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At least 19 records
All sourcesNASA NTRSDOE OSTI
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 NTRS1992

Theory for low-frequency modulated Langmuir wave packets

Langmuir wave packets with low frequency modulations (or beats) observed in the Jovian foreshock are argued to be direct evidence for the Langmuir wave decay L yields L-prime + S. In this decay, 'pump' Langmuir waves L, driven by an electron beam, produce backscattered product Langmuir waves L-prime and ion sound waves S. The L and L-prime waves beat at the frequency and wavevector of the S waves, thereby modulating the wave packets. Beam speeds calculated using the modulated Jovian wave packets (1) are reasonable, at 4-10 times the electron thermal speed, (2) are consistent with theoretical limits on the decay process, and (3) decrease with increasing foreshock depth, as expected theoretically. These results strongly support the theory. The modulation depth of some wave packets suggests saturation by the decay L yields L-prime + S. Applications to modulated Langmuir packets in the Venusian and terrestrial foreshocks and in a type III radio source are proposed.

Cairns, Iver H.↗
NASA NTRS2018

Observational Evidence for Langmuir Wave Collapse in the Source Region of a Solar Type III Radio Burst

High-time-resolution in situ wave observations show that Langmuir waves associated with solar type III radio bursts often occur as coherent localized one-dimensional magnetic-field-aligned wave packets with short durations of a few milliseconds and peak intensities well above the strong turbulence thresholds. In this paper, we report observations of a wave packet obtained by the time domain sampler of the STEREO WAVES experiment, which is unique in the sense that it is the most intense wave packet ever detected in association with a solar type III radio burst, with a peak intensity Et ∼ 107 mVm(sup-1). We show that this wave packet provides evidence for (1) oscillating two-stream instability (OTSI), (2) a collapsing soliton formed as a result of OTSI, (3) the formation of a soliton-caviton pair, and (4) excitation of second and third harmonic electromagnetic waves. We also show that the peak intensity and spatial width satisfy the threshold condition for this wave packet to be the collapsing Langmuir wave packet formed as a result of nucleation processes even when δn(sub b) > δn(sub p), where δn(sub b) and δn(sub p) are the levels of background and ponderomotive-force-induced density fluctuations, respectively. Thus, these observations provide unambiguous evidence for the spatial collapse of Langmuir waves in the source region of a type III radio burst, and the observed spectral evidence for OTSI and the ponderomotive-force-induced density cavity strongly suggest that OTSI is mostly likely responsible for the collapse of the observed wave packet.

Radio↗
NASA NTRS1995

Beat-type Langmuir wave emissions associated with a type III solar radio burst: Evidence of parametric decay

Recent measurements from the plasma wave instrument on the Galileo spacecraft have shown that Langmuir waves observed in conjunction with a type III solar radio burst contain many beat-type waveforms, with beat frequencies ranging from about 150 to 650 Hz. Strong evidence exists that the beat pattern is produced by two closely spaced narrowband components. The most likely candidates for these two waves are a beam-generated Langmuir wave and an oppositely propagating Langmuir wave produced by parametric decay. In the parametric decay process, nonlinear interactions cause the beam-driven Langmuir wave to decay into a Langmuir wave and a low-frequency ion sound wave. Comparisons of the observed beat frequency are in good agreement with theoretical predictions for a three-wave parametric decay process. Weak low-frequency emissions are also sometimes observed at the predicted frequency of the ion sound wave.

Hospodarsky, G. B.↗
DOE OSTI2021

Evidence for Trapping-Induced Nonlinear Frequency Shifts in Langmuir Waves Driven via Stimulated Raman Scattering

Thomson scattering is used to detect the spectra of Langmuir waves driven through the backward stimulated Raman scattering process in a diffraction limited laser focal spot. Measured Langmuir wave spectral frequencies are found to vary in time and have broadened spectral power, consistent with a nonlinear frequency shift of the driven Langmuir wave due to electron-trapping. Broadening of the Langmuir wave spectral power is observed to decrease in time, consistent with measured variations in the frequency shift of the driven Langmuir waves. Furthermore, the observed spectral broadening is consistent with the temporally short (ps), bursty nature of backward stimulated Raman scattered light observed in simulations that cannot be resolved by the Thomson scattering diagnostic. Comparison of the broadened spectrum with time integrated spectra from two-dimensional particle-in-cell simulations shows favorable comparison in the broadened spectral widths, supporting the supposition of electron-trapping induced, nonlinear shifting of daughter Langmuir wave frequencies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1986

Clumpy Langmuir waves in type III solar radio bursts

A model is developed for type III radio emission in the interplanetary medium based on recent data on Langmuir waves, associated with ion sound waves, density fluctuations in the interplanetary plasma, streaming electrons and radio emission. In this model, Langmuir wave growth is suppressed by refraction in field-aligned density irregularities except near density minima where clumps of Langmuir waves form. Quasi-linear relaxation limits the growth of the Langmuir waves in the clumps. The radio emission, which is attributed to coalescence of the Langmuir waves with associated ion sound waves, saturates at a brightness temperature equal to the effective temperature of the Langmuir waves, estimated to be between 10 to the 15th and 10 to the 16th K from obserrvational data. The model is consistent with all the relevant data on type III events. In particular, it accounts naturally for observed brightness temperatures of type III bursts.

Melrose, D. B.↗
NASA NTRS1990

Simulation studies of plasma waves in the electron foreshock - The generation of Langmuir waves by a gentle bump-on-tail electron distribution

Particle simulation experiments were used to study the basic physical ingredients needed for building a global model of foreshock wave phenomena. In particular, the generation of Langmuir waves by a gentle bump-on-tail electron distribution is analyzed. It is shown that, with appropriately designed simulations experiments, quasi-linear theory can be quantitatively verified for parameters corresponding to the electron foreshock.

Dum, C. T.↗
NASA NTRS2012

Interpretation of Vector Electric Field Measurements of Bursty Langmuir Waves in the Cusp

An analysis of auroral Langmuir waves in the cusp observed by two high-frequency electric field instruments on the TRICE high-flyer sounding rocket shows many examples of Langmuir wave bursts modulated at approximately 10 kHz. Previous studies have explained these and similar observations as the result of beating between waves with very close frequencies near the Langmuir cutoff, resulting from either wave-wave interactions or independent linear excitations. The unique three-dimensional (3-D) data set provided by the NASA Goddard Space Flight Center TAEFWD instrument shows that up to 25% of waveforms selected from the most intense bursts exhibit anisotropic modulations, i.e., the beat nulls and peaks are not aligned in time across the three perpendicular electric field components. Modulations of this type arise when superposed wave normal modes possess differing polarizations, and simulations using wave modes calculated with the J-WHAMP numerical dispersion code show (1) that waves with differing polarizations can exist in conditions like those observed by TRICE-High and (2) that superpositions of such waves can produce anisotropic modulation. Fourier analysis of the 3-D waveform data suggests that both linear and elliptically polarized waves are present near the Langmuir cutoff at these times. These observations illustrate how 3-D measurements can give valuable insight into the nature of wave interactions in the auroral plasma environment.

Dombrowski, M. P.↗
NASA NTRS1994

The fine structure of Langmuir waves observed upstream of the bow shock at Venus

Highly structured Langmuir waves, also known as electron plasma oscillations, have been observed in the foreshock of Venus using the plasma wave experiment on the Galileo spacecraft during the gravity assist flyby on February 10, 1990. The Galileo wideband sampling system provides digital electric field waveform measurements at sampling rates up to 201,600 samples per second, much higher than any previous instrument of this type. The main Langmuir wave emission band occurs near the local electron plasma frequency, which was approximately 43 kHz. The Langmuir waves are observed to shift above and below the plasma frequency, sometimes by as much as 20 kHz. The shifts in frequency are closely correlated with the downstream distance from the tangent field line, implying that the shifts are controlled by the electron beam velocity. Considerable fine structure is also evident, with time scales as short as 0.15 milliseconds, corresponding to spatial scales of a few tens of Debye lengths. The frequency spectrum often consists of beat-type waveforms, with beat frequencies ranging from 0.2 to 7 kHz, and in a few cases, isolated wavepackets. The peak electric field strengths are approximately 1 mV/m. These field strengths are too small for strongly nonlinear processes to be important. The beat-type waveforms are suggestive of a parametric decay process.

Hospodarsky, G. B.↗
NASA NTRS1994

Fine structure of Langmuir waves observed upstream of the bow shock at Venus

Highly structured Langmuir waves, also known as electron plasma oscillations, have been observed in the foreshock of Venus using the plasma wave experiment on the Galileo spacecraft during the gravity assist flyby on February 10, 1990. The Galileo wideband sampling system provides digital electric field waveform measurements at sampling rates up to 201,600 samples per second, much higher than any previous instrument of this type. The main Langmuir wave emission band occurs near the local electron plasma frequency, which was approximately 43 kHz. The Langmuir waves are observed to shift above and below the plasma frequency, sometimes by as much as 20 kHz. The shifts in frequency are closely correlated with the downstream distance from the tangent field line, implying that the shifts are controlled by the electron beam velocity. Considerable fine structure is also evident, with timescales as short as 0.15 ms, corresponding to spatial scales of a few tens of Debye lengths. The frequency spectrum often consists of beat-type waveforms, with beat frequencies ranging from 0.2 to 7 kHz, and in a few cases, isolated wave packets. The peak electric field strengths are approximately 1 mV/m. These field strengths are too small for strongly nonlinear processes to be important. The beat-type waveforms are suggestive of a parametric decay process.

Hospodarsky, G. B.↗
NASA NTRS1996

A Study of Uranus' Bow Shock Motions Using Langmuir Waves

During the Voyager 2 flyby of Uranus, strong electron plasma oscillations (Langmuir waves) were detected by the plasma wave instrument in the 1.78-kHz channel on January 23-24, 1986, prior to the inbound bow shock crossing. Langmuir waves are excited by energetic electrons streaming away from the bow shock. The goal of this work is to estimate the location and motion of Uranus' bow shock using Langmuir wave data, together with the spacecraft positions and the measured interplanetary magnetic field. The following three remote sensing analyses were performed: the basic remote sensing method, the lag time method, and the trace-back method. Because the interplanetary magnetic field was highly variable, the first analysis encountered difficulties in obtaining a realistic estimation of Uranus' bow shock motion. In the lag time method developed here, time lags due to the solar wind's finite convection speed are taken into account when calculating the shock's standoff distance. In the new trace-back method, limits on the standoff distance are obtained as a function of time by reconstructing electron paths. Most of the results produced by the latter two analyses are consistent with predictions based on the standard theoretical model and the measured solar wind plasma parameters. Differences between our calculations and the theoretical model are discussed.

Xue, S.↗
NASA NTRS1981

Parametric interaction and spatial collapse of beam-driven Langmuir waves in the solar wind

Observations are presented of the parametric decay and spatial collapse of Langmuir waves driven by an electron beam streaming into the solar wind from the Jovian bow shock. Long wavelength Langmuir waves upstream of the bow shock are effectively converted into short wavelength waves no longer in resonance with the beam. The conversion is shown to be the result of a nonlinear interaction involving the beam-driven pump, a sideband emission, and a low level of ion-acoustic turbulence. The beam-driven Langmuir wave emission breaks up into a complex sideband structure with both positive and negative Doppler shifts. In some cases, the sideband emission consists of isolated wave packets with very short duration bursts, which are very intense and are thought to consist of envelope solitons which have collapsed to spatial scales of only a few Debye lengths.

Gurnett, D. A.↗
NASA NTRS1992

Low frequency magnetic signals associated with Langmuir waves

With the URAP experiment on Ulysses, low frequency signals with a magnetic component in close time correlation with electrostatic Langmuir waves at the plasma frequency are observed. In most, if not all, of these cases, the Langmuir waves are part of a Type III solar burst. This effect is investigated and it is shown that the low frequency waves are in the whistler mode and are most likely due to nonlinear effects involving Langmuir waves.

Kellogg, Paul J.↗
NASA NTRS1990

Simulation of the collapse and dissipation of Langmuir wave packets

Particle-in-cell (PIC) simulations and Zakharov's partial differential equations (PDEs) are used to investigate the collapse of isolated Langmuir wave packets in two dimensions. Collapse thresholds are determined numerically, and the roles of enhanced Langmuir wave damping and nonlinearities not included in the standard Zakharov equations are discussed. The Langmuir wave and ion dynamics in PIC simulations are compared with the predictions of PDE simulations incorporating enhanced Langmuir damping. Electron heating and coherent acceleration in the PIC simulations are discussed and compared with predictions of the transit-time theory.

Newman, D. L.↗
NASA NTRS1987

A theory for the Langmuir waves in the electron foreshock

A comprehensive theory for the Langmuir waves in the earth's foreshock involving saturation of the kinetic version of the beam instability by quasi-linear relaxation is proposed in this paper. Reactive and kinetic beam instabilities are shown to be the two limiting versions of a single instability whose analytically and numerically derived descriptions are shown to form a consistent picture. It is pointed out that the reactive instability gives rise only to narrow-band growth while the kinetic instability gives rise to wide-band growth. Arguments for describing the Langmuir wave growth in terms of the kinetic instability are given, and four suppression mechanisms for the kinetic instability are discussed. It is suggested that quasi-linear relaxation limits the Langmuir growth and gives rise to distributions qualitatively similar to the observed distribution functions, and arguments in favor of this hypothesis are presented.

Cairns, Iver H.↗
NASA NTRS2000

Generalized Langmuir Waves in Magnetized Kinetic Plasmas

The properties of unmagnetized Langmuir waves and cold plasma magnetoionic waves (x, o, z and whistler) are well known. However, the connections between these modes in a magnetized kinetic plasma have not been explored in detail. Here, wave properties are investigated by numerically solving the dispersion equation derived from the Vlasov equations both with and without a beam instability present. For omega(sub p)>Omega(sub e), it is shown that the generalized Langmuir mode at oblique propagation angles has magnetic z-mode characteristics at low wave numbers and thermal Langmuir mode characteristics at high wave numbers. For omega(sub p)<Omega(sub e), it is shown that the (oblique) Langmuir mode instead connects to the whistler mode at low wave numbers. The transition from the Langmuir/z mode to the Langmuir/whistler mode near omega(sub p) = Omega(sub e) is rapid. In addition, the effects on wave dispersion and polarization after adding a beam are investigated. Applications of this theory to magnetized Langmuir waves in Earth's foreshock and the solar wind, to waves observed near the plasma frequency in the auroral regions, and to solar type III bursts are discussed.

Willes, A. J.↗
NASA NTRS2000

Thermal and Driven Stochastic Growth of Langmuir Waves in the Solar Wind and Earth's Foreshock

Statistical distributions of Langmuir wave fields in the solar wind and the edge of Earth's foreshock are analyzed and compared with predictions for stochastic growth theory (SGT). SGT quantitatively explains the solar wind, edge, and deep foreshock data as pure thermal waves, driven thermal waves subject to net linear growth and stochastic effects, and as waves in a pure SGT state, respectively, plus radiation near the plasma frequency f(sub p). These changes are interpreted in terms of spatial variations in the beam instability's growth rate and evolution toward a pure SGT state. SGT analyses of field distributions are shown to provide a viable alternative to thermal noise spectroscopy for wave instruments with coarse frequency resolution, and to separate f(sub p) radiation from Langmuir waves.

Cairns, Iver H.↗