James B. Brundell

VLF lightning location by time of group arrival (TOGA) at multiple sites

Abstract Lightning is located by using the time of group arrival (TOGA) of the VLF (3– 30 kHz ) radiation from a lightning stroke. The dispersed waveform (“sferic”) of the lightning impulse is processed at each receiving site. The TOGA is determined relative to GPS at each site from the progression of phase versus frequency using the whole wave train. Unlike current VLF methods which require transmission of the whole wave train from each site to a central processing site, the TOGA method requires transmission of a single number (the TOGA) for lightning location calculation. The stable propagation and low attenuation of VLF waves in the Earth–ionosphere waveguide (EIWG) allows a wide spacing of receiver sites of several thousand kilometer so that a truly global location service could be provided using only ∼10 receiver sites.

Rapid onset, rapid decay (RORD), phase and amplitude perturbations of VLF subionospheric transmissions

Abstract Rapid onset (few ms), rapid decay (~ls) perturbations or RORDs occur frequently on the west-to-east signal from NWC to Dunedin, more often than not with classic Trimpis. They do not appear on an NWC mimic signal directly injected into the antenna and so cannot be broadband bursts. There is no delay between the initiating sferic and RORD start, implying that they are produced not by whistler-induced electron precipitation but directly by lightning. Observations on a multi element array show that classic Trimpis and RORDs initiated by the same sferic usually come from measurably different directions, so the lightning-induced ionisation enhancements (LIEs) which cause them must be laterally displaced. They may also be vertically displaced to explain the differing decay rates (30s versus 1 s). We conclude that RORDs are VLF echoes from vertical columns of ionisation at around 40km altitude and having vertical dimensions of some tens of km and horizontal dimensions of 1–2km, since such a column would scatter sufficient signal to fit observed amplitudes. Cloud-to-ionosphere (CID) lightning discharges (also called “cloud-to-space” and “cloud-to-stratosphere” discharges) of these visible dimensions have been observed on mountain observatories and on board the Space Shuttle.

Growing Detection Efficiency of the World Wide Lightning Location Network

An experimental Very Low Frequency (VLF) World‐Wide Lightning Location Network (WWLLN) is being developed through collaborations with research institutions across the globe. In this paper we report on the steady improvement in the Detection Efficiency (DE) of the WWLLN due to increasing station number, which led to a doubling in locations provided from 2003–2007. In addition, a new algorithm has recently been implemented which leads to DE improvements of 63%.

Far-Field Power of Lightning Strokes as Measured by the World Wide Lightning Location Network

AbstractThe World Wide Lightning Location Network (WWLLN) is a long-range network capable of locating lightning strokes in space and time. While able to locate lightning to within a few kilometers and tens of microseconds, the network currently does not measure any characteristics of the strokes themselves. The capabilities of the network are expanded to allow for measurements of the far-field power from the root-mean-square electric field of the detected strokes in the 6–18-kHz band. This is accomplished by calibrating the network from a single well-calibrated station using a bootstrapping method. With this technique the global median stroke power seen by the network is 1.0 × 106 W, with an average uncertainty of 17%. The results are validated through comparison to the return-stroke peak current as measured by the New Zealand Lightning Detection Network and to the previous ground wave power measurements in the literature. The global median stroke power herein is found to be four orders of magnitude lower...

Sprite observations in the Northern Territory of Australia

Sprites, a form of brief luminous discharge in the upper atmosphere above a thunderstorm, were observed and imaged on two video cameras in Australias Northern Territory. These were the first such ground-based observations made outside the United States. Sprite discharges typically took place between the altitudes of 50 km and 80 km and spanned an average width of 44 km. Many of the sprite events were of long duration, with an average of 145 ms. These spatial and temporal features were similar to those observed from the ground and the air in the United States. During the longer events, some luminous discharge elements were observed to decay as other new elements formed. As the new elements were often laterally displaced from the old, the sprites sometimes appeared to dance across the sky. This phenomenon has been observed in Colorado and named “dancing sprites.” The lateral progression of sprite elements observed in the Northern Territory was overwhelmingly in one direction and covered distances of up to 90 km.

Temporal evolution of very strong Trimpis observed at Darwin, Australia

Very strong phase and amplitude perturbations (Trimpis) of the very strong VLF transmission from NWC received in Darwin, Australia, enabled accurate measurement of the amplitude and phase of the scattered signal and of the time variation of these. The amplitude of the scattered signal decays as the logarithm of time, quite at odds with the exponential decay observed on classic Trimpis. During the amplitude decay, the phase of the scattered signal decreased at a decreasing rate. This is shown to be consistent with scattering from a bundle of sprite-like, conducting columns extending some 50 km below the base of the ionosphere.

High-resolution in situ observations of electron precipitation-causing EMIC waves

Electromagnetic ion cyclotron (EMIC) waves are thought to be important drivers of energetic electron losses from the outer radiation belt through precipitation into the atmosphere. While the theoretical possibility of pitch angle scattering-driven losses from these waves has been recognized for more than four decades, there have been limited experimental precipitation observations to support this concept. We have combined satellite-based observations of the characteristics of EMIC waves, with satellite and ground-based observations of the EMIC-induced electron precipitation. In a detailed case study, supplemented by an additional four examples, we are able to constrain for the first time the location, size, and energy range of EMIC-induced electron precipitation inferred from coincident precipitation data and relate them to the EMIC wave frequency, wave power, and ion band of the wave as measured in situ by the Van Allen Probes. These observations will better constrain modeling into the importance of EMIC wave-particle interactions.

Energetic electron precipitation during substorm injection events: high-latitude fluxes and an unexpected midlatitude signature

[1] Geosynchronous Los Alamos National Laboratory (LANL-97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low-frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e-folding energy of 50 keV for energies <400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50–90% of the peak fluxes observed by LANL-97A. Additionally, there is a concurrent and previously unreported substorm signature at L < 2.8, observed as a substorm-associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave-induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high-energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study.

The structure of red sprites determined by VLF scattering

Red sprites occur high above the stratosphere, just under the ionosphere. Although the first reported observation was over 100 years ago, and the first theory was 40 years ago, only over the last year or so has the subject spread into the popular science magazines, and into the secular media. Most of the studies of the sprite structure have been optical, using the light they emit for a few tens of milliseconds for imaging (low-light video and photography) and spectroscopy. Here, we concentrate on the scattering by sprites of man-made VLF radio waves. This scattering shows that the columnar elements of sprites have a substantial electrical conductivity.

Are VLF rapid onset, rapid decay perturbations produced by scattering off sprite plasma?

Rapid onset, rapid decay perturbations (RORDs) of subionospheric VLF propagation require highly localized or laterally structured plasma at low altitudes to explain the wide angle scattering observed and the rapid decay. Simultaneous occurrence of RORDs and red sprites, illustrated by a single event here, together with VLF phase and group delay measurements from a pair of spaced receivers suggest that RORDs are produced by scattering from conducting columns at the position and with the lateral shape of the sprite. The sprite luminosity decays much faster than the RORDs which depend on the sprite conductivity and so plasma density. Plasma is also produced near the sprite plasma by energetic electrons precipitated from the magnetosphere by ducted whistlers and after the expected whistler and electron propagation delay. This whistler-induced electron precipitation (WEP) plasma produces wide angle VLF scattering similar to that by sprite plasma, implying similar lateral fine structure. This suggests that the processes leading to sprites also produce whistler ducts in the magnetosphere.