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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
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↗
NASA NTRS1945

Effect of Normal Pressure on the Critical Compressive and Shear Stress of Curved Sheet

Results are presented of tests of two sets of 20 curved-sheet specimens to determine the effect of normal pressure on the critical compressive and shear stress of curved sheets. It was found that normal pressure raised the critical compressive and shear stress of curved sheets except when outward bulging occurred in compression, in which case the critical stress was lowered by normal pressure. (author)

Rafel, Norman↗
NASA NTRS1944

Fatigue Strength and Related Characteristics of Joints in 24s-t Alclad Sheet

Report includes tension fatigue test results on the following types of samples of 0.040-inch alclad 24s-t: (1) monoblock sheet samples as received and after a post-aging heat treatment, (2) "sheet efficiency" samples (two equally stressed sheets joined by a single transverse row of spot welds) both as received and after post-aging, (3) spot-welded lap-joint samples as received and after post-aging, and (4) roll-welded lap-joint samples. (author)

Russell, H W↗
NASA NTRS1938

The Effective Width of Curved Sheet After Buckling

This report describes experiments made for the purpose of ascertaining the effective width of circularly curved sheet under pure flexural stress. A relation for the effective width of curved sheets is established. Experiments were made with circular cylinders compressed in longitudinal direction. The sheets were rigidly built in at the sides parallel to the axis of the cylinder.

Wenzek, W A↗
NASA NTRS1936

Experimental Studies of the Effective Width of Buckled Sheets

The object of the present experiments is a more exact determination of the effective width for the case of pure compression and of the sheet clamped at the angle section. From the experimental data of the effective width the calculation of the buckling load of an angle joined to a thin sheet is then deduced. The test material consisted of spring-hardened brass sheet.

Lahde, R↗
NASA NTRS1993

Spatial patterns in backscatter strength across the Greenland Ice Sheet

The relationship between the physical properties of the Greenland ice sheet and Synthetic Aperture Radar (SAR) data collected from aircraft and from ERS-1 is addressed. Limited aircraft data are combined with a description of the glacier surface to predict qualitatively the spatial and seasonal variation in backscatter strength across the ice sheet. In particular the model predicts relatively low backscatter near the ice edge where scattering is dominated by rough surface effects. Backscatter increases through the lake zone as volume scattering becomes important. Strongest backscatter is found in the percolation facies where volume scatter from snow grains and volume scatter from large, buried ice bodies becomes important. Backscatter weakens in the interior ice sheet where fine grained snow is the only mechanism producing backscatter.

Jezek, K. C.↗
NASA NTRS1993

Sheet flow fields on Venus

A survey of flow fields with areas greater than 50,000 sq km revealed fields with sheet-like morphology. These sheet flow fields are distinguished by their relatively uniform backscatter, lack of internal flow structure such as well defined lava streams or channels, absence of flow lobes, and irregular boundaries. Internal flow boundaries are essentially absent in these fields, and as such they cannot usually be divided into separate stratigraphic units or eruptive episodes. This is unlike other flow fields with more digitate morphologies, which are made up of discrete flow lobes, and can usually be divided into several episodes of flow emplacement. Five sheet flow fields were studied so far, and other candidates were found. A flow field associated with Lauma Dorsa is taken as an example and examined in detail.

Lancaster, M. G.↗
NASA NTRS1994

Experimental investigation of possible geomagnetic feedback from energetic (0.1 to 16 keV) terrestrial O(+) ions in the magnetotail current sheet

Data from energetic ion mass spectrometers on the ISEE 1 and AMPTE/CCE spacecraft are combined with geomagnetic and solar indices to investigate, in a statistical fashion, whether energized O(+) ions of terrestrial origin constitute a source of feedback which triggers or amplifies geomagnetic activity as has been suggested in the literature, by contributing a destabilizing mass increase in the magnetotail current sheet. The ISEE 1 data (0.1-16 keV/e) provide in situ observations of the O(+) concentration in the central plasma sheet, inside of 23 R(sub E), during the rising and maximum phases of solar cycle 21, as well as inner magnetosphere data from same period. The CCE data (0.1-17 keV/e) taken during the subsequent solar minimum all within 9 R(sub E). provide a reference for long-term variations in the magnetosphere O(+) content. Statistical correlations between the ion data and the indices, and between different indices. all point in the same direction: there is probably no feedback specific to the O(+) ions, in spite of the fact that they often contribute most of the ion mass density in the tail current sheet.

Lennartsson, O. W.↗
NASA NTRS1993

A radiating one-dimensional current sheet configuration

The structure of the x-independent (one-dimensional) forced current sheet including a self consistent By component is investigated for the case of small normal field component, Bz/B0 much less than 1. A hybrid (kinetic ions, massless fluid electrons) simulation model is used to demonstrate that such a current sheet has a time-dependent structure which radiates incompressible Alfven waves with amplitude of the order of the asymptotic (lobe) field strength B0. The central density enhancement acts as the source of a propagating wavetrain in which Bx rotates into By and back again. One of the characteristic signatures of the radiating current sheet is the presence of a reversal in Bx (or By) without a corresponding increase in density.

Pritchett, P. L.↗
NASA NTRS1993

Electron generation of electrostatic waves in the plasma sheet boundary layer

Broadband electrostatic noise (BEN) has been shown to occur in conjunction with ion beams; extensive investigations of possible ion beam-related instabilities that could generate the observed wave spectra have been conducted. It has also been demonstrated that unstable electron distribution functions are sometimes measured in the plasma sheet boundary layer. We present simultaneous observations of ion and electron distribution functions and electric field wave spectra measured by ISEE 1 and ISEE 2 in the Earth's magnetotail. As the spacecraft moved from the tail lobe toward the plasma sheet, the fast indication of boundary layer plasma was seen in the electron distributions, followed some minutes later by the detection of boundary layer ions. The onset of large-amplitude electrostatic waves at frequencies up to the electron plasma frequency was coincident with the onset of the boundary layer electrons, suggesting that broadband electrostatic waves may often be generated by unstable electron distributions in the plasma sheet boundary layer, particularly the higher frequency portion of the wave spectrum. The observed changes in the electron distribution functions indicate that the plasma was not heated locally by the waves.

Onsager, T. G.↗