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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Forced magnetic reconnection in a plasma sheet with localized resistivity profile excited by lower hybrid drift type instability

A forced magnetic reconnection process with a temporal evolution of resistivity is studied for a plasma sheet with a nonuniform resistivity profile based on the nonlocal mode structure of the lower hybrid drift type instability. The growth rate of the mode found is almost independent of the resistivity at the neutral sheet, but depends on the resistivity of the region of maximum density gradient away from the neutral sheet. This is studied by using both a nonlinear numerical MHD simulation and a linear theory. The mode may be relevant to the prevalent theoretical concept of MHD reconnection and the localized anomalous resistivity profile based on the lower hybrid drift instability.

Hoshino, M.↗
NASA NTRS1992

Analysis of surface wave propagation in a grounded dielectric slab covered by a resistive sheet

Both parallel and perpendicular polarized surface waves are known to propagate on lossless and lossy grounded dielectric slabs. Surface wave propagation on a grounded dielectric slab covered with a resistive sheet is considered. Both parallel and perpendicular polarizations are examined. Transcendental equations are derived for each polarization and are solved using iterative techniques. Attenuation and phase velocity are shown for representative geometries. The results are applicable to both a grounded slab with a resistive sheet and an ungrounded slab covered on each side with a resistive sheet.

Shively, David G.↗
NASA NTRS1992

The mechanical behavior of cross-rolled beryllium sheet

In response to the failure of a conical section of the Insat C satellite during certification testing, the use of beryllium for payload structures, particularly in sheet product form, is being reevaluated. A test program was initiated to study the tensile, shear, and out-of-plane failure modes of beryllium cross-rolled sheet and to apply data to the development of an appropriate failure criterion. Tensile test results indicated that sanding the surface of beryllium sheet has no significant effect on yield strength but can produce a profound reduction in ultimate strength and results obtained by finite element analysis. Critical examination of these test results may contribute to the modification of a JSC policy for the use of beryllium in orbiter and payload structures.

Henkener, J. A.↗
NASA NTRS1992

Nonadiabatic heating of the central plasma sheet at substorm onset

Heating events in the plasma sheet boundary layer and central plasma sheet are found to occur at the onset of expansive phase activity. The main effect is a dramatic increase in plasma temperature, coincident with a partial dipolarization of the magnetic field. Fluxes of energetic particles increase without dispersion during these events which occur at all radial distances up to 23 RE, the apogee of the ISEE spacecraft. A major difference between these heating events and those observed at geosynchronous distances lies in the heating mechanism which is nonadiabatic beyond 10 RE but may be adiabatic closer to earth. The energy required to account for the increase in plasma thermal energy is comparable with that required for Joule heating of the ionosphere. The plasma sheet must be considered as a major sink in the energy balance of a substorm. Lobe magnetic pressures during these events are estimated. Change in lobe pressure are generally not correlated with onsets or intensifications of expansive phase activity.

Huang, C. Y.↗
NASA NTRS1992

A generalized hinged-magnetodisc model of Jupiter's nightside current sheet

A nonaxial hinged magnetodisk model of Jupiter's nightside current sheet is presented. The model organizes the current sheet crossings equally successfully for all three of the spacecraft that have visited the nightside of Jupiter. The model assumes that the hinging is caused by the action of the solar wind forcing on the magnetotail of Jupiter. It is found necessary to include both the hinging of the current sheet and the propagation delay to obtain good fits to the observations.

Khurana, Krishan K.↗
NASA NTRS1992

Processing and microstructure of Nb-1 percent Zr-0.1 percent C alloy sheet

A systematic study was carried out to evaluate the effects of processing on the microstructure of Nb-1 wt. pct. Zr-0.1 wt. pct. C alloy sheet. The samples were fabricated by cold rolling different sheet bars that were single-, double- or triple-extruded at 1900 K. Heat treatment consisted on one- or two-step annealing of different samples at temperatures ranging from 1350 to 1850 K. The assessment of the effects of processing on microstructure involved characterization of the precipitates including the type, crystal structure, chemistry and distribution within the material as well as an examination of the grain structure. A combination of various analytical and metallographic techniques were used on both the sheet samples and the residue extracted from them. The results show that the relatively coarse orthorhombic Nb2C carbides in the as-rolled samples transformed to rather fine cubic monocarbides of Nb and Zr with varying Zr/Nb ratios upon subsequent heat treatment. The relative amount of the cubic carbides and the Zr/Nb ratio increased with increasing number of extrusions prior to cold rolling. Furthermore, the size and the aspect ratio of the grains appear to be strong functions of the processing history of the material. These and other results obtained will be presented with the emphasis on a possible relationship between processing and microstructure.

Uz, Mehmet↗
NASA NTRS1993

Reconstruction of late Wisconsinan Ice Sheet and sea-level implications

The Ross Sea exhibits north-south oriented troughs associated with modern ice streams and outlet glaciers. Seismic reflection profiles across the troughs show evidence that they were glacially eroded. Seismic records show morphologic features interpreted as till tongues, morainal banks, and possibly glacial deltas formed near the grounding line of the former marine ice sheet. Piston cores from the continental shelf penetrated diamictons whose origin and age is problematic. Detailed petrographic analyses of the minerals and rocks comprising these diamictons were conducted to determine subglacial versus glacial marine origin, and to reconstruct the glacial setting of the Ross Sea during the most recent glacial maximum. The most detailed work, conducted in the western Ross Sea, shows that diamictons do occur in distinct petrologic provinces. This is consistent with deposition from the basal debris zone of either an ice sheet or an ice shelf. Overcompaction, in conjunction with the widespread nature of these deposits, favors deposition from marine ice sheets; ice shelves are believed to deposit their basal debris close to the grounding lines. Other results from the investigation are briefly discussed.

Anderson, John B.↗
NASA NTRS1993

Lithology and chronology of ice-sheet fluctuations (magnetic susceptibility of cores from the western Ross Sea)

The goals of the marine geology part of WAIS include reconstructing the chronology and areal extent of ice-sheet fluctuations and understanding the climatic and oceanographic influences on ice-sheet history. As an initial step toward attaining these goals, down-core volume magnetic susceptibility (MS) logs of piston cores from three N-S transects in the western Ross Sea are compared. The core transects are within separate petrographic provinces based on analyses of till composition. The provinces are thought to reflect the previous locations of ice streams on the shelf during the last glaciation. Magnetic susceptibility is a function of magnetic mineral composition, sediment texture, and sediment density. It is applied in the western Ross Sea for two purposes: (1) to determine whether MS data differentiates the three transects (i.e., flow lines), and thus can be used to make paleodrainage reconstructions of the late Wisconsinan ice sheet; and (2) to determine whether the MS data can aid in distinguishing basal till diamictons from diamictons of glacial-marine origin and thus, aid paleoenvironmental interpretations. A comparison of the combined data of cores in each transect is presented.

Jennings, Anne E.↗
NASA NTRS1992

Formation and stability of the self-consistent one-dimensional tail current sheet

The paper investigates the formation, the structure, and the stability of self-consistent one-dimensional current sheets in which the ions carry most of the current and momentum (the occurrence of which was suggested by observations of Mitchell et al., 1990; and Sergeev et al., 1990). Results of the analysis showed that, for the case of a cold current sheet, the characteristic thickness lamba equals to about (Bz/B0) exp 4/3 c/omega(p0), where Bz is the normal field component, B0 is the asymptotic magnitude of the reversing field, and c/omega(p0)is the collisionless ion skin depth based on lobe density. A two-dimensional self-consistent dynamical simulation model is developed, which demonstrates that these idealized current sheets are unstable to kink perturbations driven by the anisotropic pressure distribution produced by the chaotic nature of the particle orbits in a field-reversal region.

Pritchett, P. L.↗
NASA NTRS1992

Effects of ion demagnetization in the plasma sheet

The processes in the plasma sheet in the earth's magnetotail that can generate the field-aligned currents associated with auroral arcs are investigated using a two-and-a-half-dimensional nonradiative electromagnetic particle code in the Darwin approximation. All three components of the magnetic field, as well as the effect of electrostatic interaction, are calculated. The results include the response of the plasma sheet to the application of a convection electric field, the heating of plasma sheet ions, and the generation of y-directed magnetic fields and currents in the plane of simulation. The relation of these results to other morphological features of auroral/magnetospheric substorms is discussed.

Swift, Daniel W.↗
NASA NTRS1993

Prediction of the heliospheric current sheet tilt - 1992-1996

Heliospheric current sheet tilt evolves systematically over the solar cycle. Here we show that this evolution is different than the sunspot cycle and that tilt for the period 1992-1996 can be predicted using persistence. That is, the tilt over the coming cycle will be the same as for the past cycle. The Ulysses spacecraft has passed Jupiter and is moving out of the plane of the ecliptic, so we use the prediction of the changing heliospheric current sheet tilt to predict that Ulysses will pass beyond the envelope, or maximum latitude, of the heliospheric current sheet in November 1993.

Suess, S. T.↗
NASA NTRS1993

On the 'delta-equations' for vortex sheet evolution

We use a set of equations, sometimes referred to as the 'delta-equations', to approximate the two-dimensional inviscid motion of an initially circular vortex sheet released from rest in a cross-flow. We present numerical solutions of these equations for the case with delta-square = 0 (for which the equations are exact) and for delta-square greater than 0. For small values of the smoothing parameter delta, a spectral filter must be used to eliminate spurious instabilities due to round-off error. Two singularities appear simultaneously in the vortex sheet when delta-square = 0 at a critical time t(c). After t(c), the solutions do not converge as the computational mesh is refined. With delta-square greater than 0, converged solutions were found for all values of delta-square when t is less than t(c), and for all but the two smallest values of delta-square used when t is greater than t(c). Our results show that, when delta-square is greater than 0, the vortex sheet deforms into two doubly branched spirals some time after t(c). The limiting solution as delta approaching 0 clearly exists and equals the delta = 0 solution when t is less than t(c).

Rottman, James W.↗
NASA NTRS1992

A combined surface/volume scattering retracking algorithm for ice sheet satellite altimetry

An algorithm that is based upon a combined surface-volume scattering model is developed. It can be used to retrack individual altimeter waveforms over ice sheets. An iterative least-squares procedure is used to fit the combined model to the return waveforms. The retracking algorithm comprises two distinct sections. The first generates initial model parameter estimates from a filtered altimeter waveform. The second uses the initial estimates, the theoretical model, and the waveform data to generate corrected parameter estimates. This retracking algorithm can be used to assess the accuracy of elevations produced from current retracking algorithms when subsurface volume scattering is present. This is extremely important so that repeated altimeter elevation measurements can be used to accurately detect changes in the mass balance of the ice sheets. By analyzing the distribution of the model parameters over large portions of the ice sheet, regional and seasonal variations in the near-surface properties of the snowpack can be quantified.

Davis, Curt H.↗
NASA NTRS1993

Surface waves on a grounded dielectric slab covered by a resistive sheet

This paper examines surface wave propagation in a grounded dielectric slab covered with a resistive sheet. Transcendental equations are derived for each polarization and are solved using iterative techniques. Attention and phase velocity are shown for a representative geometry. The results are applicable to both a grounded slab covered with a resistive sheet and an ungrounded slab covered on each side with a resistive sheet.

Shively, David↗
NASA NTRS1993

Characteristics of ion flow in the quiet state of the inner plasma sheet

We use AMPTE/IRM and ISEE 2 data to study the properties of the high beta plasma sheet, the inner plasma sheet (IPS). Bursty bulk flows (BBFs) are excised from the two databases, and the average flow pattern in the non-BBF (quiet) IPS is constructed. At local midnight this ensemble-average flow is predominantly duskward; closer to the flanks it is mostly earthward. The flow pattern agrees qualitatively with calculations based on the Tsyganenko (1987) model (T87), where the earthward flow is due to the ensemble-average cross tail electric field and the duskward flow is the diamagnetic drift due to an inward pressure gradient. The IPS is on the average in pressure equilibrium with the lobes. Because of its large variance the average flow does not represent the instantaneous flow field. Case studies also show that the non-BBF flow is highly irregular and inherently unsteady, a reason why earthward convection can avoid a pressure balance inconsistency with the lobes. The ensemble distribution of velocities is a fundamental observable of the quiet plasma sheet flow field.

Angelopoulos, V.↗
NASA NTRS1943

Bearing Tests of Magnesium-alloy Sheet

Bearing tests of AM-3S, AM-52S, and AM-C57S magnesium-alloy sheet in various thicknesses and tempers were made. Bearing yield and ultimate strengths were determined and compared for various edge distances and for various ratios of loading-pin diameter to sheet thickness. Tensile strengths were determined and ratios of average bearing yield and ultimate strength to tensile strength are given. The results of the tests indicated that ultimate bearing strengths increased with edge distances up to 1.5 to 2 times the diameter of the loading pin; that ultimate bearing strengths are a function of the ratio of pin diameter to sheet thickness; and that these properties are effected only slightly by increases in edge distance greater than 1.5 diameters.

Sharp, W H↗
NASA NTRS1943

Large-deflection theory of curved sheet

Equations are given for the elastic behavior of initially curved sheets in which the deflections are not small in comparison with the thickness, but at the same time small enough to justify the use of simplified formulas for curvature. These equations are solved for the case of a sheet with circular cylindrical shape simply supported along two edges parallel to the axis of the generating cylinder. Numerical results are given for three values of the curvature and for three ratios of buckle length to buckle width. The computations are carried to buckle deflections of about twice the sheet thickness. It was concluded that initial curvature may cause an appreciable increase in the buckling load but that, for edge strains which are several times the buckling strain, the initial curvature causes a negligibly small change in the effective width.

Levy, Samuel↗
NASA NTRS1947

Effect of Brake Forming in Various Tempers on the Strength of Alclad 75S-T Aluminum-alloy Sheet

Results are presented of tests to determine the effect of brake forming in various tempers on the strength of Alclad 75S-T aluminum alloy sheet in the direction parallel to the brake. The tensile and compressive strengths of Alclad 75S-T sheet, formed in the O and W tempers, were either increased or little affected compared with those of similarly treated unformed material. When Alclad 75S-T sheet 'as received' was formed, however, the tensile yield stress was reduced about 7 percent for the with-grain direction and 1 percent for the cross-grain direction, whereas the tensile ultimate and compressive yield stresses were increased somewhat. The elongation was always slightly reduced as a result of forming.

Woods, Walter↗