Richard L. Dowden

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.

Performance Assessment of the World Wide Lightning Location Network (WWLLN), Using the Los Alamos Sferic Array (LASA) as Ground Truth

Abstract The World Wide Lighting Location Network (WWLLN) locates lightning globally, using sparsely distributed very low frequency (VLF) detection stations. Due to WWLLN’s detection at VLF (in this case ∼10 kHz), the lightning signals from strong strokes can propagate up to ∼104 km to WWLLN sensors and still be suitable for triggering a station. A systematic evaluation of the performance of WWLLN is undertaken, using a higher-frequency (0–500 kHz) detection array [the Los Alamos Sferic Array (LASA)] as a ground truth during an entire thunderstorm season in a geographically confined case study in Florida. It is found that (a) WWLLN stroke-detection efficiency rises sharply to several percent as the estimated lightning current amplitude surpasses ∼30 kA; (b) WWLLN spatial accuracy is around 15 km, good enough to resolve convective-storm cells within a larger storm complex; (c) WWLLN is able to detect intracloud and cloud-to-ground discharges with comparable efficiency, as long as the current is comparable;...

ELF and VLF wave generation by modulated HF heating of the current carrying lower ionosphere

Abstract This paper presents further experimental results on ionospheric current modulation, using powerful amplitude modulated HF waves produced by the new heating facility at Ramfjordmoen near Tromso, Norway. As a result of the current modulation, waves in the ULF, ELF and VLF range can be efficiently generated. The experiments discussed here cover the range from low ELF up to 7 kHz. The observed signal strengths are of the order 1 pT. Decomposition of the received ELF/VLF waves into R- and L-mode shows that both modes are usually of comparable strength. The signal strength as a function of modulation frequency shows pronounced maxima at multiples of approximately 2 kHz. The paper also presents a brief theoretical discussion of the processes involved in the generation of ELF/VLF waves by HF induced current modulation.

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.

VLF line radiation observed by satellite

VLF line radiation received by the ISIS 1 and 2 satellites over New Zealand is found to fall into two distinct classes. The first of these consists of magnetospheric lines (MLs) which are characterized by a broadband appearance and by frequency drifts of a few tens of hertz per minute, similar to those reported elsewhere. Both their initial frequencies and their frequency spacings were, however, found to be essentially random rather than multiples of 50 or 60 Hz. The hourly variation in ML occurrence showed no correlation with electrical load in possible electrical mains systems sources. No clear decrease in occurrence on weekends was evident. This first satellite survey with significant numbers of MLs found no evidence of a relationship with power line harmonics. The second class of satellite-received VLF lines consists of “tram lines” (TLs) which are characterized by their very narrow bandwidth and zero frequency drift. TLs appear to lie close to harmonics of 50 or 60 Hz. An example of this class is presented.

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.

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.

Determining the size of lightning‐induced electron precipitation patches

VLF trimpi signatures are analysed at 4 sites in the Antarctic Peninsula, looking at VLF transmissions from 4 US VLF transmitters, namely NAA,NSS,NPM and NLK. The Trimpi effect is numerically modelled on all of these paths, concentrating particularly on actual events observed on many paths simulataneously. Very good agreement with observation is secured. By careful analysis of the results it is concluded that the physical size of precipitation patches is greater than previously thought, typically some 800km wide and 2000km long.