Search NASA⌕ Search

SEARCH · Search NASA

Results for “Plasma fluctuations”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The study of coronal plasma structures and fluctuations with Faraday rotation measurements

We report dual-frequency, polarimetric measurements of Faraday rotation of extragalactic radio sources viewed through the solar corona. The observations were made at the Very Large Array in 1990 during solar maximum. Of the nine observed, an excess rotation measure of -12.6 rad/sq m was detected for one source (0010+005), which was observed at an elongation of about 9 solar radii. This measurement is in fair agreement with an a priori model rotation measure of -8.6 rad/sq m estimated from coronal potential field models and the electron density model of Paetzold et al. (1992). Our measurement provides a value for the coronal magnetic field strength at 9 solar radii given a knowledge of the magnetic field sector structure, of 12.5 +/- 2.3 mG. Rotation measurements of 0010+005 were made approximately once per hour over an 11 hr period. During this interval, a slow change of about 1 rad/sq m/hr in rotation measure was detected. Although we are not absolutely certain that this drift is not unremoved ionospheric Faraday rotation, extensive analysis of data from the other sources suggests that this is not the case (Sakurai & Spangler 1994). The very long timescale for this variation argues against the agency of magnetohydrodynamics (MHD) waves, and we suggest occultation of 0010+005 by relatively static plasma structures in the corona. We filtered our rotation measure time series to search for variations on an hourly timescale, such as those reported by Hollweg et al. (1992), which could be attributed to coronal MHD waves. We were unable to detect such fluctuations and can report only an upper limit to the rms variation of 1.6 rad/sq m. This upper limit is of the same order, but slightly larger than the values typically reported by Hollweg et al. (1982). This upper limit to the rotation measure fluctuations limits the dimensionless wave amplitude (delta B)/B in the corona to be less than 0.7. Using the number, we estimate the MHD wave flux at the coronal base to be less than 1.6 x 10(exp 5) ergs/sq cm/s. This is less than the amount of wave energy flux required by wave-driven models of the solar wind. Finally, we discuss a number of ways in which such observations could be improved in the future.

Sakurai, Takayuki↗
NASA NTRS1972

Charged-particle transport in turbulent astrophysical plasmas

The effect of electromagnetic fluctuations, or plasma turbulence, on the motion of the individual particles in a plasma is investigated. Two alternative methods are used to find a general equation governing the time-evolution of a distribution of charged particles subject to both an external force field and the random fields of the fluctuations. It is found that, for the high-temperature, low-density plasmas frequently encountered in the study of astrophysics, the presence of even a small amount of turbulence can have a very important effect on the behavior of the plasma. Two problems in which turbulence plays an important role are treated.

Newman, C. E., Jr.↗
NASA NTRS1972

Resonant diffusion in strongly turbulent plasmas.

The effect of turbulent fluctuations on plasma particles is considered, and equations are derived which describe the evolution of macroscopic properties such as temperature and flow speed of the turbulent plasma. Initially, a diffusion equation for a single-particle distribution function averaged over an ensemble of plasmas is derived for an unmagnetized plasma. For the resonant diffusion in strongly turbulent plasmas, an ensemble of three-dimensional plasmas is considered with an approximately homogeneous and stationary distribution of random electromagnetic fluctuations. For each realization, the single-particle distribution function satisfies the Vlasov equation.

Birmingham, T. J.↗
NASA NTRS1972

The plasmapause as measured in positive ions.

Use of extensions of the existing theory of magnetospheric convection to describe the dynamics of the plasmasphere and the variation in the plasmapause location. The equatorial local time plane is divided into three separate and physically distinct regions, the bulge region, the nightside region, and the dayside region, corresponding to local times of 1500 to 2200 hrs, 2200 to 0600 hrs, and 0600 to 1500 hrs, respectively. The characteristics predicted by the model in the bulge region are: presence of the bulge at dusk, 1/R to the 4th power radial dependence of plasma concentration, and large fluctuations in plasma density at the plasmapause and plasma detachment. In the nightside region the predicted characteristics are: rapid response to magnetic activity changes, formative region for plasmasphere ripples, and the formative region for the dayside plasmapause location. The model also predicts the dayside plasmasphere characteristics, such as the slow response to magnetic activity changes, and filling of the plasmasphere from the ionosphere, including the formation of a 'double plasmapause.'

Sharp, G. W.↗
NASA NTRS1978

Fluctuation spectra in the NASA Lewis bumpy-torus plasma

The electrostatic potential fluctuation spectrum in the NASA Lewis bumpy-torus plasma was studied with capacitive probes in the low pressure (high impedance) mode and in the high pressure (low impedance) mode. Under different operating conditions, the plasma exhibited electrostatic potential fluctuations (1) at a set of discrete frequencies, (2) at a continuum of frequencies, and (3) as incoherent high-frequency turbulence. The frequencies and azimuthal wave numbers were determined from digitally implemented autopower and cross-power spectra. The azimuthal dispersion characteristics of the unstable waves were examined by varying the electrode voltage, the polarity of the voltage, and the neutral background density at a constant magnetic field strength.

Singh, C. M.↗
NASA NTRS2017

Critical Fluctuations in Beam-Plasma Systems and Solar Type III Radio Bursts

It is shown that the Langmuir waves are excited similar to critical fluctuations during phase transitions when the negative absorption due to electron beam traveling radially outward in the solar atmosphere is balanced by the positive absorption due to collisions in the corona and due to scattering on electron density inhomogeneities in the interplanetary medium. The effective temperature of the Langmuir fluctuations range from 1011 to 1013 K, explaining the majority of the type III bursts. The Rayleigh scattering and direct coupling due to density gradient as well as due to density inhomogeneities are discussed in the context of fundamental radiation and the combination scattering for second harmonic. The number density of electrons in type III beams is estimated and compared with observations. It is also shown that the stabilization of type III beams is achieved automatically since the instability does not develop in the case of critical fluctuations.

Fluctuations↗
NASA NTRS2017

Critical Fluctuations in Beam-Plasma Systems and Solar Type III Radio Bursts

The type III radio bursts are the most intense radio emissions from the sun. In Figure 1, we present a typical type III burst observed by the STEREO spacecraft. Ginzburg and Zheleznyakov (1958) were the first to suggest that Langmuir waves excited by the solar flare accelerated electrons are the source of these bursts. The in situ detection of electron beams and Langmuir waves in association with type III bursts together with tracking of type III burst sources in the interplanetary medium confirmed this plasma hypothesis. However, the dynamics of type III electron beams and their interaction with the ambient plasma through the excited Langmuir waves is an unsolved problem. In order that the electron beams don't lose their energy by resonantly interacting with Langmuir waves, various nonlinear processes, such as the induced scattering of Langmuir waves off the background ions in weak turbulence regime and oscillating two-stream instability in strong turbulence regime were invoked. However, the observed electron density fluctuations do not allow Langmuir waves to grow to very high intensities as pointed out by several authors. As far as the conversion of Langmuir wave energy into electromagnetic energy is concerned, there is no consensus either for the fundamental or for the second harmonic emission. In this paper, we address these issues in terms of critical fluctuations near the boundary of the beam-plasma instability which is analogous to the anomalously growing fluctuations near the phase transition points called critical points. We show that this phenomenon can account for the survival of electron beams over large distances as well as the intensities of Langmuir waves excited during this phase transition regime are sufficient to explain the peak intensities of the fundamental as well as harmonic emissions

Fluctuations↗
NASA NTRS1978

Quiet-time plasma irregularities at 1400 km in the cleft region

The quiet north polar cleft at 1400 km was studied by Isis-2 instruments, and data from the retarding potential analyzer and the cylindrical electrostatic probe show that thermal plasma density fluctuations are distributed in a region between 75 deg and 82 deg invariant latitude and approximately dawn to dusk. Cleft shape and shape variations are described. Thermal ions and thermal electrons usually fluctuated together, but suprathermal electrons fluctuated independently. Data on thermal plasma patterns correlates fairly well with observations of soft particles and auroral optical emissions and not as well with measurements of high-energy particles. The data suggest that the energy source for the thermal irregularities is associated with soft particles and that precipitating high-energy particles do not drive the thermal plasma at these altitudes.

Kayser, S. E.↗
NASA NTRS1987

Density fluctuations induced by nonlinear Alfven waves

The generation of plasma density fluctuations by nonlinear Alfven waves is considered. Of particular interest is the modification to the 'static approximation', describing the relationship between wave intensity and plasma density, when the Alfven wave packet is rapidly steepening. The study concentrates on a forced wave equation for the plasma density, which yields the static approximation in a select approximnation. An analytic solution to this equation is obtained for a reasonable model of a steeping wave packet. The principal results are: (1) the static approximation should give a surprisingly robust description of the coupling between Alfven wave intensity and plasma density; (2) departures of the plasma density perturbation from the static approximation result are determined by a parameter that is a simple function of the wave amplitude and plasma beta; (3) for rapidly evolving wave packets, the position of maximum density change is displaced from that of maximum wave intensity, and (4) the magnitude of the density fluctuation will not be much different than the static approximation estimate.

Spangler, Steven R.↗
NASA NTRS1975

Direct observations of higher frequency density fluctuations in the interplanetary plasma

Direct observations from the plasma spectrometers on Pioneer 6 at 1 AU in December, 1965, have been used to obtain the power associated with fluctuations in the number density of solar-wind protons in the frequency range from .001 to .01 Hz. A power-law spectrum is obtained in this frequency range. The extension of the power-law density spectrum based on direct observations to these higher frequencies is consistent with previous extrapolations of both spacecraft and interplanetary scintillation observations and with the dominance of large-scale turbulence in the solar wind. This result is also consistent with direct observations of the solar-wind proton speed and the interplanetary magnetic field.

Intriligator, D. S.↗
NASA NTRS1978

Energetic plasma ions within the earth's magnetosheath

Observations in the dawn magnetosheath with the LASL/MPI fast plasma experiment on ISEE 1 and 2 reveal the existence of two distinct states of plasma flow within the sheath: a quiet state and a disturbed state. Energetic ions in the range of 3-40 keV are the distinguishing feature of the disturbed state. Long period (about 1 min) fluctuations in plasma density, temperature, flow speed, and flow direction occur simultaneously with the appearance of these high-energy particles. The most likely explanation of these observations is that the energetic ions are produced in the vicinity of the bow shock under certain (unspecified) interplanetary conditions, and that the fluctuations in plasma flow are produced locally by the interaction of the energetic ions with the ambient plasma.

Asbridge, J. R.↗
NASA NTRS1997

Ion isotropy and fluctuations in the solar wind

The effects of measured fluctuations, with only general considerations as to their source, are considered. Data from interplanetary scintillations and fluctuations in plasma density provided data on electric fields, while fluctuations in magnetic fields are measured directly. Data from the unified radio and plasma experiment (URAP) on Ulysses is used to fill in higher frequency ranges, to assess the variations in the fluctuations with time and space, and to help to identify wave modes. It is shown that electric field fluctuations are of the right order of magnitude to maintain ion isotropy.

Kellogg, Paul J.↗