Showing papers on "Whistler published in 1974"

Pitch angle and energy diffusion coefficients from resonant interactions with ion-cyclotron and whistler waves

Abstract: General relations for quasi-linear diffusion coefficients in pitch angle and energy are applied to resonant particle interactions with ion–cyclotron and whistler waves. Expressions for the diffusion coefficients, valid for any distribution of wave energy with frequency and wave normal angle, are derived and normalized to be independent of the ambient magnetic field intensity and the electron density. The results illustrate how resonant particle diffusion rates vary with pitch angle and energy, and how the diffusion rates depend upon the distribution of wave energy.

Whistler waves observed upstream from collisionless shocks

Abstract: Waves in the frequency range 0.5 - 4 Hz were studied in the region upstream of the earth's bow shock using data from the fluxgate magnetic field experiment on IMP-6. Analysis of 150 examples of these waves during a three month interval indicates that amplitudes are generally less than 1 or 2 gammas and propagation directions generally make angles of between 20 and 40 degrees with the field direction. The waves as measured in the spacecraft frame of reference are either left or right hand polarized with respect to the average field direction. It is concluded that the observed waves are right handed waves in the plasma frame of reference with wavelengths of approximately 100 km propagating upstream in the whistler mode. Doppler shifting reduces the observed frequencies in the spacecraft frame and reverses the observed polarization for those waves propagating more directly upstream. Similar waves are seen ahead of most interplanetary shocks.

Some features of plasma distribution in the plasmasphere deduced from Antarctic whistlers

Abstract: Whistlers recorded in Antarctica were used to study the plasma distribution in the plasmasphere between L ≅ 2 and 6 Certain features of the plasma distribution can be understood in terms of rapid (∼1 day or less) depletion of the plasmasphere during geomagnetic disturbances, followed by slow refilling from the underlying ionosphere At a given L value, the filling proceeds steadily until the plasmasphere reaches a well-defined density level where an equilibrium with the underlying ionosphere prevails The time required to reach this equilibrium is related to plasmaspheric tube volume and varies from ∼1 day at L = 25 to ∼8 days at L = 4 Because of these long recovery times the plasmasphere usually consists of two distinct regions: an ‘inner’ plasmasphere, which is in equilibrium with the underlying ionosphere over a 24-hr period, and an ‘outer’ plasmasphere, which is still recovering from the previous disturbance The monthly median data for June 1965 show that the transition from inner to outer plasmasphere occurred at L ≅ 45 In November and December 1964 the transition occurred at L ≅ 40, and the density levels were higher than they were in June by factors of 15–3 The latter result confirms the existence of large annual electron density variations in the plasmasphere, first discovered by the whistler technique more than a decade ago

Observations of radiation from an electron beam artificially injected into the ionosphere

Abstract: This paper reports the observations of waves generated by a controlled beam of particles artificially injected into the ionosphere and magnetosphere. The measurements were made during the Electron Echo 1 experiment, in which an electron accelerator was carried to a height of 350 km in the ionosphere from Wallops Island, Virginia, on an Aerobee 350 sounding rocket. It injected into the earth's magnetic field over 3000 16-ms pulses of electrons with 40-keV energy and a current of 70 mA at pitch angles between 70 and 110 deg. The ejected fiber glass nose cone carried antennas and receivers to measure the electric field of waves generated by the beam. Associated with the electron beam was radiation detected at frequencies near the electron plasma frequency of the background ionosphere, near twice the electron cyclotron frequency in the whistler mode, and near zero frequency. Associated with the operation of an argon plasma generator used to keep the accelerator neutralized were continuous emissions detected at frequencies near the lower-hybrid resonance (LHR).

Further remarks on plasma instabilities produced by ions born in the solar wind

Abstract: Whistler and Alfven wave instabilities produced in the solar wind by newly born ions of planetary or interstellar origin are examined, and it is shown that the dispersion equation considered by Wu and Davidson (1972) contains another class of complex roots which imply stronger instabilities than previously noted. The applicability of the proposed theoretical models is discussed together with the implications of the results for the interaction of planetary and interstellar He+ ions with the solar wind.

Parametric Instabilities Induced by the Coupling of High and Low Frequency Plasma Modes

Abstract: A general dispersion relation is derived for parametric instabilities caused by a large amplitude pump wave which couples high and low frequency modes of a magnetized plasma. The nonlinear effects coupling the high and low frequency waves are: (1) the low‐frequency pondermotive force due to the coupling of the pump with the high frequency perturbations, and (2) the high frequency nonlinear source current density produced by coupling of the pump with the low frequency perturbations. The dispersion relation contains decay instabilities, purely growing instabilities, modulational instabilities, self‐focusing and nonlinear Landau damping. As examples the dispersion relation is applied to self‐focusing, magnetosonic decay and purely growing instabilities, whistler decay and nonlinear Landau dumping of electron plasma waves and transverse electromagnetic waves.

Energetic electrons in Jupiter's magnetosphere

Abstract: A theoretical model for the energetic electron fluxes in the Jovian magnetosphere is developed. Electrons are transported inward from the solar wind or Jovian magnetospheric tail by radial diffusion. The radial diffusion is driven by fluctuating ionospheric dynamo electric fields associated with a neutral-wind tidal eigenmode at ionospheric altitudes. The tidal mode is excited by the electromagnetic coupling of the solar wind to the polar ionosphere. Two injection models are considered: (1) electron penetration through the dayside magnetopause - low-energy model; and (2) injection of electrons from an assumed magnetospheric tail - high-energy model. Both thermal solar-wind electrons and energetic solar-flare electrons are considered.

Resonant scattering of particles and second phase acceleration in the solar corona

Abstract: Effective acceleration of particles by hydromagnetic turbulence requires that the particles be scattered at a rate ν comparable with the frequency ω of the turbulence. The only effective scattering process is due to resonant wave-particle interactions. The resonant waves are HM waves for ions with β≫βA(βc = particle speed, βAc = Alfven speed) and for electrons with γβ ⩾ 43β0(β0 ≈ 43βA), and are whistlers for electrons with β0 ≪ γβ≲ 43β0. The resonant waves can be generated by an anisotropic distribution of particles provided that the anisotropy factor A exceeds a threshold anisotropy A0 ≈ βA/β for HM waves and A0 ≈ β02/β2γ for whistlers. Turbulence with relative magnetic amplitude ɛ causes acceleration at a rate {ie0353-02} provided the following conditions are satisfied: (a) β ≫ βA for ions, β ≫ β0 for electrons; (b) ɛ ≫ A0; (c) n1/ne≲ω/gWi or n1/ne ≫(ω/Ωi) (γβ/43β0)2 for scattering by HM waves or whistlers respectively (n1 = number density of accelerated particles, Ωi = ion gyrofrequency).

On the enhancement of the whistler mode instability in the magnetosphere by cold plasma injection

Abstract: The modifications in the electron cyclotron electromagnetic (whistler) instability produced by the addition of cold plasma to an infinite uniform anisotropic plasma with parameters similar to those observed in the equatorial magnetosphere at 6.6 RE are discussed. The discussion is restricted to waves propagating parallel to the static magnetic field. We show that cold plasma greatly increases the growth rate if P ≡ β · A(1 + A)² ≪ 1, where β = 4πnκT∥/B² and A = (T⊥/T∥) - 1. For small values of P the enhancement of the maximum rate of growth is proportional to α−1/2 · exp (½ αP), where α = 1 + (nc/nw) and nc and nw are the particle densities of the cold and warm electron populations, respectively. There is an optimum value of nc/nw for maximum growth rate, i.e., ≃ 1/P. The addition of cold plasma also increases the instability range in k space by a factor approximately proportional to (nc/nw)1/2. The recent confirmation of the analytical results above by numerical solution of the dispersion equation as well as by computer simulation experiments is indicated.

Wave-Particle Interactions Near the Geostationary Orbit

Abstract: It has become abundantly clear during the past decade of experimental observation of the near-Earth magnetosphere that many of the gross characteristics of its particle population and their temporal behavior can at least be qualitatively understood in terms of a large-scale convection of plasma from the geomagnetic tail towards the dayside, as originally postulated by Axford and Hines (1961) and Dungey (1961). Using the very simplest of models for the convective flow i.e., a uniform dawn-to-dusk electric field across the magnetosphere associated with the tail-to-dayside motion, plus a radial electric field associated with the rotation of the Earth, leads to a basic understanding of the populations of low energy (1 to 104 eV) plasma within the magnetosphere, for the resulting flow pattern is divided into two parts. The flux tubes close to the Earth which corotate never become ‘open’ and are characterized by high densities of cold plasma from the ionosphere, while at larger distances flux tubes flow from the tail to the dayside carrying with them the hot plasma sheet particles which form the outer radiation zone and ring current. Thus the magnetosphere is divided into two regimes dominated by hot and cold plasma, respectively, and whose mutual boundary is the plasmapause (see, for example Russell and Thorne, 1970).

Derivation of magnetospheric electric fields from whistler data in a dynamic geomagnetic field

Abstract: The whistler method of determining magnetospheric electric fields has until recently been applied on the assumption of a static dipole geomagnetic field. We consider the effect on the whistler analysis of including both departures of the field from a dipole and temporal variations in the field. Departures from a dipole field appear to have relatively small effects on the analysis or can be relatively easily taken into account when it is necessary. Temporal changes present a more serious problem. In the presence of a changing geomagnetic field the temporal change in equatorial gyrofrequency of a drifting whistler path will consist of two parts: the variation due to radial drift of the path in the presence of the inhomogeneous B field and the variation due to the changes in B with time. Given knowledge of the temporal variation of B, it is possible to infer the total (induced plus potential) electric field associated with the radial drift. A substorm event is analyzed in which cross-L inward drifts near L = 4 occurred as the night side B field (as reflected in the low-latitude H component) exhibited a rapid increase. It was found that on a time scale of 15 min or greater the inferred total east-west electric field differed by ∼20% from the field estimated on the assumption of a static magnetic field. It is possible that effects of B may be relatively more important on a shorter time scale or during disturbances much larger than those thus far investigated. Also, in one quiet period preceding a substorm it appeared that all or a major part of the inferred cross-L motions of the plasma could have been due to fluctuations in the magnetic field. It is stressed that both equatorial and ionospheric measurements of electric fields are needed, since certain types of distortions of the high-altitude magnetosphere are not readily observed at ionospheric heights.

Controlled VLF Wave Injection Experiments in the Magnetosphere

Abstract: Whistler-mode waves injected into the magnetosphere from ground sources (e.g., lightning discharge, vlf transmitters) are used to probe the distribution of ions and electrons in the magnetosphere. They also cause wave growth (vlf emissions) and precipitation of electrons. Bursts of X-rays (> 30 keV) and enhancements of D-region ionization are examples of precipitation effects caused by lightning-generated waves. Growing narrowband wave trains are triggered by manmade coherent waves. Growth rates of ∼ 100 dB s-1 and total growths up to 30 dB have been measured using 5.5 kHz signals transmitted from Siple Station, Antarctica. Another source of coherent wave input to the magnetosphere are the harmonics from commercial power line systems. Power line harmonic radiation may suppress triggered emissions or change their frequency-time slope. Exponential growth of narrowband emissions is explained in terms of cyclotron resonance between the waves and trapped energetic electrons, with feedback included. Applications of wave injection experiments include: (1) study of emission mechanisms, (2) control of energetic particle precipitation, (3) diagnostics of cold and hot plasma, and (4) vlf communications.

A morphological study of substorm‐associated disturbances in the ionosphere

Abstract: The whistler technique was used in an earlier study to observe a rapid depletion of the day side plasmasphere during isolated substorm activity on June 25, 1965 (Park, 1973). It was inferred that this depletion was due to dumping of plasma into the underlying ionosphere. The average downward flux was estimated to be of the order of 109 el/cm² s across the 1000-km level, large enough to cause significant enhancements in electron concentrations in the ionospheric F region. In this paper the ionospheric behavior during the same event is examined by using ionosonde data from some 40 stations in the northern hemisphere. The results show that the F2 layer critical frequency ƒoF2 was enhanced up to 25% above the monthly median level in the same general area where downward fluxes were inferred from whistlers. This was followed by ∼20% depressions in ƒoF2 the next day, when geomagnetic conditions were quiet. These substorm effects were observed in a wide range of latitude but were limited in local time and longitude. In the area affected, the ionospheric disturbance caused by this moderate substorm activity (∼500 γ maximum AE index and ∼6-hour duration) was similar in many respects to major worldwide ionospheric storms. These results support the view that ionospheric storms can be understood in terms of the superposed effects of many substorms.

A theoretical investigation of banded chorus

Abstract: This paper is a computational study of nonlinear resonant particle trajectories in wave fields consisting of an array of narrow-band waves of closely spaced frequencies and wavenumbers. It looks at two analogous systems, cyclotron resonance with a whistler wavefield and Landau resonance with an electrostatic wave field. It is found that the wave array is able to trap particles in much the same way as a single mode. Inhomogeneity plays a vital role by causing the energy of trapped particles to change.The nonlinear resonant particle current is such as to preserve the modal structure of the wave field, and it does not change the frequency of an individual mode. Power distribution amongst the modes is far from even. Most of the energy goes into the mode at one end of the array, depending on the direction of the inhomogeneity. Also, nonlinear resonant particle excitation of a broad band signal was found to cause spectral structuring to develop automatically.The theory was applied to one of Coroniti's FTRS analyses of banded chorus elements. The observed spectral behaviour closely accorded with the computational results. The numbers involved fitted the theory very well, and strongly suggested that nonlinear cyclotron resonant particle excitation is the mechanism for banded chorus.

Resonance cone and Bernstein wave interference patterns in a magnetoplasma

Abstract: General dispersion relations for a hot magnetoplasma are used to explain the interference observed between electrostatic waves and the electromagnetic field in an afterglow plasma. Observations are made near both oblique resonance cones (connected with whistler and Z-propagation modes) and in the frequency region corresponding to “Bernstein” waves. Independent measurements of the lower (whistler) and upper (Z-mode) resonance cones, for a particular case, lead to the same isotropic temperature in the late afterglow, namely 540 K. Independent measurements of the lower resonance cone and of Bernstein waves in the early afterglow also lead to consistent isotropic temperatures. As the real and imaginary components of the wave number increase, a point of inflection is reached on the various dispersion curves for homogeneous plane waves beyond which these curves can no longer be used to explain the observed interference fringes. For homogeneous waves with a wave number smaller than that corresponding to the inflection point, reasonable values of Landau damping are obtained which agree roughly with observed damping.

Rocket measurement of wave normal directions of low‐latitude sunset whistlers

Abstract: The measurement of the wave normal direction of short whistlers is made in the ionosphere by means of a three-dimensional crossed loop antenna on a rocket. The results of the whistler wave normal direction are presented, and some relationships to the propagation characteristics in the magnetosphere and also to the penetration through the ionosphere are discussed.

Optimum cold plasma density for maximum whistler instability: Numerical versus analytical

Abstract: Approximate analytical predictions have been given by Cuperman and Landau (1974) for the parallel propagating whistler mode instability in an infinite uniform warm anisotropic plasma in the presence of various amounts of cold plasma. Here the analytical results are compared with exact results that we obtained by solving numerically the corresponding linear dispersion equation. The satisfactory agreement found indicates the reliability of the simple closed analytical expressions mentioned above. In particular, the analytical prediction for the optimum cold plasma density for the maximum whistler growth is shown to be in very good agreement with the numerical calculation result.

Generation of Alfven waves in the magnetosphere by parametric interaction between whistlers

Abstract: A theory is developed for the generation of Alfven waves by the mixing of whistler waves. Calculations are carried out for interaction in the magnetosphere where the whistlers fall in the VLF frequency range and the Alfven wave falls in the ULF(1-5 Hz) range. Typical amplitudes of 2-5 milligammas for the Alfven wave are calculated, and it is shown that these values might be increased by one order of magnitude through suitable variation of the experimental parameters. The study thereby develops a possible explanation for naturally occurring Pc 1 micropulsations and demonstrates the feasibility of artificial generation of such micropulsations by ground-launched whistlers.

Two types of phase bunching in the whistler mode wave-particle interaction

Abstract: Importance of the “phase bunching” of resonant electrons in the whistler mode (WM) wave-particle interactions was pointed out by Brice. In this paper, we call attention on the point that there are two different types of phase bunching with two physically distinct phasing mechanisms in the case of the WM wave-particle interaction. One is a longitudinal phase bunching (the Brice type) which is caused by a longitudinal acceleration, while the other is a transverse phase bunching which is caused by a transverse acceleration. The both types are considered separately and examined mathematically and physically. In order to visualize the different characteristics of the two, numerical results on the phasing patterns and the resultant resonant current are shown by computing the nonlinear motion of electrons in a monochromatic WM wave with various initial velocities (36 different initial phase angles ×15 different speed |υ|).

Low‐latitude cutoff for whistlers observed on the ground

Abstract: Ground observations of whistlers have been analyzed to yield a low-latitude cutoff around 16° geomagnetic latitude. It is suggested that this cutoff signifies a latitude below which, irrespective of the magnetic activity, no whistlers can be heard or when a few are heard, they can readily be identified as originating at higher latitudes.

The coalescence of coupling points in the theory of radio waves in the ionosphere

Abstract: When a plane radio wave is obliquely or vertically incident on a horizontally stratified ionosphere, there are certain points in the complex height plane, called coupling points (which include reflexion points) where there is a breakdown in the independent propagation of the four waves, ordinary upgoing and downgoing, and extraordinary upgoing and downgoing. If a coupling point is far enough away from other coupling points and from singularities, it is said to be isolated and the electromagnetic field near it can be expressed in terms of Airy Integral functions. Then the phase integral method can be used with suitably chosen contours, to calculate reflexion coefficients and coupling coefficients. If two coupling points are too close together, however, the procedure needs modification. This paper studies the theory when two coupling points approach coalescence. It is confined to the cases where the same two waves are coupled at the adjacent coupling points, since this is the most important in practice. It is found that there are two kinds of coalescence called coalescences of the first kind C1, and of the second kind C2. For C1 the coupling remains strong when the coupling points move to coalescence. For C2 the coupling gets weaker, and disappears completely at exact coalescence. The electromagnetic fields near a point of coalescence can be expressed in terms of solutions of Weber’s equation, but the form of this equation is different in the two cases. The type C1 is important in the theory of partial penetration and reflexion for frequencies near the penetration frequency of an ionospheric layer. The type C2 is important in the theory of ʻcrossover’ for ion-cyclotron whistlers, and in the theory of the Ellis window and related phenomena. These applications are worked out as illustrations.

Direct measurement of the propagation of whistler wave packets

Abstract: The group velocity of whistler wave packets was measured in an overdense cylindrical plasma. The phase velocity was measured using c.w. excitation. The waves are excited and received by small antennae which excite and detect an oscillating B field in the plasma. The dispersion relation for whistler waves in a cylindrical plasma is calculated. It is seen that for omega < omega ce/2, it is necessary to include collisions to account for the experimental results for the real part of the dispersion relation. In the limit of large collisions the dispersion relation goes to the plane wave result. It is found that there exists a range of plasma parameters for which two modes of the whistler waves are excited. It is also shown that whistler waves can be used in an interferometer system to measure plasma density.

Simultaneous electric field measurements near L = 4 from conjugate balloons and whistlers

Abstract: Electric field measurements have been obtained simultaneously in conjugate ionospheres by balloons near L = 4 and near the equator by analyses of the motion of whistler ducts near L = 4 and L = 2.7. The observations were conducted from Siple, Antarctica, and Rouyn, Canada, in December 1969. The westward ionospheric electric field components measured by the two conjugate balloons are in agreement with each other, as are the westward equatorial measurements on the two ducts. The equatorial data agree in general with the mapped ionospheric results, although the equatorial measurements are smaller in average magnitude. The southward ionospheric electric field components in the conjugate ionospheres do not bear any simple resemblance to each other.

Excitation of the Earth-Ionosphere Waveguide by Downgoing Whistlers. II. Propagation in the Magnetic Meridian

Abstract: The excitation of the Earth–ionosphere waveguide by a downgoing whistler with wave normal in the magnetic meridian is discussed. It is suggested that the wave normal is scattered into the acceptance cone by spread-F irregularities. The model of the ionosphere used is a slowly varying magnetoionic medium terminated by a sharp boundary at the bottom. The ground is treated as a perfect conductor. Reflexion and coupling coefficients are defined and derived. It is shown that for some angles of incidence the signal is trapped in the waveguide. The range in the waveguide is computed as a function of angle of incidence. The properties of the waveguide modes are discussed and their excitation for a plausible incident signal computed. It is shown that the least attenuated modes are very weakly excited while those modes which are strongly excited are rapidly attenuated. This explains why the field of view of a receiver is limited to about 15° of latitude.

Whistlers and Discrete ELF/VLF Emissions

Abstract: The purpose of this paper is to review recent work in this field. In the first part, whistlers are discussed both from an observational and analytical viewpoint; in the second part, attention is confined to discrete emissions believed to be generated by the cyclotron resonance instability of energetic electrons in the magnetosphere. This paper can thus be regarded as being a condensed and updated version of the earlier, and more comprehensive, review by Rycroft (1972).

Wave-wave coupling in multiple-ion plasmas

Abstract: The purpose of this note is to point out that the presence of more than one ion in a plasma opens up several possibilities for wave-wave coupling that are not available in a single-ion plasma. This can be demonstrated by using the ‘parallelogram construction’ on an ω-k diagram as discussed for whistler waves by Harker and Crawford [1969]. The basic requirement to satisfy energy and momentum conservation in wave-wave coupling is that the frequencies ω and wave vectors k of three waves satisfy ω3=ω2±ω1k3=k2±k1((1))