (PDD) and VHF instrument allows for the investigation of optical properties as a function of lightning type. General trends in the peak optical irradiance and characteristic pulse widths of PDD‐VHF coincident events are studied as a function of lightning type, using previously established techniques to identify lightning type based on VHF spectogram‐power time series. While lightning type cannot be identified from optical data alone, there are several notable features in the optical record. The distribution of observed characteristic widths of PDD events has a cutoff near 200 μs, which represents a lower limit on the combination of intrinsic optical emission time and pulse broadening due to photon scattering in the intervening clouds. Events with the highest peak optical irradiances observed at FORTE are typically positive initial return strokes. Also, the median value of peak optical irradiance for cloud‐to‐ground lightning events is more than double that for in‐cloud lightning.

The Los Alamos Sferic Array: A research tool for lightning investigations

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 μs) 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.

Coordinated observations of optical lightning from space using the FORTE photodiode detector and CCD imager

This paper presents an overview of the coordinated observation of optical lightning from space using the photodiode detector (PDD) and CCD‐based imager known as the Lightning Location System (LLS) aboard the Fast On‐Orbit Recording of Transient Events (FORTE) satellite. PDD/LLS coincidence statistics are presented and show that both the detected energy density and the detected peak irradiance of optical lightning events are proportional to the number of LLS pixels (pixel multiplicity) which are activated during the event. The inference is that LLS pixel multiplicity is more a function of the detected intensity and horizontal extent of the optical event rather than a direct indicator of the degree of scattering. PDD/LLS event coincidence is also used to improve upon traditional recurrence/clustering algorithms that discriminate against false LLS events due to energetic particles and glint. Energy density measurements of coincident events show that about 4% of the optical energy detected by the broadband PDD appears in the narrowband LLS. This is in general agreement with ground‐based measurements and with assumptions incorporated into the design of current and planned CCD‐imaging sensors.

FORTE observations of simultaneous VHF and optical emissions from lightning: Basic phenomenology

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 μs. This delay is composed of a transit time delay (mean of 105 μs) 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 μs (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.

A FORTE for lightning

A newly launched satellite designed to test technology to monitor compliance with arms control treaties is detecting thousands of more radio bursts from lightning strikes and other phenomena than previously reported from other satellites, according to a poster presented at the AGU Fall Meeting in December.Instruments on board the Fast On‐orbit Recording of Transient Events (FORTE) satellite—a joint project of the United States' Department of Energy's Los Alamos and Sandia National Laboratories—are detecting powerful radio bursts that are 100 times stronger and 1,000 times briefer than radio emissions generated by lightning, and are believed to originate from an altitude of 3–10 miles in the vicinity of thunderstorm complexes.