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At least 217 records · Page 12
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NASA NTRS1987

On the generation of broadband electrostatic noise

Linear theory analysis and particle simulation studies were carried out to understand the mechanisms generating broadband electrostatic noise (BEN) and the effects of BEN on particles as well as the nonlinear saturation level in the geomagnetic tail region. Streaming ion beams as well as warm ions and electrons are used to model the plasma-sheet particle population. When the beam ion temperature is comparable to the temperature of the warm plasma-sheet particles, electrostatic ion cyclotron instabilities become unstable, giving rise to low-frequency noise at omega less than about n(Omega i), where Omega i is the ion gyrofrequency.

Ashour-Abdalla, M.↗
NASA NTRS1978

Nonlinear stability of solar type 3 radio bursts. 2: Application to observations near 1 AU

A set of rate equations including strong turbulence effects and anomalous resitivity are solved using parmeters which model several solar type 3 bursts. Exciter distributions observed at 1 AU are excitation of the linear bump-in-tail instability, amplifying Langmuir waves above the threshold for the oscillating two stream instability (OTSI). The OTSI, and the attendant anomalous resistivity produce a rapid spectral transfer of Langmuir waves to short wavelengths, out of resonance with the electron exciter. Further energy loss of the beam is thus precluded. The various parameters needed to model the bursts are extrapolated inside 1 AU with similar results. Again, the OTSI is excited and decouples the electron beam from the Langmuir radiation. Reabsorption of the Langmuir waves by the beam is shown to be unimportant in all cases, even at 0.1 AU. The theory provides a natural explanation for the observed realationship between radio flux, and the electron flux.

Goldstein, M. L.↗
NASA NTRS1985

Return currents in solar flares - Collisionless effects

If the primary, precipitating electrons in a solar flare are unstable to beam plasma interactions, it is shown that strong Langmuir turbulence can seriously modify the way in which a return current is carried by the background plasma. In particular, the return (or reverse) current will not be carried by the bulk of the electrons, but by a small number of high velocity electrons. For beam/plasma densities greater than 0.01, this can reduce the effects of collisions on the return current. For higher density beams where the return current could be unstable to current driven instabilities, the effects of strong turbulence anomalous resistivity is shown to prevent the appearance of such instabilities. Again in this regime, how the return current is carried is determined by the beam generated strong turbulence.

Rowland, H. L.↗
NASA NTRS1986

Particle propagation, wave growth and energy dissipation in a flaring flux tube

Wave amplification by downgoing particles in a common flare model is investigated. The flare is assumed to occur at the top of a coronal magnetic flux loop, and results in the heating of plasma in the flaring region. The hot electrons propagate down the legs of the flux tube towards increasing magnetic field. It is simple to demonstrate that the velocity distributions which result in this model are unstable to both beam instabilities and cyclotron maser action. An explanation is presented for the propagation effects on the distribution, and the properties of the resulting amplified waves are explored, concentrating on cyclotron maser action, which has properties (emission in the z mode below the local gyrofrequency) quite different from maser action by other distributions considered in the context of solar flares. The z mode waves will be damped in the coronal plasma surrounding the flaring flux tube and lead to heating there. This process may be important in the overall energy budget of the flare. The downgoing maser is compared with the loss cone maser, which is more likely to produce observable bursts.

White, S. M.↗
NASA NTRS1989

Nonlinear generation of whistler waves by an ion beam

An electromagnetic hybrid code is used to simulate a new mechanism for whistler wave generation by an ion beam. First, a field-aligned ion beam becomes unstable to the electromagnetic ion/ion right-hand resonant instability which generates large amplitude MHD-like waves. These waves then trap the ion beam and increase its effective temperature anisotropy. As a result, the growth rates of the electron/whistler instability are significantly enhanced, and whistlers start to grow above the noise level. At the same time, because of the reduced parallel drift speed of the ion beam, the frequencies of the whistlers are also downshifted. Full simulations were performed to isolate and separately investigate the electron/ion whistler instability. The results are in agreement with the assumption of fluid electrons in the hybrid simulations and with the linear theory of the instability.

Akimoto, K.↗
NASA NTRS1992

The CIV processes in the CRIT experiments

A qualitative analysis is conducted to reconcile the experimental data from critical ionization velocity (CIV) studies with CIV theories. The experimental data are reviewed demonstrating that: (1) the wave frequency is variable and low; (2) the wave polarization is almost isotropic; (3) electron energization is not easily reconciled with the observed wave spectrum; and (4) ambient electron density plays a role in determining CIV triggering conditions. Analytical treatment is given to the dispersion relation of the lower hybrid wave (LWH) instability driven by the streaming of an ion beam generated by the interaction of the neutral cloud with the ambient atmosphere. By incorporating the LWH instabilities of strong turbulence and finite-size effects into theoretical CIV relationships, the observations can be interpreted. The issues raised by the experimental data are understood within the context of a hypothesis of backward propagating nonlinearly collapsing wavepackets.

Papadopoulos, K.↗
NASA NTRS1987

Numerical study of the spectrum of broadband electrostatic noise in the magnetotail

A numerical study of the spectrum of broadband electrostatic noise (BEN) generated by ion beams in the plasma sheet boundary layer (PSBL) is presented, in which two cases are analyzed. The first case is concerned with a single hot-electron species. In the second case, a second population of cold electrons (arising from diffusion from the lobe into the PSBL) is introduced. The presence of the second electron component in the dispersion relation for generating BEN was found to significantly increase the upper frequency range of the unstable spectrum, enhancing the associated growth rates. It is shown that ion beam instabilities can explain most major features of the observed BEN spectrum. The results explain the gradual rise in the upper frequency range of BEN as a spacecraft approaches the plasma sheet from the lobe, and the sudden jump in that frequency up to near the plasma frequency upon crossing the PSBL.

Grabbe, Crockett L.↗
NASA NTRS1995

Low-frequency waves associated with Langmuir waves in solar wind

The Ulysses spacecraft has detected several events of low-frequency electromagnetic waves in association with Langmuir waves in the solar wind. The high time resolution observations show that the Langmuir waves are very intense and occur as broad peaks superposed by collapsing millisecond spikes. The low-frequency waves are identified as electromagnetic lower hybrid waves. The observed energy densities of these waves often exceed the strong turbulence thresholds. It is shown that none of the parametric decay instabilities involving Langmuir and low-frequency waves are energetically favorable to explain the present observations. The low-frequency waves are proposed to arise from currents associated with gradients in the electron beam originating at sites where Langmuir waves scatter the beam electrons.

Thejappa, G.↗
NASA NTRS1985

Collisionless effects on beam-return current systems in solar flares

A theoretical study of the beam-return current system (BRCS) in solar flares shows that the precipitating electrons modify the way in which the return current (RC) is carried by the background plasma. In particular it is found that the RC is not carried by the bulk of the electrons but by a small number of high-velocity electrons. For beam/plasma densities exceeding approximately 0.001, this can reduce the effects of collisions and heating by the RC. For higher-density beams, where the RC could be unstable to current-driven instabilities, the effects of strong turbulence anomalous resistivity prevent the appearance of such instabilities. The main conclusion is that the BRCS is interconnected, and that the beam-generated strong turbulence determines how the RC is carried.

Vlahos, L.↗
NASA NTRS1979

Artificial stimulation of auroral electron acceleration by intense field aligned currents

A cesium-doped high explosion was detonated at 165 km altitude in the auroral ionosphere during quiet conditions. An Alfven wave pulse with a 200-mV/m electric field was observed, with the peak occurring 135 ms after the explosion at a distance of about 1 km. The count rate of fixed energy 2-keV electron detectors abruptly increased at 140 ms, peaked at 415 ms, and indicated a downward field-aligned beam of accelerated electrons. An anomalously high-field aligned beam of backscattered electrons was also detected. The acceleration is interpreted as due to production of an electrostatic shock or double layer between 300 and 800 km altitude. The structure was probably formed by an instability of the intense field-aligned currents in the Alfven wave launched by the charge-separation electric field due to the explosion.

Holmgren, G.↗
NASA NTRS1981

Electromagnetic-wave excitation in a large laboratory beam-plasma system

The mechanism by which unstable electrostatic waves of a beam-plasma system are converted into observed electromagnetic waves is of current interest in space physics and in tokamak fusion research. The process involved in the conversion of electrostatic to electromagnetic waves at the critical layer is well understood. However, the radiation from uniform plasmas cannot be explained on the basis of this process. In connection with certain difficulties, it has not yet been possible to establish the involved emission processes by means of experimental observations. In the considered investigation these difficulties are overcome by employing a large laboratory plasma in a parameter range suitable for detailed diagnostics. A finite-diameter electron beam is injected into a uniform quiescent afterglow plasma of dimensions large compared with electromagnetic wavelengths. The considered generation mechanism concerning the electromagnetic waves is conclusively confirmed by observing the temporal evolution of an instability

Whelan, D. A.↗
NASA NTRS1987

Wave generation and particle transport in the plasma sheet and boundary layer

The one and two ion beam instability was considered as a possible explanation for the observations of broadband electrostatic noise in the plasma sheet region of the geomagnetic tail. When only hot streaming plasma sheet boundary layer ions were present, no broadband waves were excited. Cold, streaming ionospheric ions can generate electrostatic broadband waves propagating in the slow beam-acoustic mode, but the growth rates of the waves were significantly enhanced when warm boundary layer ions were presented. (Both the slow and fast beam-acoustic modes can be excited, depending on the relative ion drift.) This mode predicted that the wave intensity of the broadband noise should peak in the plasma sheet boundary layer (PSBL). Observations of less intense electrostatic waves in the lobes and plasma sheet were likely a result of the absence of warm ion beams or large ion temperatures, respectively, which resulted in smaller growth rates. The model dependence of the ion beam instability has also been studied. For cold and warm ions streaming in the same direction, researchers found wave growth peaked for wave normal angles theta = 0 deg. and wave frequencies approx. 0.1 x the electron plasma frequency. However, for anti-parallel streaming cold and warm ions, wave growth peaks near theta = 90 deg. and wave frequencies were an order of magnitude smaller.

Dusenbery, P. B.↗
NASA NTRS1988

Simulation of electrostatic turbulence in the plasma sheet boundary layer with electron currents and bean-shaped ion beams

Plasma data from ISEE-1 show the presence of electron currents as well as energetic ion beams in the plasma sheet boundary layer. Broadband electrostatic noise and low-frequency electromagnetic bursts are detected in the plasma sheet boundary layer, especially in the presence of strong ion flows, currents, and steep spacial gradients in the fluxes of few-keV electrons and ions. Particle simulations have been performed to investigate electrostatic turbulence driven by a cold electron beam and/or ion beams with a bean-shaped velocity distribution. The simulation results show that the counterstreaming ion beams as well as the counterstreaming of the cold electron beam and the ion beam excite ion acoustic waves with a given Doppler-shifted real frequency. However, the effect of the bean-shaped ion velocity distributions reduces the growth rates of ion acoustic instability. The simulation results also show that the slowing down of the ion bean is larger at the larger perpendicular velocity. The wave spectra of the electric fields at some points of the simulations show turbulence generated by growing waves.

Nishikawa, K.-I.↗
NASA NTRS2011

Observations and Effects of Dipolarization Fronts Observed in Earth's Magnetotail

Dipolarization fronts in Earth's magnetotail are characterized by sharp jumps in magnetic field, a drop in density, and often follow earthward fast plasma flow. They are commonly detected near the equatorial plane of Earth s tail plasma sheet. Sometimes, but not always, dipolarization fronts are associated with global substorms and auroral brightenings. Both Cluster, THEMIS, and other spacecraft have detected dipolarization fronts in a variety of locations in the magnetotail. Using multi-spacecraft analyses together with simulations, we have investigated the propagation and evolution of some dipolarization events. We have also investigated the acceleration of electrons and ions that results from such magnetic-field changes. In some situations, the velocities of fast earthward flows are comparable to the Alfven speed, indicating that the flow bursts might have been generated by bursty reconnection that occurred tailward of the spacecraft. Based on multi-spacecraft timing analysis, dipolarization fronts are found to propagate mainly earthward at 160-335 km/s and have thicknesses of ~900-1500 km, which corresponds to the ion inertial length or gyroradius scale. Following the passage of dipolarization fronts, significant fluctuations are observed in the x and y components of the magnetic field. These peaks in the magnetic field come approximately 1-2 minutes after passage of the dipolarization front. These Bx and By fluctuations propagate primarily dawnward and earthward. Field-aligned electron beams are observed coincident with those magnetic field fluctuations. Non-Maxwellian electron and ion distributions are observed that are associated with the dipolarization that may be unstable to a range of electrostatic and/or whistler instabilities. Enhanced electrostatic broadband noise at frequencies below and near the lower-hybrid frequency is also observed at or very close to these fronts. This broadband noise is thought to play a role in further energizing the particles. Such studies provide insights into the particle acceleration mechanisms associated with substorm dipolarization, and, in turn, the effects of those acceleration mechanisms on the structure and evolution of dipolarization fronts.

Goldstein, Melvyn L.↗
NASA NTRS1985

Multiple Raman up-conversion of radiation from pre-existing Langmuir turbulence

Little is known regarding the mechanisms involved in the production of radiation at frequencies which are much higher than the maximum plasma frequency in nonmagnetic plasmas. It is pointed out that radiation at high multiples of the plasma frequency has been observed in laser-pellet experiments, in relativistic beam-plasma interactions, and in low-energy beam-plasma interactions. Radiation at twice the plasma frequency can be produced by coalescence of two Langmuir waves. Another approach is based on radiation near the plasma frequency or its second harmonic, produced by instability or by external sources. Raman scatter from preexisting Langmuir turbulence is permitted repeatedly. At each step such scattering will produce frequency upshifts (anti-Stokes process) and downshifts (Stokes process). The present paper is concerned with this process. Attention is given to equilibrium spectra, analytic approximations, laser-plasma interaction and coherent radiation from relativistic electron beams.

Russell, D.↗
NASA NTRS1981

Instabilities of low frequency, parallel propagating electromagnetic waves in the earth's foreshock region

An instability analysis is presented for parallel and antiparallel propagating electromagnetic waves generated by reflected and diffuse suprathermal ions upstream of the earth's bow shock. Calculations are performed on the basis of upstream particle observations made by the ISEE 1 Quadrispheric Lepedea instrument and low-energy electron measurements made by the ISEE 1 electron spectrometer for a single period. The electromagnetic dispersion relation is computed and the unstable modes and growth times of the fastest growing waves are determined. It is found that the reflected ions destabilize the plasma most strongly at a wave frequency 0.1 that of the ion gyrofrequency by a resonant ion beam instability for waves propagating upstream and by a nonresonant firehose-like instability for waves propagating downstream. The diffuse ions also destabilize the plasma most strongly at the same frequency by means of resonant instabilities of both right- and left-hand polarized waves propagating away from the bow shock.

Sentman, D. D.↗
NASA NTRS1995

Linear and nonlinear properties of the ULF waves driven by ring-beam distribution functions

The problem of the exitation of obliquely propagating magnetosonic waves which can steepen up (also known as shocklets) is considered. Shocklets have been observed upstream of the Earth's bow shock and at comets Giacobini-Zinner and Grigg-Skjellerup. Linear theory as well as two-dimensional (2-D) hybrid (fluid electrons, particle ions) simulations are used to determine the properties of waves generated by ring-beam velocity distributions in great detail. The effects of both proton and oxygen ring-beams are considered. The study of instabilities excited by a proton ring-beam is relevant to the region upstream of the Earth's bow shock, whereas the oxygen ring-beam corresponds to cometary ions picked up by the solar wind. Linear theory has shown that for a ring-beam, four instabilities are found, one on the nonresonant mode, one on the Alfven mode, and two along the magnetosonic/whistler branch. The relative growth rate of these instabilities is a sensitive function of parameters. Although one of the magnetosonic instabilities has maximum growth along the magnetic field, the other has maximum growth in oblique directions. We have studied the competition of these instabilities in the nonlinear regime using 2-D simulations. As in the linear limit, the nonlinear results are a function of beam density and distribution function. By performing the simulations as both initial value and driven systems, we have found that the outcome of the simulations can vary, suggesting that the latter type simulations is needed to address the observations. A general conclusion of the simulation results is that field-aligned beams do not result in the formation of shocklets, whereas ring-beam distributions can.

Killen, K.↗
NASA NTRS1987

Effect of wave localization on plasma instabilities

The Anderson model of wave localization in random media is involved to study the effect of solar wind density turbulence on plasma processes associated with the solar type III radio burst. ISEE-3 satellite data indicate that a possible model for the type III process is the parametric decay of Langmuir waves excited by solar flare electron streams into daughter electromagnetic and ion acoustic waves. The threshold for this instability, however, is much higher than observed Langmuir wave levels because of rapid wave convection of the transverse electromagnetic daughter wave in the case where the solar wind is assumed homogeneous. Langmuir and transverse waves near critical density satisfy the Ioffe-Reigel criteria for wave localization in the solar wind with observed density fluctuations -1 percent. Numerical simulations of wave propagation in random media confirm the localization length predictions of Escande and Souillard for stationary density fluctations. For mobile density fluctuations localized wave packets spread at the propagation velocity of the density fluctuations rather than the group velocity of the waves. Computer simulations using a linearized hybrid code show that an electron beam will excite localized Langmuir waves in a plasma with density turbulence. An action principle approach is used to develop a theory of non-linear wave processes when waves are localized. A theory of resonant particles diffusion by localized waves is developed to explain the saturation of the beam-plasma instability. It is argued that localization of electromagnetic waves will allow the instability threshold to be exceeded for the parametric decay discussed above.

Levedahl, William Kirk↗