132,896 research outputs found
Trimpi Perturbations from Large Ionisation Enhancement Patches
No full textA number of increasingly sophisticated and realistic models have been developed in order to investigate the interaction between sub-ionospherically propagating VLF waves and regions of enhanced electron density in the D-region caused by lightning induced electron precipitation (LIEs). These LIEs can result in phase and amplitude perturbations on received VLF signals that are referred to as Trimpis. It is important, for comparison with experimentally observed Trimpi effects, that the spatial extent of the D-region electron density perturbation is modeled accurately. Here, it is argued that most previous modeling has used patch sizes that are typically up to 100 km in both latitudinal and longitudinal extent, which are generally smaller than those that actually occur for real lightning induced electron precipitation events. It would also appear that maximum ?Ne values assumed have often been too large and patches have been incorrectly modeled as circular rather than elliptical in horizontal extent. Consequently, in the present work, Trimpi perturbations are determined for LIEs with smaller maximum ?Ne, larger spatial extent and elliptical shape. Calculations of VLF Trimpis have been made as a function of the horizontal coordinates of the LIE centre, over the whole rectangular corridor linking transmitter and receiver. The Trimpi modelling program is fully 3D, and takes account of modal mixing at the LIE. The underlying theory assumes weak Born scattering, but the code calculates a non-Born skin depth attenuation function for the LIE in question. The LIE is modelled as an electron density enhancement with a Gaussian profile in all coordinates. Results for a large elliptical LIE ~ 200 x 600 kms show that significant Trimpis, ~-0.4dB in amplitude and ~+4 degrees in phase are predicted, using modest maximum ?Ne values ~ 1.5 el/cc. Such an electron density enhancement is well within the range that would be expected to result from experimentally observed fluxes of electron precipitation following wave particle interactions with whistler-mode waves. This shows the continued viability of the original explanation of whistler-induced electron precipitation as the mechanism for the “Classical Trimpi”
The numerical simulation of the performance of a robustised broadband frequency LMS adaptive beamformer
No full textThe paper is concerned with the optimal processing of data from an array of sensors/antennas. Such sensors may be sonar, radar, VHF/HF radio or 'acoustics in air'. The processing aims might be any of the following: (a) detection and identification of weak wanted signals. (b) Bearing estimation of weak wanted signals. (c) Presentation of the time series of wanted signals at maximum S/N ratio for further processing/display. (d) Accurate bearing estimation and discrimination of strong signal sources. All array beamformer techniques suffer difficulties when applied in the field. Highly optimised algorithms are quickly degraded by multipathing, array deformation and by sensor errors. Time varying noise fields and finite integration time exact a further toll. Eigenvector methods such as MUSIC are rather expensive when applied to broadband environments. The authors consider a broad band robustised LMS frequency domain adaptive algorithm as described in Nunn (1989). Its performance is analysed in numerical simulations incorporating multipathing, array distortion, sensor errors and finite integration time
VLF emission triggering by a highly anisotropic electron plasma
No full textA recent paper by Bell et al (Bell et al,2000) reports observations from the POLAR spacecraft of highly anisotropic hot electron distribution functions in the equatorial region of the magnetosphere at L=3.4. The particle instrument HYDRA measures electron fluxes from 1-20 keV. VLF emissions triggered by pulses from Omega (Norway) are found to coincide with 'pancake' type electron distributions with average pitch angles >70 degrees, such distributions being effectively confined to the equatorial zone. We examine the linear and non linear wave particle interaction process between pancake distributions and CW ducted VLF signals. It is concluded that the pitch angle range 67-76 degrees dominates the interaction process, and that with in duct wave saturation amplitudes of 6pT strong non linear trapping occurs for these particles. It is difficult to avoid the impression that highly anisotropic pitch angle distributions don’t have a great effect on resonant particle dynamics. High anisotropy has raised the pitch angle of maximum non linear contribution from 61->72 degrees, and reduced particle non linearity somewhat, in that the onset of trapping occurs at 2pT rather than 1.6pT. Using this data a 1D Vlasov Hybrid Simulation (VHS) VLF code was run to numerically simulate risers triggered by a 1 s Omega pulse. The VHS algorithm defines a time varying phase space simulation box covering the trans-equatorial nonlinear trapping region and a segment of parallel velocity space centred on the local resonance velocity. The simulation particles have F defined as a constant on their trajectories by Liouville's theorem. At each time step F is interpolated from the particles onto the fixed phase space grid, allowing resonant particle current to be calculated. The VHS method is extremely efficient since at each step particles leaving the phase box are discarded, and fresh particles are embedded into the phase fluid where the latter flows into the phase box. Successful numerical triggering of emissions by Omega is shown, and examples of risers, fallers and hooks are shown. The integrated linear trans-equatorial amplification of ~10dB agreed well with figures calculated by Bell from HYDRA data. These successful simulations of Omega emissions with highly anisotropic distribution functions confirm that non linear trapping of cyclotron resonant electrons in the geomagnetic field is the root plasma physical mechanism behind the triggering of VLF emissions
Wave Particle Interactions in Electrostatic Waves in an Inhomogeneous Medium
No full textThe system studied is that of a narrow-band electrostatic wave packet in a collision-free plasma. Inhomogeneous effects are represented by a wave-number, which varies linearly with distance. The system is excited by a weak resonant beam, and, to first order in a smallness parameter associated with the weakness of the beam, the resonant-particle distribution function and charge densities are calculated. It is found that second-order resonant particles become stably trapped in the wave, and, after a few trapping periods, make a dominant contribution to the resonant particle charge density. The growth rate due to the resonant beam was found to increase linearly with trapping time, and typically a pulse which traps particles for n trapping periods exhibits a growth rate ˜ n times the linear Landau value. Furthermore, a reactive component of charge density was found that was able to cause a steady change in wave frequency and wave-number. These features of large growth rates and changing frequency should appear in parallel problems involving other wave types. An obvious application is that of VLF emissions in the whistler mode
The contested and contingent outcomes of Thatcherism in the UK
No full textThe death of Margaret Thatcher in April 2013 sparked a range of discussions and debates about the significance of her period in office and the political project to which she gave her name: Thatcherism. This article argues that Thatcherism is best understood as a symbolically important part of the emergence of first-phase neoliberalism. It engages with contemporary debates about Thatcherism among Marxist commentators and suggests that several apparently divergent positions can help us now reach a more useful analysis of Thatcherism’s short- and long-term outcomes for British political economy. The outcomes identified include: an initial crisis in the neoliberal project in the UK; the transformation of the party political system to be reflective of the politics of neoliberalism, rather than its contestation; long-term attempts at the inculcation of the neoliberal individual; de-industrialisation and financial sector dependence; and a fractured and partially unconscious working class. In all long-term outcomes, the contribution of Thatcherism is best understood as partial and largely negative, in that it cleared the way for a longer-term and more constructive attempt to embed neoliberal political economy. The paper concludes by suggesting that this analysis can inform current debates on the left of British politics about how to oppose and challenge the imposition of neoliberal discipline today
Cyclotron amplification of whistler-mode waves: a parametric study relevant to discrete VLF emissions in the earth's magnetosphere
No full textWe study the non linear amplification of VLF waves in the earth's magnetosphere. Particular attention is paid to zero order distribution functions of electrons in which there is a sharp step with respect to parallel velocity. It is shown that such steps strongly favour both the linear and non linear amplification processes. The results obtained are discussed in the light of recent observations of VLF emissions, particularly those of Bell and co workers at Stanford University
Modelling the relaxation of red sprite plasma
No full textRed sprites consist of multiple ionised columns extending above a thunderstorm from ~30 km to ~90 km. Electron densities in these columns are very much larger than the ambient background, perhaps fives times at 70 km. These highly ionized structures cause observable perturbations in subionospheric VLF transmissions known as "VLF Sprites". Three models of initial sprite electron density are considered, and using a realistic ionization relaxation model the time dependence of electron density is derived. A 3-D Born propagation code of is used to compute the time profile of a VLF sprite. Two profiles show good agreement with the time signature experimentally observed, in that scattered amplitude and phase decrease linearly log(t). These simulations provide insight into the nature and structure of sprite columns, and indicate an additional constraint which should be applied to red sprite creation models
Cyclotron acceleration of radiation belt electrons by whistlers
No full textThe work considers the non linear scattering of energetic electrons in the earth's radiation belts due to cyclotron interaction with VLF whistlers. In particular we consdier the acceleration of electrons which may result from trapping in the inhomogeneous medium. It is shown that considerable electron heating may result, and that the very anisotorpic electron distribution functions observed by Bell etal may be explained
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