Results 1 - 10 of 1635
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[en] The guided propagation of whistler waves along cylindrical density depletion ducts in a magneto-plasma is studied. It is shown that, under certain conditions, such ducts can support volume and surface eigenmodes. The dispersion properties and field structure of whistler modes guided by density depletion ducts are analyzed. The effect of collisional losses in the plasma on the properties of modes is discussed.
[en] Guidance of azimuthally symmetric waves by cylindrical density ducts in magnetoplasma in the nonresonant region of the whistler frequency range is investigated. It is demonstrated that eigenmodes existing at the studied frequencies in ducts with enhanced plasma density allow simplified description that makes analysis of the features of their guided propagation much easier. The results of calculation of the dispersion characteristics and field structure of the whistler modes supported by such ducts are presented
[en] Charge separation effects in a strongly magnetized plasma are shown to reduce the rate of magnetic field line reconnection in the whistler frequency range. A system of two eigenmode equations describing the linear growth of the reconnection instability is derived and solved numerically, using the compressible electron-magnetohydrodynamic theory first developed by Kuvshinov et al. [Phys. Lett. A 241, 287 (1998)]. Charge separation introduces a 'renormalized skin depth' λe, larger than the electron skin depth de, which results in the broadening of the eigenmode structure
[en] Whistlers observed on the ground, in several stations at different latitudes and longitudes, are used to compare various methods of determining whistler parameters in the absence of causative spheric and nose frequency. The Corcuff (1977) method (or CH method) is applied to a large number of whistlers recorded at Kerguelen during a period of several hours. The results show that this method is sufficiently accurate to detect slow variations of the characteristic whistler parameters, and thence the motions of the cold plasma in the magnetosphere
[fr]Des sifflements observes au sol, en des stations de latitudes et de longitudes tres diverses, sont utilises pour comparer differentes methodes d'analyse ne necessitant la connaissance ni de la frequence de 'nose', ni celle de l'atmospherique origine. La methode proposee par Corcuff P. (1977), ou methode CH, est appliquee a un nombre important de sifflements enregistres a Kerguelen au cours de plusieurs heures consecutives. L'etude des resultats montre que cette methode est suffisamment precise pour que des variations lentes des parametres fsub(n), tsub(n) des sifflements soient mises en evidence, variations qu'il est important de connaitre pour determiner les mouvements du plasma froid dans la magnetosphere
[en] A one-dimensional electromagnetic particle-in-cell simulation is performed to study wave excitation due to an isotropic electron beam in a plasma system. The waves excited by the electron beam are analysed with the fast Fourier transformation and the wavelet transformation. The results show that the isotropic beam electrons can excite the whistler and the Langmuir instabilities. Also, the strong damping of whistler modes due to the emergence of electrostatic waves together with the appearance of temperature anisotropy in the electron beam can be found. Furthermore, the velocity distributions indicate that beam electrons are heated in the direction parallel to the ambient magnetic field. The background electrons still exhibit an isotropic velocity distribution, which is different from that of the beam electrons.
[en] Motivated by the upcoming Solar Orbiter and Solar Probe Plus missions, qualitative and quantitative predictions are made for the effects of the violation of the Taylor hypothesis on the magnetic energy frequency spectrum measured in the near-Sun environment. The synthetic spacecraft data method is used to predict observational signatures of the violation for critically balanced Alfvénic turbulence or parallel fast/whistler turbulence. The violation of the Taylor hypothesis can occur in the slow flow regime, leading to a shift of the entire spectrum to higher frequencies, or in the dispersive regime, in which the dissipation range spectrum flattens at high frequencies. It is found that Alfvénic turbulence will not significantly violate the Taylor hypothesis, but whistler turbulence will. The flattening of the frequency spectrum is therefore a key observational signature for fast/whistler turbulence
[en] The parametric decay of a large amplitude electromagnetic wave in the ion cyclotron range of frequency into a compressional Alfven wave and an electromagnetic sideband wave in a magnetized plasma is investigated. The pump wave propagates in the direction of ambient magnetic field whereas the decay waves propagate at oblique angles. When the pump wave is left circularly polarized the decay is not permitted kinematically as the momentum of pump photon always exceeds the sum of momenta of the decay wave photons. For the right circularly polarized whistler mode pump the decay is permitted with sideband nearly right circularly polarized. The density perturbation associated with the Alfven wave couples with the pump driven oscillatory velocities of ions and electrons to produce a current driving the sideband. The sideband and the pump exert pondermotive force on ions and electrons that drive the Alfven wave. The frequency and growth rate of the Alfven wave increase with the normalized pump frequency. The threshold power density, determined by the collisional damping rates of the decay waves is rather modest.
[en] Using the relation proposed by Ho (1974) and a property of the function Q(f)=(t√f)-1 relative to a whistler (Corcuff P. and Corcuff Y., 1973) the method described in this paper gives, with a good approximation, the nose frequency fsub(n) and the minimum group delay tsub(n) when fsub(n) and the causative spheric are unknown. Different relations used in this method enable us to give an interpretation of the empirical relation found by Dowden and Allcock (1971), and to justify the corrections proposed by some authors using the linear approximation for the function Q(f). Theoretical whistlers are used to compare the different methods and to determine the uncertainties due respectively to the measurements, to the extrapolation factor, to the ionospheric propagation and to the electron concentration models
[fr]Utilisant la relation de Ho (1974) et une propriete caracteristique de la fonction Q(f)=(t√f)-1 d'un sifflement electronique (Corcuff P. et Corcuff Y., 1973), la methode decrite permet de determiner avec une bonne precision la frequence de 'nose' fsub(n) et le temps de propagation tsub(n) correspondant, fsub(n) n'etant pas directement observable et l'atmospherique initial n'etant pas identifiable sur le spectrogramme du sifflement. Les relations utilisees permettent de donner une interpretation de la relation empirique trouvee par Dowden et Allcock (1971) et de justifier les corrections proposees par certains auteurs utilisant l'approximation lineaire de la fonction Q(f). Des sifflements theoriques sont utilises pour comparer differentes methodes d'analyse et pour determiner les incertitudes dues respectivement aux mesures, au facteur d'extrapolation, a la propagation ionospherique et aux modeles d'ionisation choisis
[en] Properties of magnetic loop antennas for exciting electron whistler modes have been investigated in a large laboratory plasma. The parameter regime is that of large plasma frequency compared to the cyclotron frequency and signal frequency below half the cyclotron frequency. The antenna diameter is smaller than the wavelength. Different directions of the loop antenna relative to the background magnetic field have been measured for small amplitude waves. The differences in the topology of the wave magnetic field are shown from measurements of the three field components in three spatial directions. The helicity of the wave magnetic field and of the hodogram of the magnetic vector in space and time are clarified. The superposition of wave fields is used to investigate the properties of two antennas for small amplitude waves. Standing whistler waves are produced by propagating two wave packets in opposite directions. Directional radiation is obtained with two phased loops separated by a quarter wavelength. Rotating antenna fields, produced with phased orthogonal loops at the same location, do not produce directionality. The concept of superposition is extended in a Paper II to generate antenna arrays for whistlers. These produce nearly plane waves, whose propagation angle can be varied by the phase shifting the currents in the array elements. Focusing of whistlers is possible. These results are important for designing antennas on spacecraft or diagnosing and heating of laboratory plasmas