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At least 127 records · Page 7

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NASA NTRS1976

Whistler wave propagation in a large magnetoplasma

A large collisionless quiescent plasma source is developed for investigating the phase and amplitude distribution of antenna-launched whistler waves in a specified parameter regime relating wave frequency to electron cyclotron frequency. Wave dispersion is studied both by interferometer techniques with monochromatic waves and by propagation of short phase-coherent wave bursts. The wave damping mechanism is examined by propagating perfectly ducted whistler waves. The dispersion of single frequency waves and wave packets is demonstrated. Trough ducting for wave frequency to electron cyclotron frequency ratio greater than 1/2 is verified, and new eigenmodes in nonuniform plasmas at ratio values less than 1/2 are observed. It is shown that geometric effects due to ray divergence and wave refraction dominate over collisional damping.

Stenzel, R. L.↗

NASA NTRS1978

Whistlers and VLF noises propagating just outside the plasmapause

Ground-based recordings have been made of broadband whistler mode signals propagating along the plasmasphere outer surface, i.e., beyond the region of steep plasmapause density gradients. In contrast to features found in other field line regions, the extension of signal frequencies into the 0.5-0.8 f(Heq) range is observed, (where f(Heq) is the equatorial electron gyrofrequency of the path). The effects are exhibited by the fine structure of the knee trace and other plasmapause irregularities, amplitude peaks and emissionlike behavior above the nose frequency, whistler mode echoing above 0.5 f(Heq), and narrowband noises and noise bursts propagating along the plasmapause outer surface.

Carpenter, D. L.↗

NASA NTRS1980

Whistler propagation in the Jovian magnetosphere

The paper deals with computer ray tracing results for lightning-generated whistlers propagating in the Jovian magnetosphere. The waves are launched from a point on the Jovian surface at 66 degrees latitude and propagate approximately along L = 6 out to near the equatorial plane and into the Io plasma torus. The results clearly indicate that the whistlers propagate with very little dispersion until they reach the torus, at which time the dispersion starts to increase very rapidly. Good agreement between the computed and observed dispersions is established.

Menietti, J. D.↗

NASA NTRS1980

Lightning on Venus - Orbiter detection of whistler signals

Taylor et al. (1979) presented preliminary evidence for lightning on Venus, based on Pioneer Venus orbiter detection of whistler mode signals as the spacecraft first traversed the nightside ionosphere near periapsis. The initial periapsis eclipse season for the orbiter has been completed, and the plasma wave instrument obtained low-altitude nightside data for about 100 orbits. An analysis is presented of the impulsive whistler mode signals measured during these orbits, and the connection with atmospheric lightning is discussed. It is shown that the signals are detected in the 100-Hz channel when the local magnetic field is sufficiently strong and steady and when the field is oriented to point down below the ionosphere.

Scarf, F. L.↗

NASA NTRS1982

Whistler mode turbulence in the disturbed solar wind

The magnetic field fluctuations with frequencies lying between the ion and electron cyclotron frequenices are enhanced downstream of interplanetary shocks in fast streams. Although spectra synthesized from ISEE 3 magnetometer and plasma wave instrument data can be described by one power law below 1 Hz, and another one above, measurement behind four shocks of the spectral index above the ion cyclotron frequency showed it to be twice the figure below, while no clear relationship is apparent in the weaker fast stream events. Although the data base is limited, the ratios of the average wave magnetic amplitude to electric field amplitudes confirm that the waves are whistler mode emissions, as suggested by their frequency range. It is indirectly deduced that the whistler waves are generated in such a way as to propagate at large angles to the local interplanetary field.

Coroniti, F. V.↗

NASA NTRS1982

The whistler nozzle phenomenon

The whistler nozzle is a simple device which can induce jet self-excitations of controllable amplitudes and frequencies and appears highly promising for many applications involving turbulent transport, combustion and aerodynamic noise. The characteristics of this curious phenomenon are documented for different values of the controlling parameters and attempts to explain the phenomenon. It is shown that the whistler excitation results from the coupling of two independent resonance mechanisms: shear-layer tone resulting from the impingement of the pipe-exit shear layer on the collar lip, and organ-pipe resonance of the pipe-nozzle. The crucial role of the shear-layer tone in driving the organ-pipe resonance is proven by reproducing the event in pipe-ring and pipe-hole configurations in the absence of the collar. It is also shown that this phenomenon is the strongest when the self-excitation frequency matches the preferred mode of the jet.

Hussain, A. K. M. F.↗

NASA NTRS1983

Ion cyclotron waves in the Io plasma torus - Polarization reversal of whistler mode noise

Because of the presence of multiple ion species in the Io plasma torus, whistler mode noise can be converted to ion cyclotron waves via a polarization reversal process at the local crossover frequency. Using whistler mode intensity measurements in the Jovian magnetosphere from Voyager 1, the pitch-angle diffusion rates that would occur if the noise is converted to ion cyclotron waves is estimated. Typical pitch-angle diffusion coefficients range from 0.000001/sec for protons resonating near the equator to 0.0001/sec for 10-keV O(+) ions resonating at high latitudes. Although complete bounce averaged diffusion coefficients have not yet been computed, preliminary estimates indicate that the energetic ion precipitation caused by these waves may be able to account for the EUV auroral emissions at the foot of the torus field lines.

Gurnett, D. A.↗

NASA NTRS1983

Electron acceleration by Landau resonance with whistler mode wave packets

Recent observations of electrostatic waves associated with whistler mode chorus emissions provide evidence that electrons are being trapped by Landau resonance interactions with the chorus. In this paper, the trapping, acceleration and escape of electrons in Landau resonance with a whistler mode wave packet are discussed. It is shown that acceleration can occur by both inhomogeneous and dispersive effects. The maximum energy gained is controlled by the points where trapping and escape occur. Large energy changes are possible if the frequency of the wave packet or the magnetic field strength increase between the trapping and escape points. Various trapping and escape mechanisms are discussed.

Gurnett, D. A.↗

NASA NTRS1983

The 'whistler-nozzle' phenomenon

Hussain, A. K. M. F.↗

NASA NTRS1984

A magnetohydrodynamic model of whistler duct structure in the magnetosphere

In this study, the physical structure for the propagation of whistler waves within a duct in the earth's magnetosphere is investigated by means of magnetohydrodynamic (MHD) theory. Expressions for the current density and induced magnetic field are determined analytically and evaluated in terms of two models for the duct plasma density distribution. It is found that once the duct is formed, forces associated with the current structure will maintain it. MHD instabilities are examined briefly and found to be unlikely to threaten duct maintenance in regions where whistlers are typically observed. Examination of some effects of field-aligned currents suggest that this may be a viable mechanism for duct formation.

Wang, S.↗

NASA NTRS1984

Perturbations of subionospheric LF and MF signals due to whistler-induced electron precipitation bursts

Increasing attention is now being devoted to the problem of the pitch angle scattering and resulting precipitation of magnetospheric energetic electrons by coherent waves. The present investigation has the objective to report the first evidence of a correlation between whistlers and amplitude perturbations on low-frequency (LF) signals at 37.2 kHz and medium-frequency (MF) signals at 780 kHz. Whistler-correlated amplitude perturbations were observed on a 780-kHz MF signal propagating on an approximately 1800 km path from South America to Palmer. The observed MF perturbations were of order 50 percent in amplitude and developed much more quickly than other changes of comparable magnitude on the signal.

Carpenter, D. L.↗

NASA NTRS1986

Whistler-mode radiation from the Spacelab 2 electron beam

During the Spacelab 2 mission the Plasma Diagnostics Package (PDP) performed a fly-around of the Shuttle at distances of up to 300 meters while an electron beam was being ejected from the Shuttle. A magnetic conjunction of the Shuttle and the PDP while the electron gun was operating in a steady (DC) mode is discussed. During this conjunction, the PDP detected a clear funnel-shaped emission that is believed to be caused by whistler-mode emission from the beam. Ray-path calculations show that the shape of the funnel can be accounted for by whistler-mode waves propagating near the resonance cone. Because the beam and waves are propagating in the same direction, the radiation must be produced by a Landau interaction with the beam. Other types of waves generated by the beam are also described.

Gurnett, D. A.↗

NASA NTRS1987

Electron/ion whistler instabilities and magnetic noise bursts

Two whistler instabilities are investigated by means of the linear Vlasov dispersion equation. They are called the electron/ion parallel and oblique whistler instabilities, and are driven by electron/ion relative drifts along the magnetic field. It is demonstrated that the enhanced fluctuations from these instabilities can explain several properties of magnetic noise bursts in and near the plasma sheet in the presence of ion beams and/or field-aligned currents. At sufficiently high plasma beta, these instabilities may affect the current system in the magnetotail.

Akimoto, K.↗

NASA NTRS1987

Proton beam generation of whistler waves in the earth's foreshock

It is shown that proton beams, often observed upstream of the earth's bow shock and associated with the generation of low-frequency hydromagnetic fluctuations, are also capable of generating whistler waves. The waves can be excited by an instability driven by two-temperature streaming Maxwellian proton distributions which have T (perpendicular)/T(parallel) much greater than 1. It can also be excited by gyrating proton beam distributions. These distributions generate whistler waves with frequencies ranging from 10 to 100 times the proton cyclotron frequency (in the solar wind reference frame) and provide another mechanism for generating the '1-Hz' waves often seen in the earth's foreshock.

Wong, H. K.↗

NASA NTRS1988

Proton beam generation of oblique whistler waves

It is known that ion beams are capable of generating whistler waves that propagate parallel to the mean magnetic field. Such waves may have been observed both upstream of the earth's bow shock and in the vicinity of comets. Previous analyses are extended to include propagation oblique to the mean magnetic field. The instability is generated by the perpendicular component of free energy in the ions, which can arise either via a temperature anisotropy or via a gyrating distribution. In the former case, the generation of whistler waves is confined to a fairly narrow cone of propagation directions centered about parallel propagation; in the latter case, the maximum growth of the instability can occur at fairly large obliquities (theta equal to about 50 deg).

Wong, H. K.↗

NASA NTRS1987

Ion and relativistic electron acceleration by Alfven and whistler turbulence in solar flares

A model is proposed in which turbulent Alfven and whistler waves simultaneously produce the proton and electron spectra implied by the gamma-ray observations noted during the impulsive phase of the June 3, 1982 flare. The results demonstrate that protons can be accelerated to several GeV in less than about 10 sec by Alfven turbulence whose energy density is greater than a few erg/cu cm. It is also found that electrons may be accelerated to tens of MeV on similar time scales by whistler and Alfven turbulence. A lower limit on the energy density of the Alfven turbulence is obtained which is small compared to the total magnetic energy density.

Miller, James A.↗

NASA NTRS1989

Three-dimensional simulation of whistler mode excited by the Spacelab 2 electron beam

During the Spacelab 2 mission, while an electron beam was being ejected from the Shuttle, the Plasma Diagnostics Package (PDP) detected a clear funnel-shaped emission that is believed to be caused by whistler-mode emission from the electron beam. In order to understand the mechanism of this emission, simulations with a three-dimensional partially magnetostatic code have been performed. The simulation results show that whistler-mode and lower hybrid waves are excited by the electron beam, which is initially localized in the column in the three-dimensional simulation system, and that they propagate away from the beam. The wave spectra of the electric and magnetic fields diagnosed at some points show several peaks due to the waves excited by the electron beam. The frequency range of these spectra is in qualitative agreement with the PDP data. The intense narrowband electrostatic emission near the electron plasma frequency is observed by the simulations. The simulation results show that the beam instability is responsible for the generation mechanism of these emissions.

Nishikawa, K.-I.↗

NASA NTRS1989

Whistler wave bursts upstream of the Uranian bow shock

Observations of magnetic field wave bursts upstream of the Uranian bow shock are reported which were recorded prior to the inbound shock crossing. Three wave types are identified. One exhibits a broad spectral enhancement from a few millihertz to about 50 mHz and is seen from 17 to 10 hr prior to the inbound shock crossing. It is argued that these waves are whistler waves that have propagated upstream from the shock. A second wave type has a spacecraft frame frequency between 20 and 40 mHz, is seen only within or immediately upstream of the shock pedestal, is right-hand polarized in the spacecraft frame, and has a typical burst duration of 90 s. The third wave type has a spacecraft frame frequency of about 0.15 Hz, is seen exclusively within the shock pedestal, is left-hand polarized in the spacecraft frame, and has a burst duration lasting up to 4 min. It is argued that the low-frequency bursts are whistler waves with phase speed comparable to, but in excess of, the solar wind speed.

Smith, Charles W.↗