共查询到20条相似文献，搜索用时 328 毫秒
1.
Natural VLF emissions received by a single antenna can be characterised at each point in the emissions' frequency-time domain by a power and a phase. Emissions received at a single point by two antennae with a fixed relative orientation in space can be similarly described by the cross-spectral power and relative phase. It is shown that the cross-spectral phase contains information on the propagation characteristics of the waves which is better utilised in wave analysis than the power. In fact, the phase information allows weak signals to be identified more readily than is possible from a power spectrogram. It also allows the recognition of waves propagating with different wave normal directions. Data from the Geos-1 electric and magnetic antennae, pre-processed by the on-board correlator, are used to study the cross-spectral characteristics of VLF hiss and chorus in the Earth's magnetosphere. 相似文献
2.
A ray tracing computer program for non-ducted whistler mode waves in a warm plasma in the magnetosphere is developed, where electron temperature effects are taken into account. The refractive index is calculated from the warm-plasma approximation and is used in the ray tracing after its accuracy has been checked by comparison with the hot-plasma solution without approximation. The ray paths do not depend appreciably on electron temperature. However, there are regions where the waves are heavily damped by Landau damping. By paying attention to this damping region, the electron temperature can be estimated from a satellite observation of the Doppler shift and damping of a ground-based VLF signal. 相似文献
3.
R.N. Srivastava 《Planetary and Space Science》1974,22(11):1545-1564
Comparison of the low altitude polar orbiting Injun 5 Satellite data with the ground VLF data has revealed that there is a definite scarcity of VLF/ELF emissions at the ground level compared with the extent to which they are present at or above the auroral altitudes. Reasons for this have been investigated by performing ray path computations for whistler mode VLF propagation in an inhomogeneous and anisotropic medium, such as the magnetosphere and the ionosphere. Based on wave normal computations in the lower ionosphere, it has been found that many of the near-auroral zone VLF/ELF events are frequently either reflected from, or heavily attenuated in, the lower ionosphere. Besides collisional loss, severe attenuation of VLF signals in the lower ionosphere is also caused by the divergence of ray paths from the vertical (spatial attenuation). Cone of wave normal angles for the wave, within which VLF/ ELF signals are permitted to reach the ground, has been established. Wave normals lying outside this transmission cone are reflected from the lower ionosphere and do not find exit to the Earth-ionosphere cavity. Computations for VLF signals produced at auroral zone distances in the equatorial plane of the magnetosphere indicates that these signals are more or less trapped in the magnetosphere at altitudes > 1RE. 相似文献
4.
P. C. W. Fung 《Astrophysics and Space Science》1972,16(2):249-273
In this investigation, we attempt to analyze the quasi-linear cyclotron instability (under the weak turbulence regime) for whistler-mode waves due to pitch angle anisotropy of nonthermal electrons. The motivation of this study is to explain the triggered discrete VLF emissions occurring in the terrestrial magnetosphere. The time evolution of the growth rate and the induced waves spectrum for a loss cone type of nonthermal electrons is analyzed numerically. The diffusion of particles in pitch angles due to quasilinear cyclotron instability is illustrated. It is shown that several major features of triggered VLF emissions can be explained by the stated instability. Some predictions of the theory is given and suggestions for further research are presented.On special leave during the summer of 1971 from the Physics Department, Faculty of Science, University of Hong Kong. 相似文献
5.
The nonlinear frequency shift arising from the interaction of the quasimono- chromatic whistler-mode wave with resonant particles in an inhomogeneous plasma is derived. The modulational instability caused by this shift is investigated. The results are applied to the propagation of long-duration VLF whistler-mode signals along the magnetic field in the magnetosphere. It is shown that the modulational instability of these waves in the equatorial region leads to pulsations very similar to those observed experimentally 相似文献
6.
Extremely low frequency (ELF)/Very low frequency (VLF) hiss is whistler mode wave that interacts with energetic electrons
in the magnetosphere. The characteristics features of ELF/VLF hiss observed at low latitude ground station Jammu (Geomag.
lat. 22°16′ N, L=1.17) are reported. It is observed that most of hiss events first propagate in ducted mode along higher L-values (L = 4–5), after reaching lower edge of ionosphere excite the Earth-ionosphere waveguide and propagate towards equator to be
received at low-latitude station Jammu. To understand the generation mechanism of ELF/VLF hiss, incoherent Cerenkov radiated
power from the low-latitude and mid-latitude plasmasphere are evaluated. Considering this estimated power as an input for
wave amplification through wave–particle interaction, the growth rate and amplification factor is evaluated which is too small
to explain the observed wave intensity. It is suggested that some non-linear mechanism is responsible for the generation of
ELF/VLF hiss. 相似文献
7.
K. Morrison 《Planetary and Space Science》1990,38(12):1555-1565
The first observations are presented from Halley, Antarctica, of quasi-periodic (QP)_VLF intensity variations modulated at the frequency of concurrent Pc3 magnetic pulsations. Seen on broadband frequency-time plots, the QP emissions are of both the dispersive and non-dispersive types. From the frequency and phase variation with time of the QP emissions and magnetic pulsations, estimates are obtained of the travel times of the ULF waves from the interaction region to the ground. The observations appear consistent with the idea of modulation of a pre-existing VLF hiss source in the magnetosphere by the compressional components of ULF waves. A significant change in the travel time during one event is consistent with a crossing of the plasmapause by the Halley fieldline. 相似文献
8.
9.
Yu.V. Golikov V.V. Pustovalov A.B. Romanov V.P. Silin V.T. Tikhonchuk 《Planetary and Space Science》1977,25(10):899-906
A theory of VLF noise excitation by electron beams in the polar magnetosphere is proposed. Two modes of excited oscillations are considered: waves with frequencies in the vicinity of the lower hybrid resonance (LHR) from about 50 to 1000 kHz and whistler-mode waves in the frequency range of several kHz.The spectral distribution and the level of turbulent noise, having been excited by means of two counterstreaming electron beams, are deduced in magnetized plasma at the LHR frequency. It is also shown that the growth of noise up to the quasistationary level oscillates with time. Energy density of oscillations at the LHR frequency in the region of the dayside polar cusp agrees with the experimental data.The processes of whistler excitation by electron beams are discussed. The growth rate of excitation of whistler-mode by electrostatic oscillations at the LHR frequency is calculated. 相似文献
10.
The particle energy required to generate the observed VLF hiss in the Jovian magnetosphere has been computed under longitudinal and transverse resonance condition. It is shown that the minimum energy required by electrons to generate VLF hiss under the longitudinal resonance condition lies in the range of 100eV–1keV for the wave frequencies of 2–10 kHz, while the corresponding energy range for the transverse resonance condition for the same frequency range comes out to be 8 keV–40 keV. Further, the average radiated power by the erenkov process in the Jupiter's magnetosphere atL=5.6 Rj by electrons of energy 10 eV, 100 eV, and 1 keV for the wave frequency of 5 kHz has also been computed. 相似文献
11.
Sekiko Yoshida 《Planetary and Space Science》1965,13(12):1165-1170
It is shown that VLF emissions are greatly affected by the polar cap absorption caused by the bombardment of solar protons. Characteristics of the PCA effect on VLF emissions are examined and they are in agreement with those obtained by other studies, such as the polar blackout and the cosmic radio absorption. Therefore, the earlier conclusion that the occurrence frequency of VLF emissions decreases in high latitudes during magnetic storms is likely to be due to the PCA effect.
Taking this PCA effect into account, it is established that an enhancement of occurrence of VLF emissions occurs at geomagnetic latitudes lower than 67° during the magnetic storm. This suggests that enhanced VLF emissions during geomagnetic storms are generated in the co-rotating region of the magnetosphere or in the outer radiation belt, but not in the tail region. 相似文献
12.
Roger Gendrin 《Astrophysics and Space Science》1974,28(1):245-266
We consider the movement of individual electrons in a magnetized plasma in which a monochromatic wave is propagating in the whistler mode. We derive simple expressions which give the displacement of the electrons as a function of time, the phase angle that their velocity vector makes with the magnetic component of the wave, their pitch angle and energy changes. A useful formula is obtained which gives the velocity range over which particles remain trapped inside the wave, as a function of the wave intensity and of the initial phase angle of the particle. It is shown that even strictly resonant particles can escape from the wave when their initial phase angle is very small. From the derived expressions, it is possible to compute the phase-bunching effect which occurs approximately at one trapping wavelength behind the leading edge of the interaction region. We deduce also the total amount of energy which is taken from (or given to) the wave by magnetospheric electrons in both cases of naturally existing or artificially injected particles. It is shown that these non-linear amplification processes can lead to very large VLF amplitude in the magnetosphere. 相似文献
13.
N.A. Smirnova 《Planetary and Space Science》1984,32(4):425-438
The results of the investigations of chorus type VLF emissions both in the subauroral conjugate regions and along the meridional profile near the plasmapause projection are summarized in order to connect the peculiarities of ground-observed VLF chorus with the plasma processes in the magnetosphere during substorms development. The method for determination of the location of the instability region in the magnetosphere is suggested which is based on the measurement of the upper boundary frequency of the ground-observed VLF chorus (UBF-method). Dynamics of chorus spectra during the substorm development is investigated. On the basis of the revealed regularities the phenomenological model of the VLF chorus source development is constructed. The calculations of some magnetospheric plasma parameters such as plasma density, large-scale electric field etc. are carried out. The conformity between the calculated and typical measured parameters is shown. The diagnostic possibilities of VLF chorus are discussed. 相似文献
14.
V.Yu. Trakhtengerts 《Planetary and Space Science》1973,21(3):359-366
The paper deals with a nonlinear instability of quasi-monochromatic VLF signals and whistlers in the Earth's magnetosphere due to induced scattering. The instability growth rates and the threshold values of the signal amplitude at which the instability occurs have been found. The instability is shown to be more effectively excited when the initial transverse VLF wave transforms into plasma oscillations at the lower hybrid resonance (LHR) frequency and may be responsible for the phenomena such as trigger LHR emission, the amplitude and phase modulation of artificial VLF signals and be the origin of some types of discrete VLF signals. 相似文献
15.
An analysis is made to calculate input impedance of a loop antenna for radiation of the VLF whistler mode in the magnetosphere. The magnetosphere is assumed to be represented by a cold, uniform and collisionless magnetoplasma medium. Assuming a uniform current distribution of a circular loop, oriented at an arbitrary angle with respect to the Earth's magnetic field line, several closed-form expressions for the loop impedance have been derived. It is found that the loop input reactance is in substantial agreement with the self-inductance of a loop in free space and that the radiation resistance for a small loop can be as large as ～10 2 Ω. It is also found that a second order quasi-static theory is quite valid for determining the input impedance for small loops radiating VLF whistlers in the magnetosphere. 相似文献
16.
D. I. Mendzhul O. V. Agapitov O. K. Cheremnykh 《Kinematics and Physics of Celestial Bodies》2013,29(3):107-119
The ray trajectories of waves in the very low frequency (VLF) range in the case of nonducted propagation in the earth’s inner magnetosphere are studied as functions of location of their source region, frequency, and initial angle between the vector of wave normal and intensity vector of external magnetic field. Simulation is performed on the basis of geometric ray tracing approach in multicomponent plasma. The parameters of the magnetospheric medium were calculated using a diffusion model of the concentration distribution of plasma components and the International Geomagnetic Reference Field (IGRF) model. It is shown that the magnetospheric wave reflection can occur if the lower hybrid resonance frequency is greater than its own wave frequency (ω LHF > ω), i.e., at the latitudes λ ≈ 50°. The simulation results confirm that the quasi-longitudinal approximation cannot be used to describe the magnetospheric whistler propagation. We present simulations of propagation of chorus-type wave magnetospheric emissions that were performed using realistic wave distributions over initial parameters. In particular, we present distributions of chorus waves over directions of wave vector as functions of geomagnetic latitude; these distributions are required to study the particle scattering and acceleration processes in the radiation belts. Our results well agree with CLUSTER satellite measurements. 相似文献
17.
Vladimir N. Damgov 《Earth, Moon, and Planets》1993,61(2):87-117
Simple modelling systems and mechanisms of oscillation excitation are presented that may contribute to the revealing of mechanisms of planetary magnetosphere radiosources generation and wave interactions mechanisms in the Earth ionosphere and magnetosphere as well as the excitation of VLF waves in the near-Earth space.An investigation supported by The Bulgarian National Foundation Scientific Research under Contract No. TH-302/92. 相似文献
18.
D. Nunn 《Planetary and Space Science》1973,21(1):67-88
The work deals with the resonant particle excitation of two electrostatic waves with closely spaced wave numbers, when there is an inhomogeneity present in the form of a spatially dependent wave number. Resonant particle behaviour in such a field is investigated and the resonant particle current is computed for a variety of cases. If the inhomogeneity is such that resonant particles see the wave numbers of the waves increasing, then it turns out that the wave of greatest wave number is preferentially amplified. If the gradient is reversed it is the opposite wave which grows. Thus when a narrow band electrostatic wave is subject to beam excitation, only one of the sideband waves is unstable.The theory is applied to the closely analogous problem of sideband formation in the case of triggering of VLF emissions by magnetospheric whistler pulses, and seems to account for much of the observed behaviour. 相似文献
19.
Observations by Mariner 10 during its first and third flybys showed that Mercury possesses an intrinsic magnetic field resulting in a small magnetosphere that can keep the solar wind from directly interacting with the planet's surface under usual conditions. Since Mercury occupies a large fraction of its magnetosphere, regions of trapped charged particles in the inner magnetosphere, the plasmasphere and the energetic radiation belts, would all be absent. During the first flyby, energetic particle bursts were detected and interpreted as hermean substroms analogous to the terrestrial magnetosphere. Moreover, during this flyby, ULF waves and field-aligned currents were detected in the data. Earth-based observations of Na, K, and Ca populations in the exosphere strongly suggest the existence of dynamic magnetospheric processes at high latitudes interacting with the planet's surface. 相似文献
20.
Spectrograms of broad-band ELF/VLF goniometer data obtained from ground based measurements made at Halley, Antarctica (L = 4.3, conjugate near St. Anthony, Newfoundland) have shown the presence of discrete line radiation of magnetospheric origin, in the frequency range 1–4 kHz. The properties of this radiation are broadly similar to Power Line Harmonic Radiation (PLHR), studied from ground based observations made at Siple, Antarctica (L = 4.1, conjugate—Roberval, Quebec), although there are some interesting differences. Line radiation observed at Halley, is never regularly spaced in frequency by 120 Hz, as one may expect if signals from the Newfoundland power distribution system (60 Hz fundamental) are entering the magnetosphere, and being amplified. Instead, frequency spacings are widely distributed about mean values between 50 and 90 Hz. The lines are observed to trigger emissions and often exhibit 2 hop amplitude modulation, which demonstrates that they are of magnetospheric origin. Events occur mostly in quiet to moderate geomagnetic conditions, and during the late afternoon period of local time. Arrays of lines are often observed to drift upwards together in frequency. Line bandwidths are 20–30 Hz—much larger than the bandwidths of locally generated induction lines. We show that the line spacing of ～80 Hz is too large to correspond to sideband separation for waves of equatorial field strength ～10 pT, and we investigate the conditions required for effective particle trapping by the wave array, of the type described by Nunn, 1974. It is proposed that the line radiation either originates in the signals which enter the magnetosphere from Newfoundland, or is “naturally” generated, possibly by a linear instability which takes place if the electron distribution function has sharp localised gradients in pitch angle. 相似文献