Craig J. Rodger

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.

ELF and VLF radio waves

Abstract This review covers developments in ELF and VLF radio-wave propagation research over the last 50 years of the Journal of Atmospheric and Solar-Terrestrial Physics. A review of such a large field, over such a long period, cannot be fully comprehensive and the authors have therefore covered important areas which have they themselves have found interesting. The survey begins with a review of work on natural and man made sources of ELF and VLF radiation. This is followed by sections on experimental and theoretical studies of unperturbed (ambient) ELF and VLF radio propagation. Schumann resonance research, which is currently undergoing a renaissance, is then reviewed. A review of research into transient perturbations of ELF and VLF propagation follows, extending from the early work on nuclear explosions up to the current work on sprites. The review concludes with a brief summary of the VLF navigation systems of the USSR and USA, (Alpha and Omega) whose development and life-span covered most of the last 50 years.

Red sprites, upward lightning, and VLF perturbations

In the last decade there has been a great deal of interest in the detection and understanding of phenomena occurring above active thunderstorms. The discovery of the optical phenomena now termed “red sprites” is discussed, along with the properties that have been experimentally determined. Areas of disagreement between experimentalists are pointed out. Other optical and electromagnetic phenomena associated with red sprites are presented. These include blue jets, transionospheric pulse pairs, and gamma ray flashes. Particular attention is given to the work on perturbations on very low frequency radio wave transmissions (“VLF sprites”), which has provided estimates of the electrical properties of sprites. Research into activity above thunderstorms will continue to lead to a greater understanding of the coupling between thunderstorms in the troposphere to the stratosphere, mesosphere, ionosphere, and beyond.

Missing driver in the Sun-Earth connection from energetic electron precipitation impacts mesospheric ozone.

Energetic electron precipitation (EEP) from the Earth’s outer radiation belt continuously affects the chemical composition of the polar mesosphere. EEP can contribute to catalytic ozone loss in the mesosphere through ionization and enhanced production of odd hydrogen. However, the long-term mesospheric ozone variability caused by EEP has not been quantified or confirmed to date. Here we show, using observations from three different satellite instruments, that EEP events strongly affect ozone at 60–80 km, leading to extremely large (up to 90%) short-term ozone depletion. This impact is comparable to that of large, but much less frequent, solar proton events. On solar cycle timescales, we find that EEP causes ozone variations of up to 34% at 70–80 km. With such a magnitude, it is reasonable to suspect that EEP could be an important part of solar influence on the atmosphere and climate system.

Subionospheric VLF perturbations associated with lightning discharges

Abstract The use of very-low frequency (VLF) transmissions propagating inside the waveguide formed by the Earth and the lower ionosphere is a well developed technique for probing conditions within the waveguide. This paper seeks to review the current understanding of lightning discharge associated processes that lead to changes in the characteristics of the waveguide and thus variations in the received phase and/or amplitude of VLF transmissions. Particular emphasis is placed upon events which appear to be produced directly by lightning discharge processes. These include VLF sprites and early Trimpi for which significant research efforts have been made over the last 10 years. The properties and interpretation of these events are discussed in detail. QE-field produced early Trimpi are due to relatively large disturbed regions (horizontal extent ∼100 km ) in the lower ionosphere. VLF sprites are produced by red sprite discharges, leading to relatively small, dense, ionospheric modifications made up of fine-scale horizontal structures (∼300 m ) that stretch over wide altitude ranges (⩽50– 80 km ). Differences in the ionospheric modifications electron temperatures and ionisation structure produces the differing delay times and scattering patterns in the experimentally observed VLF perturbations. Lightning-EMP produced Elves lead to large (∼500 km ) , relatively smoothly varying ionospheric disturbance at high altitudes (∼85 km ) . Such a modification should create sudden step-like changes in received VLF amplitude and phase that lack a clear relaxation signature and occur along narrow forward-scattered directions. The VLF techniques described in this article are part of the larger field of research into high-altitude processes connected with thunderstorms. Lightning EMP produces a large (∼500 km ) , relatively smoothly varying ionospheric disturbance at high altitudes (∼85 km ) .

Lower ionospheric modification by lightning-EMP : Simulation of the night ionosphere over the United States

It has recently been suggested that successive intense lightning-electromagnetic pulse (EMP) events could cause significant large-scale changes to the properties of the nighttime lower ionosphere. In order to examine this quantitatively, data on lightning detected over the United States are combined with the output from a simulation code. During the course of a night strong lightning-EMP events can lead to significant (∼100% or even greater) increases in the electron density of the lower ionosphere, with the largest increases at ∼90 km altitude. Regions with significant decreases in electron density are also possible. It is shown that changes in the electron temperature of the lower ionosphere are unlikely to be significant. The time required to produce large-scale changes of ionospheric electron density above an active thunderstorm may explain the observation of a thunderstorm “modification time” before red sprite activity is initiated.

POES satellite observations of EMIC-wave driven relativistic electron precipitation during 1998-2010

[1] Using six Polar Orbiting Environmental Satellites (POES) satellites that have carried the Space Environment Module-2 instrument package, a total of 436,422 individual half-orbits between 1998 and 2010 were inspected by an automatic detection algorithm searching for electromagnetic ion cyclotron (EMIC) driven relativistic electron precipitation (REP). The algorithm searched for one of the key characteristics of EMIC-driven REP, identified as the simultaneity between spikes in the P1 (52 keV differential proton flux channel) and P6 (>800 keV electron channel). In all, 2331 proton precipitation associated REP (PPAREP) events were identified. The majority of events were observed at L-values within the outer radiation belt (3 < L < 7) and were more common in the dusk and night sectors as determined by magnetic local time. The majority of events occurred outside the plasmasphere, at L-values ~1 Re greater than the plasmapause location determined from two different statistical models. The events make up a subset of EMIC-driven proton spikes investigated by Sandanger et al. (2009), and potentially reflect different overall characteristics compared with proton spikes, particularly when comparing their location to that of the plasmapause, i.e., EMIC-driven proton precipitation inside the plasmapause, and potentially EMIC-driven REP outside the plasmapause. There was no clear relationship between the location of plasmaspheric plumes and the locations of the PPAREP events detected. Analysis of the PPAREP event occurrence indicates that high solar wind speed and high geomagnetic activity levels increase the likelihood of an event being detected. The peak PPAREP event occurrence was during the declining phase of solar cycle 23, consistent with the 2003 maximum in the geomagnetic activity index, Ap.

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%.

Radiation belt electron precipitation due to VLF transmitters: Satellite observations

In the Earth’s inner magnetosphere, the distribution of energetic electrons is controlled by pitch-angle scattering by waves. A category of Whistler waves originates from powerful ground-based VLF transmitter signals in the frequency range 10–25 kHz. These transmissions are observed in space as waves of very narrow bandwidth. Here we examine the significance of the VLF transmitter NWC on the inner radiation belt using DEMETER satellite global observations at low altitudes. We find that enhancements in the 100–600 keV drift-loss cone electron fluxes at L values between 1.4 and 1.7 are linked to NWC operation and to ionospheric absorption. Waves and particles interact in the vicinity of the magnetic equatorial plane. Using Demeter passes across the drifting cloud of electrons caused by the transmitter; we find that 300 times more 200 keV electrons are driven into the drift-loss cone during NWC transmission periods than during non-transmission periods. The correlation between the flux of resonant electrons and the Dst index shows that the electron source intensity is controlled by magnetic storm activity.

Relaxation of transient ionization in the lower ionosphere

This paper presents a set of basic expressions and parameters to describe the relaxation of ionization in the upper atmosphere and lower ionosphere. Both spatial (diffusion) and chemical loss are included, along with the varying electrical conductivity caused by changing electron density and temperature. In particular, reasonable values for the attachment rates and recombination coefficients at these high altitudes are discussed. Example calculations are given involving studies into red sprites. However, the modeling and parameters are not restricted to these events, and might well be useful in a number of other modeling problems.