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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1983

Steady-state turbulence with a narrow inertial range

Coupled two-dimensional wave equations are solved on a computer to model Langmuir wave turbulence excited by a weak electron beam. The model includes wave growth due to beam-plasma interaction, and dissipation by Landau damping. The inertial range is limited to a relatively small number of modes such as could occur when the ratio of masses between the negative and positive ions is larger than in a hydrogen plasma, or when there is damping in long wavelength Langmuir waves. A steady state is found consisting of quasistable, collapsed wave packets. The effects of different beam parameters and the assumed narrow inertial range are considered. The results may be relevant to plasma turbulence observed in connection with type III solar bursts.

Weatherall, J. C.↗
NASA NTRS1980

Polarization and position measurements of Type III bursts

The positional and polarization characteristics of Type III bursts in the range 24-220 MHz as measured by the Culgoora radioheliograph, spectrograph and spectropolarimeter are reported. The study includes 997 bursts which are of two classes: fundamental-harmonic (F-H) pairs and 'structureless' bursts with no visible F-H structure, and concentrates on the polarization of the bursts and the variation of polarization from centre to limb. The observed centre-to-limb decrease in polarization approximately follows a cosine law. This decrease is not as predicted by simple theory but is consistent with other observations which imply that open field lines from an active region diverge strongly. The observed o-mode polarization of harmonic radiation implies that the wave vectors of Langmuir waves are always parallel, within about 20 deg, to the magnetic field, while the constancy of H polarization with frequency implies that the ratio of gyromagnetic to plasma frequency, the Alfven speed and the plasma beta are constant with height on the open field lines above an active region. Finally, it is inferred that some factor, in addition to the magnetic field strength, controls the polarization of F radiation.

Suzuki, S.↗
NASA NTRS1994

Theory of 2 omega(sub pe) radiation induced by the bow shock

A new radiation emission mechanism is proposed to explain electomagnetic radiation observed at twice the electron plasma frequency, 2 omega(sub pe), in the upstream region of the Earth's bow shock. This radiation had its origin at the electron foreshock boundary where energetic electron beams and intense narrow-band Langmiur waves are observed. The proposed emission mechanism results from the interaction of the electron beam and Langmuir waves that are backscattered off thermal ions. This interaction is described by a nonlinear dispersion equation which incorporates an effect owing to electron trajectory modulation by the backscattered Langmuir waves. Subsequent analysis of the dispersion equation reveals two important consequences. First, a long-wavelength electrostatic quasi-mode with frequency at 2 omega(sub pe) is excited, and second, the quasi-mode and the electomagnetic mode are nonlinearly coupled. The implication is that, when the excited 2 omega(sub pe) quasi-mode propagates in an inhomgeneous medium with slightly decreasing density, the quasi-mode can be converted directly into an electromagnetic mode. Hense the electomagnetic radiation at twice the plasma frequency is generated. Numerical solutions of the dispersion equation with the choice of parameters that describe physical characteristics of the electron foreshock are presented, which illustrates the viability of the new mechanism.

Yoon, Peter H.↗
NASA NTRS1992

Unusual radio and plasma wave phenomena observed in March 1991

During the intense solar flare activity in March 1991 a number of unusual radio emission and Langmuir wave phenomena were observed by the radio and plasma wave (URAP) experiment on the Ulysses spacecraft. These phenomena were associated with unusual conditions in the interplanetary medium (IPM) presumably resulting from intense solar activity. Some of these URAP observations cannot be explained by mechanisms usually attributed to interplanetary (IP) radio emissions and Langmuir wave activity and require other interpretations.

Reiner, M. J.↗
NASA NTRS1987

Aspects of interplanetary plasma turbulence

Data acquired in the free solar wind by the ISEE 1 and ISEE 2 propagation experiment were used to obtain the power spectra of fluctuations in electron density; a maximum-entropy technique has made it possible to exploit fully the intrinsic precision of the in situ measurements. The results have led to a new determination of the anisotropy of density fluctuations in the solar wind, which is in conflict with certain expectations based on the propagation of Langmuir waves, but is in agreement with multistation interplanetary-scintillation measurements: it could well be that Langmuir wave growth rates during type III solar bursts are much larger than has been inferred from the electron distribution function, whose measurement involves averaging over relatively long time intervals. The relation between the fluctuation level and the mean density is studied, and tentative evidence for a correlation between the relative fluctuation level and proton temperature is presented.

Celnikier, L. M.↗
NASA NTRS1991

Large scale motions of Neptune's bow shock: Evidence for control of the shock position by the rotation phase of Neptune's magnetic field

The Voyager 2 spacecraft observed high levels of Langmuir waves before the inbound crossing of Neptune's bow shock, thereby signifying magnetic connection of the bow shock. The Langmuir waves occurred in multiple bursts throughout two distinct periods separated by an 85 minute absence of wave activity. The times of onsets, peaks, and disappearances of the waves were used together with the magnetic field directions and spacecraft position, to perform a 'remote-sensing' analysis of the shape and location of Neptune's bow shock prior to the inbound bow shock crossing. The bow shock is assumed to have a parabolidal shape with a nose location and flaring parameter determined independently for each wave event. The remote-sensing analysis give a shock position consistent with the time of the inbound shock crossing. The flaring parameter of the shock remains approximately constant throughout each period of wave activity but differs by a factor of 10 between the two periods. The absence of waves between two periods of wave activity coincides with a large rotation of the magnetic field and a large increase in the solar wind ram pressure' both these effects lead to magnetic disconnection of the spacecraft from shock. The planetwards motion of the shock's nose from 38.5 R(sub N) to 34.5 R(sub N) during the second time period occurred while the solar wind ram pressure remained constant to within 15 percent. This second period of planetwards motion of the shock is therefore strong evidence for Neptune's bow shock moving in response to the rotation of Neptune's oblique, tilted magnetic dipole. Normalizing the ram pressure, the remotely-sensed shock moves sunwards during the first wave period and planetwards in the second wave period. The maximum standoff distance occurs while the dipole axis is close to being perpendicular to the Sun-Neptune direction. The remote-sensing analysis provides strong evidence that the location of Neptune's bow shock is controlled by Neptune's rotation phase.

Cairns, Iver H.↗
NASA NTRS2013

Solar Wind Strahl Broadening by Self-Generated Plasma Waves

This Letter reports on the results of numerical simulations which may provide a possible explanation for the strahl broadening during quiet solar conditions. The relevant processes involved in the broadening are due to kinetic quasi-linear wave-particle interaction. Making use of static analytical electron distribution in an inhomogeneous field, it is found that self-generated electrostatic waves at the plasma frequency, i.e., Langmuir waves, are capable of scattering the strahl component, resulting in energy and pitch-angle diffusion that broadens its velocity distribution significantly. The present theoretical results provide an alternative or complementary explanation to the usual whistler diffusion scenario, suggesting that self-induced electrostatic waves at the plasma frequency might play a key role in broadening the solar wind strahl during quiet solar conditions.

strahl↗
NASA NTRS1987

Microstructures in type III events in the solar wind

A model is presented for the generation and evolution of bump-in-tail driven Langmuir waves in the solar wind during type III emission which removes a number of apparent inconsistencies between theory and observations. Growth rates and energy densities of Langmuir waves are considerably enhanced, permitting growth to overcome linear scattering losses, and also allowing nonlinear decay into ion-acoustic waves, in line with observations. Estimates are made of the probability distribution p(E), of wave field strengths E, based on linear and nonlinear wave-packet evolution, yielding p(E) approximately equal to E exp -alpha, alpha greater than or equal to 3. This helps explain why very high values of E are rarely found in the measured spiky wave turbulence.

Melrose, D. B.↗
NASA NTRS1981

Energetic electrons and plasma waves associated with a solar type III radio burst

Detailed in situ observations from the ISEE 3 spacecraft of energetic electrons, plasma waves, and radio emission for the type II solar radio burst of February 17, 1979, are presented. The reduced, one-dimensional electron distribution function is constructed as a function of time. Since the faster electrons arrive before the slower ones, a bump on tail distribution forms which is unstable to the growth of Langmuir waves. The plasma wave growth computed from the distribution function agrees well with the observed onset of the Langmuir waves, and there is qualitative agreement between variations in the plasma wave levels and in the development of regions of positive slope in the function. The evolution of the function, however, predicts far higher plasma wave levels than those observed. The maximum levels observed are approximately equal to the threshold for nonlinear wave processes, such as oscillation two-stream instability and soliton collapse.

Lin, R. P.↗
DOE OSTI2022

Electron Modulational Instability in the Strong Turbulent Regime for an Electron Beam Propagating in a Background Plasma

We study collective processes for an electron beam propagating through a background plasma using simulations and analytical theory. A new regime where the instability of a Langmuir wave packet can grow locally much faster than ion frequency is clearly identified. The key feature of this new regime is an electron modulational instability that rapidly creates a local Langmuir wave packet, which in its turn produces local charge separation and strong ion density perturbations because of the action of the ponderomotive force, such that the beam-plasma wave interaction stops being resonant. Three evolution stages of the process and observed periodic burst features are discussed. In conclusion, different physical regimes in the plasma and beam parameter space are demonstrated for the first time.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1982

Beam-plasma instability in the presence of low-frequency turbulence

General equations are derived for a linear beam-plasma instability in the presence of low-frequency turbulence. Within a 'quasi-linear' statistical approximation, these equations contain Langmuir wave scattering, diffusion, resonant and nonresonant anomalous absorption, and a 'plasma laser' effect. It is proposed that naturally occurring density irregularities in the solar wind may stabilize the beam-unstable Langmuir waves which occur during type III solar emissions.

Goldman, M. V.↗
NASA NTRS1991

Evidence of a transverse Langmuir modulational instability in a space plasma

The auroral ionosphere Langmuir-wave electric field amplitude modulation observations presented may be accounted for by a transverse modulational instability that is resonant with lower hybrid of ion Bernstein waves. In independent observations, the modulation frequencies were near the characteristic frequencies of perpendicular modes. These data strongly suggest the need for two- or three-dimensional model descriptions of nonlinear Langmuir wave evolution in the low-beta plasma.

Ergun, R. E.↗
NASA NTRS1985

Electron plasma waves upstream of the earth's bow shock

Electrostatic waves are observed around the plasma frequency fpe in the electron foreshock, together with electrons backstreaming from the bow shock. Using data from the sounder aboard ISEE 1, it is shown that this noise, previously understood as narrow band Langmuir waves more or less widened by Doppler shift or nonlinear effects, is in fact composed of two distinct parts: one is a narrow band noise, emitted just above fpe, and observed at the upstream boundary of the electron foreshock. This component has been interpreted as Langmuir waves emitted by a beam-plasma instability. It is suggested that it is of sufficiently large amplitude and monochromatic enough to trap resonant electrons. The other is a broad band noise, more impulsive than the narrow band noise, observed well above and/or well below fpe, deeper in the electron foreshock. The broad band noise has an average spectrum with a typical bi-exponential shape; its peak frequency is not exactly equal to fpe and depends on the Deybe length. This peak frequency also depends on the velocity for which the electron distribution has maximum skew. An experimental determination of the dispersion relation of the broad band noise shows that this noise, as well as the narrow band noise, may be due to the instability of a hot beam in a plasma.

Lacombe, C.↗
NASA NTRS1988

Nonlinear wave interactions and evolution of a ring-beam distribution of energetic electrons

A ring-beam distribution function of moderately relativistic electrons is unstable to electromagnetic and electrostatic waves. The results obtained in numerical simulations show that electromagnetic radiation corresponding to the normal modes of the background plasma is observed to grow even in the presence of a strong electrostatic instability and becomes very strong when the growth of the electrostatic Langmuir waves is minimized, and that the instability process is best described as a beam cyclotron resonance. Another strong radiation generated by the ring beam is the Z mode which is coupled to the electrostatic Langmuir wave. Under certain circumstances, these mechanisms may be significant in astrophysical situations.

Kainer, S.↗
NASA NTRS1983

Progress and problems in the theory of type III solar radio emission

The experimental and theoretical status of type III solar radio emission is considered in detail. Very recent developments which are relevant to the underlying plasma physics are emphasized. In particular, the identity of the submegahertz emissions as fundamental, or second harmonic, the degree of correlation between emissivities, electron streams, and plasma (Langmuir) waves, paradoxes concerned with the time-ordering of these phenomena, and the role of background density irregularities and ion-acoustic turbulence in the solar wind, are discussed. From the theoretical point of view, the current picture of the underlying Langmuir turbulence, including such effects as the interaction between Langmuir waves and stream electrons, induced scatter off ions, and strong turbulence effects such as modulational instability and soliton collapse, is discussed.

Goldman, M. V.↗
NASA NTRS1991

Low-frequency radio emissions in the outer heliosphere

Progress is reported toward a model for the 2 and 3 kHz radio waves observed by Voyagers 1 and 2 during the 1983-1987 interval at radial distances from the sun of 17 and 13 AU, respectively. The brightness temperature and range of the volume emissivity for the radiation are calculated, and the results are compared with the characteristics of known radiation at multiples of the plasma frequency. The derived brightness temperatures are used to constrain the source of the Langmuir waves required to generate the observed emission and to rule out certain emission mechanisms. Minimum values of 3-30 micro-V/m are derived for the Langmuir wave electric field intensity and are found to be in reasonable agreement with observed values at planetary bow shocks. Path lengths required for the radiation to reach the observed levels are derived and discussed. The relevance of these ideas to possible direct observations of heliospheric boundaries is addressed.

Macek, W. M.↗
NASA NTRS1991

Landau damping in space plasmas

The Landau damping of electrostatic Langmuir waves and ion-acoustic waves in a hot, isotropic, nonmagnetized, generalized Lorentzian plasma is analyzed using the modified plasma dispersion function. Numerical solutions for the real and imaginary parts of the wave frequency omega sub 0 - (i)(gamma) have been obtained as a function of the normalized wave number (k)(lambda sub D), where lambda sub D is the electron Debye length. For both particle distributions the electrostatic modes are found to be strongly damped at short wavelengths. At long wavelengths, this damping becomes less severe, but the attenuation of Langmuir waves is much stronger for a generalized Lorentzian plasma than for a Maxwellian plasma. It is concluded that Landau damping of ion-acoustic waves is only slightly affected by the presence of a high energy tail, but is strongly dependent on the ion temperature.

Thorne, Richard M.↗