163,546 research outputs found
Numerical study of mode conversion between lower hybrid and whistler waves on short-scale density striations
No full textWe present a theoretical and numerical study of linear mode conversion of lower hybrid waves interacting with short-scale density striations in the Earth's ionosphere. The efficiency of the conversion process is investigated for different sets of parameters such as the angle of incidence, the wavelength of the lower hybrid wave, and the size of the striation. It is found that the most efficient whistler generation occurs at a critical angle of incidence where the whistler waves are driven resonantly along the density striations, and when the product of the striation width and the wave number of the lower hybrid wave is of the order unity. It is suggested that whistlers generated as a byproduct of upper hybrid F-region ionospheric heating can be observed on the ground and by satellites. The generated whistlers could be important for the precipitation of energetic electrons in the radiation belts
Determining the size of lightning-induced electron precipitation patches
No full text[1] We analyze Trimpi signatures during 23 and 24 April 1994 at four sites on or near the Antarctic Peninsula (Palmer, Faraday, Rothera, and Halley) on subionospheric VLF signals received from four U. S. naval transmitters (NAA, NSS, NLK, and NPM). Electron precipitation patches are found to be large, i.e., similar to1500 km x 600 km, with the longer axis orientated east-west. Calculations using a three-dimensional Born scattering model, where patch densities are 1.5 electrons cm(-3) above ambient at the center at similar to84 km altitude, provides results that are consistent with this picture. A high proportion (38%) of the Trimpi events were associated with strong lightning flashes in eastern United States. When lightning discharges had currents >65 kA (positive or negative), there was a >80% chance of seeing an associated Trimpi event. The chance of seeing any Trimpi events fell to near zero for discharges of <45 kA. The largest Trimpi perturbations occur when the center of the precipitation patch is 700-800 km from the receivers. This result is consistent with the modeling calculations for large patches. The equatorward edge of the precipitation patch was estimated to be at &SIM;60&DEG;S, close to the magnetic conjugate of the lightning. The close association of the equatorward edge of the precipitation patch with the conjugate location of the causative lightning is consistent with a quasi-ducted whistler-induced precipitation mechanism. Nonducted whistler-induced precipitation mechanisms would predict a 5&DEG;-10&DEG; latitudinal gap between the lightning and the equatorward edge of the patch. However, the lack of observed whistlers at the time of the Trimpi events is consistent with the nonducted whistler mechanism and is not consistent with the quasi-ducted mechanism, although the distances from duct exit point to receiver may have been too large (&SIM;700-1000 km) for the signals to be detectable. Using the significantly larger patch dimensions determined in this study, it is estimated that lightning may well be 10-100 times more effective at depleting the radiation belts than hiss
Electron heating by nonlinear whistler waves
No full textAbstract Nonlinear whistler-mode phenomena observed in laboratory plasmas will be addressed. Nonlinearities arise when strong waves modify the density, temperature and magnetic field, all of which affect the wave propagation. A brief review of thermal filamentation will be given. The main focus is on the magnetic nonlinearity of whistler modes whose wave magnetic fields exceed the ambient magnetic field B 0 . Such intense waves are launched from loop antennae with axial fields along B 0 and form in one polarity whistler spheromaks and whistler mirrors in the opposite polarity. Spheromaks propagate slower, mirrors faster than linear whistlers. Spheromaks show soliton-like properties. In a whistler vortex (spheromak), the toroidal electron current ring with J E converts magnetic energy into electron heat. In contrast, linear whistlers and whistler mirrors are supported by electron Hall currents and produce negligible heat (J · E = 0). The collision of counter-propagating spheromaks is inelastic, forming a stationary, helicity-free field-reversed configuration, while linear whistler modes pass through each other without interaction. These results are important for the understanding of strong whistler turbulence and reconnection
Monitoring the plasmapause using geomagnetic field line resonances
No full textThis paper discusses the use of ground magnetometer data to derive plasma mass density profiles of the dayside plasmapause region with spatial and temporal resolution in the range 0.15-0.4 R-E and 20-60 min. This is achieved using cross-phase techniques to identify field line resonance signatures that are not apparent in power spectra. Under quiet conditions, mass density profiles do not show a distinct plasmapause and closely resemble electron density profiles for similar conditions. Under more active conditions the plasmapause can be clearly identified, and its width can be resolved in about 20% of the cases. Spatial integration effects smooth the mass density profiles near the plasmapause boundaries, while comparison of the mass and electron densities allows estimates of the heavy ion mass loading. Temporal variations in the plasmapause position and plasmaspheric density depletions are readily resolved. Sudden changes in solar wind conditions cause a redistribution of plasma within similar to20 min, probably in response to penetration of the magnetospheric electric field into the plasmasphere. Field line resonances occur daily and provide a useful tool for investigating the plasmapause region, especially in conjunction with VLF whistler and in situ particle and imaging experiments. Furthermore, the extensive existing suites of magnetometer data permit retrospective studies of focus intervals
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
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”
[Report to Chief J. E. Curry, by an unknown author #1]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
Whistler Duct Formation
No full textIt is proposed that whistler ducts, field aligned ionization enhancements in the plasmasphere, are the direct result of a positive feedback loop coupling the plasmasphere and ionosphere. Estimated sizes are in accord with experimental data and whistler duct movement is shown to be a natural consequence of the theory. During their formation large fluxes of electrons will be precipitated - thus accounting for the observed anomalous electron precipitation accompanying whistler mode waves. Ionospheric irregularities will also be associated with whistler ducts
Whistler intensities above thunderstorms
No full textInternational audienceWe report a study of penetration of the VLF electromagnetic waves induced by lightning to the ionosphere. We compare the fractional hop whistlers recorded by the ICE experiment onboard the DEMETER satellite with lightning detected by the EUCLID detection network. To identify the fractional hop whistlers, we have developed software for automatic detection of the fractional-hop whistlers in the VLF spectrograms. This software provides the detection times of the fractional hop whistlers and the average amplitudes of these whistlers. Matching the lightning and whistler data, we find the pairs of causative lightning and corresponding whistler. Processing data from ∼200 DEMETER passes over the European region we obtain a map of mean amplitudes of whistler electric field as a function of latitudinal and longitudinal difference between the location of the causative lightning and satellite magnetic footprint. We find that mean whistler amplitude monotonically decreases with horizontal distance up to ∼1000 km from the lightning source. At larger distances, the mean whistler amplitude usually merges into the background noise and the whistlers become undetectable. The maximum of whistler intensities is shifted from the satellite magnetic footprint ∼1 • owing to the oblique propagation. The average amplitude of whistlers increases with the lightning current. At nighttime (late evening), the average amplitude of whistlers is about three times higher than during the daytime (late morning) for the same lightning current
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