We revisit the findings from the FORTE satellite program (1997–2004), which collected optical imaging of lightning as well as optical and radio frequency time series waveforms globally from low‐earth‐orbit. These include surveys of the earth's radio frequency anthropogenic noise environment; earth surface reflectivity at radio frequencies; a scheme for classifying lightning discharge types on the basis of their very high frequency time domain power envelope; insights into the polarization and radiation pattern characteristics of different lightning types, with implications for the underlying discharge processes; and estimates of cloud optical properties based on the statistics of scattered light. Most significantly, FORTE was uniquely suited to capture large samples of data from the rare discharge known as “narrow bipolar events,” enabling detailed examination of their basic characteristics and confirming that they appear to result from fundamentally distinct physical processes compared to other lightning. In particular, despite representing huge charge transfer they evidently produce little‐to‐no light output.

The most radio-powerful intracloud lightning emissions are associated with a phenomenon variously called "narrow bipolar events" or "compact intracloud discharges". This article examines in detail the coincidence and timing relationship between, on the one hand, the most radio-powerful intracloud lightning events and, on the other hand, optical outputs (or lack thereof) of the same discharge process. This is done, first, using coordinated very high frequency (VHF) and optical observations from the FORTE satellite and, second, using coordinated sferic and all-sky optical observations from the Los Alamos Sferic Array. In both cases, it is found that the sought coincidences are exceedingly rare. Moreover, in the handful of coincidences between optical and intense radio emissions that have been identified, the radio emissions differ from their usual behavior, by being accompanied by approximately simultaneous "conventional" lightning radio emissions. It is implied that the most radio-powerful intracloud emission process essentially differs from ordinary incandescent lightning.

For the past two decades, there has been an ongoing research effort at Los Alamos National Laboratory to learn more about the Earth's radiofrequency (RF) background utilizing satellite-based RF observations of terrestrial lightning. The Fast On-orbit Recording of Transient Events (FORTE) satellite provided a rich RF lighting database, comprising of five years of data recorded from its two RF payloads. While some classification work has been done previously on the FORTE RF database, application of modern pattern recognition techniques may advance lightning research in the scientific community and potentially improve on-orbit processing and event discrimination capabilities for future satellite payloads. We now develop and implement new event classification capability on the FORTE database using state-of-the-art adaptive signal processing combined with compressive sensing and machine learning techniques. The focus of our work is improved feature extraction using sparse representations in learned dictionaries. Conventional localized data representations for RF transients using analytical dictionaries, such as a short-time Fourier basis or wavelets, can be suitable for analyzing some types of signals, but not others. Instead, we learn RF dictionaries directly from data, without relying on analytical constraints or additional knowledge about the signal characteristics, using several established machine learning algorithms. Sparse classification features are extracted via matching pursuit search over the learned dictionaries, and used in conjunction with a statistical classifier to distinguish between lightning types. We present preliminary results of our work and discuss classification scenarios and future development.

Ongoing research at Los Alamos National Laboratory (LANL) studies the Earth's radiofrequency (RF) background utilizing satellite-based RF observations of terrestrial lightning. The Fast On-orbit Recording of Transient Events (FORTE) satellite provided a rich satellite lightning database, that has been previously used for some event classification. We now develop and implement new event classification capability on the FORTE database using state-of-the-art adaptive signal processing combined with compressive sensing and machine learning techniques. The focus of our work is improved feature extraction using sparse representations in data-adaptive dictionaries. We explore two dictionary approaches: dictionaries learned directly from data, and analytical, over-complete dictionaries. Discriminative dictionaries learned directly from data do not rely on analytical constraints or knowledge about the signal characteristics, and provide sparse representations that can perform well when used with a statistical classifier. Pursuit-type decompositions over analytical, over-complete dictionaries yield sparse representations by design and can work well for signals in the same function class as the dictionary atoms. We present preliminary results of our work and discuss performance and future development.

The lightning stroke called a "Narrow Bipolar Event", or NBE, is an intracloud discharge responsible for significant charge redistribution. The NBE occurs within 10–20 μs, and some associated process emits irregular bursts of intense radio noise, fading at shorter timescales, sporadically during the charge transfer. In previous reports, the NBE has been inferred to be quite different from other forms of lightning strokes, in two ways: First, the NBE has been inferred to be relatively dark (non-luminous) compared to other lightning strokes. Second, the NBE has been inferred to be isolated within the storm, usually not participating in flashes, but when it is in a flash, the NBE has been inferred to be the flash initiator. These two inferences have sufficiently stark implications for NBE physics that they should be subjected to further independent test, with improved statistics. We attempt such a test with both optical and radio data from the FORTE satellite, and with lightning-stroke data from the Los Alamos Sferic Array. We show rigorously that by the metric of triggering the PDD optical photometer aboard the FORTE satellite, NBE discharges are indeed less luminous than ordinary lightning. Referred to an effective isotropic emitter at the cloud top, NBE light output is inferred to be less than ~3 × 108 W. To address isolation of NBEs, we first expand the pool of geolocated intracloud radio recordings, by borrowing geolocations from either the same flash's or the same storm's other recordings. In this manner we generate a pool of ~2 × 105 unique and independent FORTE intracloud radio recordings, whose slant range from the satellite can be inferred. We then use this slant range to calculate the Effective Radiated Power (ERP) at the radio source, in the passband 26–49 MHz. Stratifying the radio recordings by ERP into eight bins, from a lowest bin (<5 kW) to a highest bin (>140 kW), we document a trend for the radio recordings to become more isolated in time as the ERP increases. The highest ERP bin corresponds to the intracloud emissions associated with NBEs. At the highest ERP, the only significant probability of temporal neighbors is during times following the high-ERP events. In other words, when participating in a flash, the high-ERP emissions occur at the apparent flash initiation.

A set of 133 positive narrow bipolar lightning events located by the Los Alamos Sferic Array (LASA) is examined. The events were selected because their fast electric field change (EFC) records show a secondary positive peak, following the primary positive peak, which is consistent with what would be expected for a propagating current pulse reflecting off an impedance discontinuity along a transmission line. With an assumed propagation speed, the time difference between the primary and secondary (reflected) peak in the field change record offers a direct technique for estimating the length of the discharge channel. If the signal amplitude is assumed to decay exponentially with time and that the reflection coefficient is near unity, the time difference between peaks and the ratio of their amplitudes yields an estimate of the temporal decay rate constant. Statistics on the 133 located events examined show an average peak‐to‐reflected‐peak time of 6.7 μs, a mean amplitude ratio estimate of 6.1, and an amplitude decay rate constant of 4.0 μs. Bound by the speed of light, the upper limit on channel length for the typical positive narrow bipolar event (NBE) in this study is shown to be approximately 2 km.

Hurricanes generally produce very little lightning activity compared to other noncyclonic storms, and lightning is especially sparse in the eye wall and inner regions within tens of kilometers surrounding the eye [Molinari et al., 1994, 1999]. (The eye wall is the wall of clouds that encircles the eye of the hurricane.) Lightning can sometimes be detected in the outer, spiral rainbands, but the lightning occurrence rate varies significantly from hurricane to hurricane as well as within an individual hurricane's lifetime.Hurricanes Katrina and Rita hit the U.S. Gulf coasts of Louisiana, Mississippi, and Texas, and their distinctions were not just limited to their tremendous intensity and damage caused. They also differed from typical hurricanes in their lightning production rate.