Use of VLF transmissions in the location and mapping of lightning-induced ionisation enhancements (LIEs)

Lightning-induced ionisation enhancements (LIEs) are usually produced by short (~ 1 s) bursts of energetic electrons precipitated from the radiation belts in the process of amplifying whistlers. During their short life (~ 30 s) LIEs diffract or otherwise modify stable transmissions of VLF waves propagating in the two-dimensional Earth-ionosphere waveguide. This causes perturbations (‘Trimpis’) on the same time scale in the phase and amplitude of these VLF waves. Unless the LIEs are large (> 100 km) and smoothly varying (e.g., Gaussian distribution of ionisation enhancement) in the horizontal directions, the LIEs need not be on the great circle path (GCP) from VLF transmitter to receiver to produce Trimpis. Large and smooth LIEs produce ‘GCP Trimpis’, while small or structured LIEs produce ‘echo Trimpis’. The two can usually be distinguished, if Trimpi phase and amplitude are monitored and if the Trimpis are observed at several frequencies or on two or more spaced receivers simultaneously.If only GCP Trimpis are considered, the causative LIEs can be located and mapped by geometric optics using a network of receivers of sufficient density (spacing ~ 100 km) and a few transmitters. Provided all Trimpis are identified as GCP, their mere detection is sufficient for location. This is equivalent to locating the LIE ‘shadows’ cast onto arrays of spaced receivers by two or more transmitters. If this GCP identification is not made, or is just assumed, location and mapping (size estimation) errors can be quite large. At VLF (λ ~ 15 km) this geometric optics approach cannot be used to study the horizontal fine structure of LIEs since LIEs producing GCP Trimpis have no fine structure.Small or structured LIEs cast a diffraction pattern onto an array of spaced receivers. If both the phase and amplitude perturbation of echo Trimpis are measured at each receiver of the array, holographic techniques can be used to reconstruct the two-dimensional map or image of the causative LIEs. It is shown that, for a single system of one transmitter and a receiver array, this allows high resolution (~ 10 km) in the azimuthal dimension only. Equally high resolution in both horizontal dimensions can be achieved with two orthogonal systems. This technique works equally well on GCP Trimpis to map the causative LIEs (which are large and structureless) without incurring location errors thereby.     

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

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.     

Lightning-induced perturbations on VLF subionospheric transmissions

Phase and amplitude perturbations on VLF subionospheric transmissions from transmitter NWC to Dunedin have been studied on both MSK frequencies and at spaced receivers, 9 km apart. In any one event (a ‘Trimpi’) the phase and amplitude perturbation can be expressed in terms of a perturbation phasor. This is generally believed to be the result of lightning-induced electron precipitation (LEP) producing a localized increase in ionization near the normal reflection height for subionospheric (waveguide) VLF waves. Most of the Trimpis received on the NWC-Dunedin path can be best explained if the LEP ionization is sufficiently localized so that it acts as a scattering centre for the subionospheric VLF wave from the transmitter. It is then this scattered wave or echo at the receiver which makes the perturbation phasor. We call these ‘echo Trimpis’. The phase of the echo relative to the direct signal will differ on spaced antennae if the angle of arrival of the two signals differ. Similarly, this relative phase will vary with frequency if the group delay of the signals differ. Thus measurement of these differences allows location of the scattering centres, and so too the LEP. Locations made show a significant grouping in a region where the lightning intensity is high. This and other features strongly suggest that these echo Trimpis originate from local (southern hemisphere) lightning. This and other reasons are suggested to explain the high proportion of echo Trimpis on this path.     

Quasi-periodic particle precipitation and Trimpi activity at Halley,Antarctica

The relationship between quasi-periodic VLF emissions and micropulsations is briefly reviewed, and then discussed with reference to an event recorded at Halley, Antarctica, on day 257 in 1986. VLF emissions at 2 kHz with a quasi-period of 9 s were observed simultaneously with Pi1 and Pe1 micropulsations. Also observed was a quasi-periodic Trimpi event on the amplitudes and phases of the VLF transmitters NAA and NSS. It is deduced that the VLF emissions are modulated in the generation region by a hydromagnetic wave, giving rise to particle precipitation. The emissions are also modulated by the bounce period of the generating particles. The Trimpi effect is due to 120 keV electrons being precipitated into the lower ionosphere by the interaction with the VLF emissions. This event shows that the Trimpi effect can be used to detect particle precipitation occurring during the ULF/VLF interaction, and can give information which helps to define the mechanism reponsible for the interaction.     

Characteristics and frequency of occurrence of Trimpi events recorded during 1982 at Sanae,Antarctica

‘Trimpi’ amplitude perturbations on VLF signals received at Sanae, Antarctica, have been identified using a new computerised technique. Our survey of 1982 data, taken during magnetically disturbed times, shows that events of short duration (<25 s) constitute 60% of all events detected and that all events found are amplitude attenuations with deviations from quiescent levels ranging up to 90%. It is unusual, at Sanae, to observe the causative whistler with a Trimpi event. This, together with further evidence from Trimpi occurrence statistics, may suggest that the gyroresonant interactions responsible for some of the events occur with non-ducted whistler mode waves. A method for estimating the extent of the precipitation region is presented.     

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban (L = 1.69, 29°53′S, 31°00′E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L (L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds (T_i ∼ 5 ms) and large wave amplitudes (B_w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.     

Experimental daytime VLF ionospheric parameters

Measured field strengths from VLF transmitters are used to determine improved daytime values of ionospheric parameters to enable improved VLF propagation predictions. These parameters are the traditional H′ (height in km) and β (sharpness in km⁻¹) as used by Wait and by NOSC in their Earthionosphere waveguide computer program. They are found by comparing the predictions of the NOSC program with the observed VLF field strengths over both long and short paths.Experimental observations from two nearly north-south paths are used to determine the solar zenith angle dependence of both H′ and β for low latitude (or summer mid-latitude) conditions. These results are then used to predict the daytime variations in VLF field strengths with solar zenith angle (and hence time) on other suitable paths and good agreement is found with measurements made on these paths.The absolute value of β for overhead Sun is found to be 0.45 km⁻¹ and is principally determined by the attenuation on the very long, west to east, fully sunlit, 14.4 Mm path from NWC (Australia, 22°S) to San Francisco (37°N), after applying small corrections for the solar zenith angle variations along the path at midday. Further support is obtained from results from the 8.6 Mm path NDT (Japan) to San Francisco, an 8.2 Mm path NPM (Hawaii) to New Zealand, and an east to west 7.5 Mm path from NPM to Townsville, Australia. The conditions studied are solar maximum. The frequencies studied are 15–30 kHz.     

Ray-tracing the paths of very low latitude whistler-mode signals

VLF whistler-mode signals with very low group delays (75–160 ms) received at night in Dunedin, N.Z., from the 23.4 kHz MSK transmissions of NPM, Hawaii (21.5°N, 158°W), are explained by ray-tracing along unducted paths. The typical vertical and horizontal electron density gradients of the night equatorial ionosphere are found to be sufficient to explain not only the typical group delays but also their decrease during the night and the typical frequency shifts observed on these signals. An important feature appears to be the decreasing starting and finishing latitudes (and the decreasing maximum height of the path) during the course of the night. The amplitude of the signals in relation to the expected collisional absorption in the ionosphere is discussed. A simple but quite accurate analytical expression suitable for ray-tracing is derived for the night electron density in the height range 170–1400 km, based on non-isothermal diffusive equilibrium and O⁺/O friction.     

ELF/VLF propagation measurements in the Atlantic during 1989

The vertical electric field component was measured by a group of the Ukrainian Institute of Radio Astronomy on board the Professor Zubov scientific vessel during April 1989 at latitudes from 30°S to 50°N. Results of the amplitude measurements in the Atlantic of natural ELF radio signals and those from the VLF navigation system “Omega” at its lowest frequency of 10.2 kHz are given. Characteristics were obtained of the moving ship as the field-site for the ELF observations. Variations in the ELF radio noise amplitude recorded at tropical latitudes agree with the computed data for the model of three continental centres of lightning activity. The VLF results were obtained by the “beat” technique providing the simplest narrow-band amplitude registration. Range dependencies of the field amplitudes from A (Norway), B (Liberia) and F (Argentina) stations have been analysed. The VLF attenuation factor was estimated for the ambient day conditions along the four cardinal directions. This allowed the detection of a statistically significant attenuation difference between the east-west and west-east propagation paths. The VLF radio signal was also used as a probe to evaluate the effective height of the vertical electric antenna and to calibrate the ELF noise amplitudes.     

SABRE observations of E-region vertical velocity structures

The variation of backscatter intensity with slant range for the Wick radar of the SABRE system consistently exhibits a minimum at approximately 800 km. Theoretical modelling of the antenna characteristics indicates that this is due to a null in the vertical polar diagram due to interference from ground reflections (Lloyd's mirror effect). By means of this “null” feature, the vertical velocity distribution within the scattering layer can be examined.A sharp discontinuity often exists in the variation of the Doppler velocity with range across the backscatter minimum. These observations can be accounted for in terms of an increase in the drift velocity of about 200 m s⁻¹ with height across the 10 km range of the scattering layer. These results are consistent with the theoretical predictions of St.-Maurice and Schlegel based on the assumption of a two-stream instability.     

A simple theoretical model of the diffraction of VLF electromagnetic waves by the Antarctic icecap

The Antarctic continent has been modelled as a spherical cap whose pole is coincident with that of the South Pole, which totally absorbs VLF radio waves attempting to propagate over it. The propagation of Omega navigation signals around this model icecap has then been computed using Kirchhoff diffraction theory. Spherical caps extending to 66.5 and 75.5°S have been found to accurately model the signals from Omega La Reunion and Argentina, respectively, received on flights between Christchurch, New Zealand and Scott Base in Antarctica, up to the boundary of the theoretical icecap. These model icecaps were found to be good fits to the boundary of the Antarctic continent, when measured at the 1–1.5 km contour of ice thickness, in the region where the VLF waves diffracted around the icecap. The good agreement obtained between the experimental field strength data and those computed theoretically, using only simple diffraction theory, suggests that coastal refraction plays at most only a secondary role in circumpolar propagation.     

In-situ observations of magnetospheric electron scattering by a VLF transmitter

High resolution pitch angle measurements of outer zone electrons in the energy range 12 keV−1.6 MeV were obtained at high altitude in the region of the high power VLF transmitter UMS [300 kW radiated at 17.1 kHz (Watt A. D., 1967, VLF Radio Engineering, Pergamon Press, Oxford)] while a resonant wave-particle interaction was in progress. Additional complementary electron measurements in the range of 36–316 keV were obtained in the drift loss cone by another satellite at low altitude along the drift path 75° east of the interaction region. The data from the low-altitude satellite confirm that UMS was precipitating particles in the inner zone, in the slot, and in the outer zone at the time that the high-altitude satellite was obtaining its data. The high-altitude pitch angle distributions indicate that, for this event, two types of scattering interactions were in progress. Particles with small pitch angles, up to 17.2° at the Equator, were being removed, resulting in an enhanced loss cone. Particles which were mirroring between 6500 km and the altitude of the spacecraft (7200) km were also being strongly scattered, resulting in a relative minimum in the pitch angle distribution around 90°. The data are interpreted as indicating that a cyclotron mode interaction with UMS waves was precipitating electrons with equatorial pitch angles up to 17.2° and that another process, perhaps electrostatic (ES) waves arising from the UMS radiations through a mode-conversion process, was present in the region above 6500 km and was efficiently scattering those particles which mirrored in that region     

Reflection of VLF radio waves from distant mountain ranges

A computer cross-correlation technique is being used to determine the group delays and directions of arrival of man-made subionospheric VLF signals which have reached the receiver by paths other than the direct great circle path. The 200 baud MSK signals transmitted by NWC, NPM and NLK allow time resolution to at least 5 ms and, with 15 min of integration, the sensitivity can be as low as about 0.1 μV m⁻¹ in quiet conditions. Reflections from the Andes, the Rockies and the mountains of S.E. Asia have now been identified at Dunedin, New Zealand. Round-the-world and round-the-world-the-other-way signals have also been observed.     

Re-radiation of VLF radio waves from mountain ranges

VLF signals transmitted from Hawaii, Japan and Australia (NPM, NDT, NWC) and scattered from large mountain ranges such as the Andes or Rockies are monitored in New Zealand in the presence of the much larger direct signals. Measurements of the amplitudes of these indirect signals are reported in the range 0.l-2.0μVm⁻¹.Sudden changes in the refractive index along the path, such as those caused by abrupt changes in ground conductivity, are shown to give rise to reflected amplitudes much smaller than those measured. However, the measured amplitudes are found to be comparable with reasonable estimates of the re-radiation from typical surface currents induced by the incident wave in mountains of suitable height, ground conductivity and steepness.     

The Politics of Knowledge: Anthropology and Māori Modernity in Mid-Twentieth-Century New Zealand

This article argues that growing confidence in anthropology as a tool for managing the adaptation of a resurgent Māori population to modernity shaped a “politics of knowledge” regarding New Zealand's indigenous people in the mid-twentieth century. We examine the relationship between anthropological discourse, state policy, and the Māori struggle to uphold traditional ways, through the prism of Ernest and Pearl Beaglehole's psycho-ethnographic study entitled Some Modern Māoris (Beaglehole, E., and P. Beaglehole. 1946. Some Modern Māoris. Wellington: New Zealand Council for Educational Research). This paper demonstrates that the study was the product of a nexus between concerns for Māori welfare, a perceived need for empirical research that could be applied to the “problem” of indigenous adjustment to contemporary conditions, and American philanthropy. For this reason, and as a detailed record of a small community when Māori society was on the cusp of post-Second World War transformations, we contend that the study deserves to be recovered from historical obscurity.     

Radical Environmentalism and “Commoning”: Synergies Between Ecosystem Regeneration and Social Governance at Tamera Ecovillage,Portugal

This article explores the scope and limitations of Radical Environmentalism as a source of practices of “commoning”. The application of the radical environmental “Healing Biotope” model in Tamera, an ecovillage located in southern Portugal, further expands the understanding of “commoning” as a social process, as well as of Radical Environmentalism as a cognitive framework. This article distinguishes between the technical and political dimensions of “commoning”. It also identifies two structuring dimensions of Radical Environmentalism, hereby called integrative rationality and the experiential action research and learning methodology. These dimensions support the technical aspect of “commoning” in Tamera by promoting epistemic and methodological coherence between social and environmental technologies. Despite their contested scientific validity, they contribute to the sustainability of the project by promoting synergies between ecological regeneration and social governance. However, they have limited capacity to address the political dimension of “commoning”, related with rank and socio‐economic inequalities among members.     

Inside “the system”: engineers,scientists, and the boundaries of social protest in the long 1960s

By the end of the 1960s, many engineers and scientists in the US questioned the social and political dimensions of science and technology. This introspection came as critics assailed science and technology as elements of the militaristic, alienating structure of modern society. Engineers and scientists repudiated, appropriated, or sometimes even acted in concert with these critics. Also relevant, however, in engineers' and scientists' evaluations of their role in society were the emergence of “engineering science,” pre‐existing political ideologies, and simply making a living in a volatile economy. This paper presents dissention from three vantages within the technical community: design engineer Steve Slaby at Princeton University, the Fluid Mechanics Laboratory at the Massachusetts Institute of Technology, and the activist organization Science for the People, located in chapters across the country. These cases differ in the actors' level of political consciousness, their involvement in military research, and the tactics they employed. All, however, suggest cause for reassessment both of the borders between “critics” and “scientists” and of the culture of “total control” ascribed to the era.     

Global study of northern hemisphere quasi-2-day wave events in recent summers near 90 km altitude

We attempt to find the northern hemisphere zonal wavenumber for a striking quasi-2-day wave “event” or “burst” observed near 90 km altitude in the summer of 1992. A unique set of data on the upper atmosphere from nine radar sites is analysed (spacings ∼400– ∼ 12,000 km), and compared with expectations from models. The 2-day wave phase comparison, which finds zonal wavenumber m = 4, is conclusive. Determination of n, which defines the meridional wave amplitude structure, is not attempted, as the sites here have only a small latitude spread (21°N to 55°N). Also the amplitude seems to be unstable showing some sort of modulation which is not simultaneous at all sites. Finally, the radars have not been “calibrated” against each other in terms of wind speed. This calibration would have to be done before small differences in wave amplitude could be believed. A similar event in 1991 for which fewer sites are available is also discussed. Here the choice between m = 3 and 4 is not as clear.     

“Moral Panic” or Pejorative Labelling? Rethinking the Mazengarb Inquiry into Underage Sex in the Hutt Valley in 1954

This article re‐examines the interpretation of widespread concern over significant underage sex in the Hutt Valley, Wellington, which resulted in a government inquiry in 1954. It challenges the typical “moral panic” interpretive lens concerning the inquiry, arguing that the term obscures more than it reveals. The term focuses on reaction to the Hutt Valley affair but fails to address sufficiently the causative question of why such concern existed in the first place. The “moral panic” framing of the Hutt Valley incidents has failed to give adequate recognize that the developments were early indicators of increasing societal shifts that threatened long‐held public views on sexuality; that manifest, societal, sexuality values changes in the next two decades showed that concerned people of 1954 were right within the framework of their worldview to have such concern; and that the so‐called “moral panic” concern of 1954 already existed prior to the Hutt Valley disclosures.