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At least 1,027 records · Page 57

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NASA NTRS1988

Observational test of shock drift and Fermi acceleration on a seed particle population upstream of earth's bow shock

The efficiency of proposed shock acceleration mechanisms as they operate at the bow shock in the presence of a seed energetic particle population was examined using data from simultaneous observations of energetic solar-origin protons, carried out by the IMP 7 and 8 spacecraft in the vicinity of the quasi-parallel (dawn) and quasi-perpendicular (dusk) regions of the earth's bow shock, respectively. The results of observations (which include acceleration effects in the intensities of the energetic protons with energies as high as 4 MeV observed at the vicinity of the dusk bow shock, but no evidence for any particle acceleration at the energy equal to or above 50 keV at the dawn side of the bow shock) indicate that the acceleration of a seed particle population occurs only at the quasi-perpendicular bow shock through shock drift acceleration and that the major source of observed upstream ion populations is the leakage of magnetospheric ions of energies not less than 50 keV, rather than in situ acceleration.

Anagnostopoulos, G. C.↗

NASA NTRS1988

Ambipolar diffusion drifts and dynamos in turbulent gases

Ambipolar drift in turbulent fluids are considered. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, it is shown that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvenic (helical) turbulence. A simple dynamo theory is made and is compared with standard theories in which dissipation is caused by turbulent diffusion. The redistribution of magnetic flux in interstellar clouds is also discussed.

Zweibel, Ellen G.↗

NASA NTRS1988

Gas scintillation drift chambers with wave shifter fiber readout

Results for a prototype xenon gas scintillation drift chamber are presented. Its operation is discussed using two types of light detection schemes: one based on an Anger camera geometry and one based on an array of wave-shifting light fibers. The results are judged to demonstrate the instrument's potential.

Sadoulet, Bernard↗

NASA NTRS1989

Low-latitude zonal and vertical ion drifts seen by DE 2

Horizontal and vertical ion drift data from the DE 2 spacecraft have been used to determine average zonal and vertical plasma flow (electric field) characteristics in the +/- 26-deg dip latitude region during a time of high solar activity. The 'average data' local time profile for an apex height bin centered at 400 km indicates westward plasma flow from 0600 to 1900 solar local time ((SLT) with a maximum westward velocity of 80 m/s in the early afternoon. There is a sharp change to eastward flow at approximately 1900 hours with an early evening peak of 170 m/s. A secondary nighttime maximum exists at 0430 SLT preceeding the reversal to westward flow. This profile is in good agreement with Jicamarca, Peru, radar measurements made under similar solar maximum conditions. Haramonic analysis indicates a net superrotation which is strongest at lower apex altitudes. The diurnal term is dominant, but higher order terms through the quatradiurnal are significant.

Coley, W. R.↗

NASA NTRS1989

A new calculation of the cosmic-ray antiproton spectrum in the Galaxy and heliospheric modulation effects on this spectrum using a drift plus wavy current sheet model

The expected interstellar antiproton spectrum arising from cosmic-ray interactions in the Galaxy is recalculated, and the modulation of both antiprotons and protons is calculated using a two-dimensional modulation model incorporating gradient and curvature drifts and a wavy current sheet as well as the usual diffusion, convection, and energy-loss effects. Significant differences in the antiproton/proton ratio for different solar magnetic field polarities are predicted as well as a 'low-energy' component for antiprotons below about 1 GeV.

Webber, W. R.↗

NASA NTRS1989

Fast Fermi and gradient drift acceleration of electrons at nearly perpendicular collisionless shocks

Electrons can be reflected and effectively energized at quasi-perpendicular shocks. At a curved shock this process is most prominent where the upstream magnetic field and the shock surface are tangent. A theoretical explanation of the underlying physical mechanism in terms of fast Fermi acceleration has been proposed in the literature. The theory uses properties of the de Hoffmann-Teller frame and assumes conservation of magnetic moment in a static, simplified shock profile. Here, the discussion is extended in order to clarify certain aspects of the reflection process and to pinpoint the physical mechanisms that are operative and dominant from the viewpoint of the normal incidence frame. By reducing the analysis to the essential physical content and solving the pertinent energy equation, the equivalence of fast Fermi and gradient drift acceleration is shown.

Krauss-Varban, D.↗

NASA NTRS1990

Lower hybrid drift instability at the inner edge of the ring current

Excitation of electrostatic waves around and above the lower hybrid frequency at the inner edge of the ring current is investigated. The model consists of cold plasmaspheric ions; hot, dilute, inhomogeneous ring current ions; and cold charge neutralizing electrons. It is shown that in the presence of a sufficiently large density gradient the coupling of the lower hybrid with the drift cyclotron harmonic mode destabilizes the plasma. Unstable interactions also occur at higher cyclotron harmonics when the lower hybrid is replaced by an ion Bernstein mode. Additional power at frequencies above the lower hybrid frequency is attributed to the thermal enhancement by the hot ring current ions, which increases with their density gradient. Particle simulations are conducted, and comparison to AMPTE data is discussed.

Roth, I.↗

NASA NTRS1990

Plasma structuring by the gradient drift instability at high latitudes and comparison with velocity shear driven processes

Using results of the in situ measurements made by the DE 2 satellite, the nature of plasma structuring at high latitudes, caused by the gradient drift instability process, is described. Using noon-midnight and dawn-dusk orbits of the DE 2 satellite, it was possible to examine the simultaneous density and electric field spectra of convecting large-scale plasma density enhancements in the polar cap known as 'patches', in directions parallel and perpendicular to their antisunward convection. The results provide evidence for the existence of at least two generic classes of instabilities operating in the high-latitude ionosphere: one driven by large-scale density gradients in a homogeneous convection field with respect to the neutrals, and the other driven by the structured convection field itself in an ambient ionosphere where density fluctuations are ubiquitous.

Basu, Sunanda↗

NASA NTRS1990

Simulated forecast error and climate drift resulting from the omission of the upper stratosphere in numerical models

Using an NCAR community climate model, Version I, the forecast error growth and the climate drift resulting from the omission of the upper stratosphere are investigated. In the experiment, the control simulation is a seasonal integration of a medium horizontal general circulation model with 30 levels extending from the surface to the upper mesosphere, while the main experiment uses an identical model, except that only the bottom 15 levels (below 10 mb) are retained. It is shown that both random and systematic errors develop rapidly in the lower stratosphere with some local propagation into the troposphere in the 10-30-day time range. The random growth rate in the troposphere in the case of the altered upper boundary was found to be slightly faster than that for the initial-condition uncertainty alone. However, this is not likely to make a significant impact in operational forecast models, because the initial-condition uncertainty is very large.

Boville, Byron A.↗

NASA NTRS1990

Long-term microwave power drift of a cesium frequency standard and its effect on output frequency

It has been shown that the long-term frequency stability of a cesium (Cs) frequency standard is affected by variations in the standard's internal microwave power source. Studies were performed on a commercial Cs frequency standard for a period of 20 days, to determine the stability of its microwave power source. The results were then analyzed statistically, and the effects of microwave power drift on the standard's frequency stability were calculated.

Johnson, W. A.↗

NASA NTRS1990

Plasma diffusion at the magnetopause? The case of lower hybrid drift waves

The diffusion expected from the quasilinear theory of the lower hybrid drift instability at the Earth's magnetopause is recalculated. The resulting diffusion coefficient is in principle just marginally large enough to explain the thickness of the boundary layer under quiet conditions, based on observational upper limits for the wave intensities. Thus, one possible model for the boundary layer could involve equilibrium between the diffusion arising from lower hybrid waves and various low processes. However, some recent data and simulations seems to indicate that the magnetopause is not consistent with such a soft diffusive equilibrium model. Furthermore, investigation of the nonlinear equations for the lower hybrid waves for magnetopause parameters indicates that the quasilinear state may never arise because coalescence to large wavelengths, followed by collapse once a critical wavelengths is reached, occur on a time scale faster than the quasilinear diffusion. In this case, an inhomogeneous boundary layer is to be expected. More simulations are required over longer time periods to explore whether this nonlinear evolution really takes place at the magnetopause.

Treumann, R. A.↗

NASA NTRS1991

Plasma diffusion at the magnetopause - The case of lower hybrid drift waves

The diffusion expected from the quasi-linear theory of the lower hybrid drift instability at the earth's magnetopause is recalculated. The resulting diffusion coefficient is marginally large enough to explain the thickness of the boundary layer under quiet conditions, based on observational upper limits for the wave intensities. Thus, one possible model for the boundary layer could involve equilibrium between the diffusion arising from lower hybrid waves and various loss processes.

Treumann, R. A.↗

NASA NTRS1992

The simulated features of heliospheric cosmic-ray modulation with a time-dependent drift model. I - General effects of the changing neutral sheet over the period 1985-1990

A time-dependent drift model is used to simulate the heliospheric modulation of galactic cosmic rays, with emphasis on the effects of the wavy heliospheric neutral sheet (HNS) as a function of time during the period 1985-1990. The model predicted a clearly defined time difference between the time when minimum modulation occurred at earth and at radial distances farther away from the sun, indicating that the HNS plays an important part in establishing this observed time delay. The movements of the Voyager 1 and 2 and Pioneer 10 spacecraft were simulated in order to calculate instantaneous radial and latitudinal gradients. The time dependencies of these gradients were found to follow the observations well. The model study indicates that the HNS cannot be ignored, especially during periods of low to moderate solar activity, when the modulation of cosmic rays is described.

Potgieter, M. S.↗

NASA NTRS1992

The simulated features of heliospheric cosmic-ray modulation with a time-dependent drift model. II - On the energy dependence of the onset of new modulation in 1987

Time-dependent heliospheric cosmic-ray modulation for the period 1985-1989 is simulated by means of a time-dependent axially symmetric drift model with an emulated wavy heliospheric neutral sheet (HNS). The model is used to extend previous calculations to other energies in order to study a possible energy-dependence of the onset of new modulation at various radial distances in 1987. The model, with the outward propagating changes of the HNS as the only time-dependent parameters, is found to predict essentially no energy-dependence in the time when new modulation started in the simulated 1987. When a more practical approach in defining 'constant' modulation in 1987 is followed, the present calculations can be interpreted to indicate that the end of the recovery period in 1987 happened progressively earlier and the onset of new modulation progressively later the higher the rigidity of the cosmic rays. This period of relatively unchanged modulation is predicted to last longer with increasing radial distance.

Le Roux, J. A.↗

NASA NTRS1993

International Reference Ionosphere: Past, present, and future. I - Electron density. II - Plasma temperatures, ion composition and ion drift

The most important investigations leading to the International Reference Ionosphere 1990 (IRI-90) are overviewed, and the latest version of the model is described. The shortcomings and limitations of the IRI-90 are pointed out, together with the ways of overcoming them. The list of studies that the IRI group has yet to carry out includes the investigations of magnetic storm effects as the highest priority. This paper discusses determinations of and the available data on the electron density, plasma temperatures, ion composition, and ion drift in the ionosphere, together with future improvements needed on these parameters.

Bilitza, D.↗

NASA NTRS1992

Alfvenic drift Kelvin-Helmholtz instability in the presence of an equilibrium electric field

The Alfvenic drift Kelvin-Helmholtz instability of a high-beta plasma in the presence of equilibrium magnetic and electric fields perpendicular to each other are studied. The plasma components are assumed to have 2D sheared velocity in y and z directions. The dispersion relation is derived, and the instability criterion is determined. It is shown that the equilibrium electric field has either stabilizing or destabilizing effect depending on certain conditions discussed in the paper.

Sharma, Avadhesh C.↗

NASA NTRS1992

GaAs single-drift flat-profile IMPATT diodes for CW operation at D band

Single-drift flat-profile GaAs IMPATT diodes were designed for CW operation in the 140 GHz range. The diodes were fabricated from MBE grown material, mounted on diamond heatsinks, and tested in a radial line full height waveguide cavity. An RF output power of 15 mW with a corresponding DC to RF conversion efficiency of 1.5 percent was obtained at 135.3 GHz.

Eisele, H.↗

NASA NTRS1992

Gamma-ray astronomy using a high pressure gas scintillation drift chamber with a waveshifting fiber readout

We describe a balloon-borne hard X-ray telescope called SIGHT (Scintillation Imaging Gas-filled Hard X-ray Telescope). SIGHT is a high sensitivity, good energy resolution instrument that images in the 30 to 300 keV region. We discuss the development of a large area, 20 atmosphere, position sensitive xenon gas scintillation drift chamber which is the gamma-ray detector at the heart of the telescope package. Results of the development of the novel waveshifting fiber readout for this chamber are presented.

Wilkerson, J.↗