R. K. Said

Geolocation of terrestrial gamma‐ray flash source lightning

[1] Terrestrial gamma-ray flashes (TGFs) are impulsive (1 ms) but intense sources of gamma-rays associated with lightning activity and typically detected via low orbiting spacecrafts. We present the first catalog of precise (<30 km error) TGF source locations, determined via ground-based detection of ELF/VLF radio atmospherics (or sferics) from lightning discharges, which enables precise geolocation of lightning locations. We present the distribution of source-tonadir distances, established due to effects of Compton scattering on the escaping photons. We find that TGFs occur in coincidence with the lightning discharge, but with a few ms variance, and that a detectable sferic at long distances is nearly always present. The properties of TGF-associated sferics and their connection to multiple-peak TGFs are highly variable and inconsistent, and are classified into two categories. Citation: Cohen, M. B., U. S. Inan, R. K. Said, and T. Gjestland (2010), Geolocation of terrestrial gamma-ray flash source lightning, Geophys. Res. Lett., 37, L02801, doi:10.1029/ 2009GL041753.

Evaluation of the GLD360 performance characteristics using rocket‐and‐wire triggered lightning data

We estimated the performance characteristics of the Global Lightning Dataset (GLD360) using rocket-and-wire triggered lightning data acquired at Camp Blanding, Florida, in 2011–2013. The data set consisted of 201 return strokes and 84 kiloampere-scale (≥1 kA) superimposed pulses (initial continuous current pulses and M components) in 43 flashes. All the events transported negative charge to ground. The GLD360 detected 75 strokes and 4 superimposed pulses in 29 flashes. The resultant detection efficiencies were 67% for flashes, 37% for strokes, and 4.8% for superimposed pulses. Out of 75 detected strokes, one (1.3%) was reported with incorrect polarity. The median location error was 2.0 km, and the median absolute current estimation error was 27%. This is the first comprehensive evaluation of GLD360 performance characteristics relative to absolute ground truth, with all previous evaluations being at least in part relative to other locating systems. The results presented in this work may be applicable to regions in and around Florida.

Climatological diurnal variation of negative CG lightning peak current over the continental United States

This study provides an 11 year climatology of the diurnal variability of the cloud-to-ground (CG) lightning peak current. The local diurnal variation of peak current for negative polarity CG (−CG) flashes exhibits a highly consistent behavior, with increasing magnitudes between the late night to early morning hours and decreasing magnitudes during the afternoon. Over most regions, an inverse relationship exists between the −CG peak current and the corresponding −CG activity, although specific details can depend on region and time of day. Overall, the diurnal variation of the −CG peak current appears to reflect fundamental differences between morning and afternoon storms, but additional studies are required to clearly identify the primary cause(s).

Shipborne LF‐VLF oceanic lightning observations and modeling

Approximately 90% of natural lightning occurs over land, but recent observations, using Global Lightning Detection (GLD360) geolocation peak current estimates and satellite optical data, suggested that cloud-to-ground flashes are on average stronger over the ocean. We present initial statistics from a novel experiment using a Low Frequency (LF) magnetic field receiver system installed aboard the National Oceanic Atmospheric Agency (NOAA) Ronald W. Brown research vessel that allowed the detection of impulsive radio emissions from deep-oceanic discharges at short distances. Thousands of LF waveforms were recorded, facilitating the comparison of oceanic waveforms to their land counterparts. A computationally efficient electromagnetic radiation model that accounts for propagation over lossy and curved ground is constructed and compared with previously published models. We include the effects of Earth curvature on LF ground wave propagation and quantify the effects of channel-base current risetime, channel-base current falltime, and return stroke speed on the radiated LF waveforms observed at a given distance. We compare simulation results to data and conclude that previously reported larger GLD360 peak current estimates over the ocean are unlikely to fully result from differences in channel-base current risetime, falltime, or return stroke speed between ocean and land flashes.

Latest developments in global and total lightning detection

In a constant will to provide the best performances in the field of lightning detection Vaisala has lately conducted significant upgrades on its whole product line. The major key points of these improvements reside in: 1) The introduction of a Global Lightning Detection network: GLD360 offering for the first time a worldwide detection of flashes with significant detection efficiency and location accuracy of the cloud to ground (CG) flashes accompanied by the polarity of the measured discharges and an estimate of the peak current. 2) The implementation of arrival-time onset and propagation corrections in the LS700X sensors family (medium range LF/VLF sensors) leading to an improvement in the CG flashes location accuracy which now reaches 100 meters in operational networks. 3) The development of a new Total Lightning Sensor: TLS200, featuring a full digital VHF interferometer for an accurate mapping in 2 dimensions of cloud flashes in addition to the latest development in the VLF/LF band for accurate and efficient CG flashes location. This paper aims to present an update on these developments.

Polarization of Narrowband VLF Transmitter Signals as an Ionospheric Diagnostic

Very low frequency (VLF, 3–30 kHz) transmitter remote sensing has long been used as a simple yet useful diagnostic for the D region ionosphere (60–90 km). All it requires is a VLF radio receiver that records the amplitude and/or phase of a beacon signal as a function of time. During both ambient and disturbed conditions, the received signal can be compared to predictions from a theoretical model to infer ionospheric waveguide properties like electron density. Amplitude and phase have in most cases been analyzed each as individual data streams, often only the amplitude is used. Scattered field formulation combines amplitude and phase effectively, but does not address how to combine two magnetic field components. We present polarization ellipse analysis of VLF transmitter signals using two horizontal components of the magnetic field. The shape of the polarization ellipse is unchanged as the source phase varies, which circumvents a significant problem where VLF transmitters have an unknown source phase. A synchronized two-channel MSK demodulation algorithm is introduced to mitigate 90∘ ambiguity in the phase difference between the horizontal magnetic field components. Additionally, the synchronized demodulation improves phase measurements during low-SNR conditions. Using the polarization ellipse formulation, we take a new look at diurnal VLF transmitter variations, ambient conditions, and ionospheric disturbances from solar flares, lightning-ionospheric heating, and lightning-induced electron precipitation, and find differing signatures in the polarization ellipse.

Lightning activity following the return stroke

Natural lightning is both frequent and variable and thus a good subject for statistical studies. A typical negative cloud-to-ground (CG) flash consists of multiple individual return strokes. The spatial and temporal distributions of various lightning events throughout the discharge provide a surrogate look inside the CG flash and offer insight into the underlying physical processes. In this study, we combine 8years of National Lightning Detection NetworkTM (NLDN) and North Alabama Lightning Mapping Array (NALMA) data to compute the spatial and temporal distributions of (i) subsequent NLDN-reported return strokes and (ii) LMA-reported sources around NLDN-reported CG strokes. Subsequent strokes are separated into those with the same contact point as the first stroke and those flowing along new lightning channels. Statistically, the distribution of strokes along new channels evolves deterministically, with ∼200 km/s propagation speed from the original channel, comparable to the speed of a stepped leader. This suggests that the −CG subsequent strokes forming new channels may be directly linked to the initial one by a propagating leader inside the cloud. We present LMA case studies and a multiyear analysis of NLDN-LMA data that support this behavior. Our results are supported by ground-truth measurements and video recordings from previous field studies.

Broadband longwave radio remote sensing instrumentation

We present the performance characteristics of a high-sensitivity radio receiver for the frequency band 0.5-470 kHz, known as the Low Frequency Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education, or LF AWESOME. The receiver is an upgraded version of the VLF AWESOME, completed in 2004, which provided high sensitivity broadband radio measurements of natural lightning emissions, transmitting beacons, and radio emissions from the near-Earth space environment. It has been deployed at many locations worldwide and used as the basis for dozens of scientific studies. We present here a significant upgrade to the AWESOME, in which the frequency range has been extended to include the LF and part of the medium frequency (MF) bands, the sensitivity improved by 10-25 dB to be as low as 0.03 fT/ Hz , depending on the frequency, and timing error reduced to 15-20 ns range. The expanded capabilities allow detection of radio atmospherics from lightning strokes at global distances and multiple traverses around the world. It also allows monitoring of transmitting beacons in the LF/MF band at thousands of km distance. We detail the specification of the LF AWESOME and demonstrate a number of scientific applications. We also describe and characterize a new algorithm for minimum shift keying demodulation for VLF/LF transmitters for ionospheric remote sensing applications.

Spatial and temporal patterns in lightning discharges as a proxy of thunderstorm characteristics

Lightning is natures way of destroying electrical buildup in thunderclouds. Thus, the pattern of lightning activity is inherently a proxy measure of the timescales for charge separation and the lateral extent of the charge reunification in a flash. Using data from lightning stroke geolocation networks such as NLDN and GLD360, we deduce statistically the charge buildup time by observing the suppression of the probability of two nearby flashes (an effect which fades away). We characterize this suppression effect for different storms, and as a function of storm phase and lightning parameters such as peak current and polarity.