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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Ice-sheet dynamics by satellite laser altimetry

Glaciological applications of a satellite laser altimeter system with a range precision of approximately 10 cm and a footprint diameter of 70 m are discussed. Available data on ice-sheet dynamics are reviewed, and the measurement errors associated with the laser altimeter are examined. It is shown that information collected with the system could make it possible to detect changes in average elevation (i.e., in average ice thickness) of less than 15 cm for areas of hundreds of sq km to a few thousand sq km. Other possible applications of the system are briefly considered, including the mapping of the positions of ice-sheet margins, the mapping of the junction between a grounded ice sheet and a floating ice shelf, and the estimation of iceberg calving rates.

Zwally, H. J.↗
NASA NTRS1983

Analysis and retracking of continental ice sheet radar altimeter waveforms

The Seasat-1 radar altimeter data set acquired over both the Antarctic and Greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface. The radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans. Causal factors identified in this complicated response include sloping surfaces, undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam-limited footprint, off-track reflections, and dynamic lag of the altimeter tracking circuit. Retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters. The entire set of Seasat-1 altimetry over the continental ice sheets is being retracked by fitting a multi-parameter function to each waveform. Many waveforms have double ramps indicating near-normal reflections from two distinct portions of the ice surface within the altimeter beam. Two independent range measurements differing by less than 25 m are obtained from retracking the double-ramp waveforms.

Martin, T. V.↗
NASA NTRS1983

Simulation of auroral current sheet equilibria and associated V-shaped potential structures

Results from numerical simulations of auroral current sheet equilibria and associated V-shaped potential structures are presented. It is shown that with allowance for both hot magnetospheric ion and cold ionospheric ion populations, the perpendicular potential drop, Phi(m), associated with a non-neutral auroral current sheet is critically controlled by the temperature of the 'heated' ionospheric ions. The heating is caused by the wave turbulence excited by the auroral current sheet. In the presence of heated ionospheric ions, a relatively large variation in the temperature of the hot magnetospheric ion population causes a very small variation in the potential drop Phi(m). The perpendicular potential drop acts to produce a V-shaped double layer with multiple potential steps parallel to the magnetic field when a zero potential boundary condition is imposed at the ionospheric boundary. Outside the V-shaped potential structure, ionospheric return currents develop self-consistently.

Singh, N.↗
NASA NTRS1984

An Improved Analysis of the Thermal Buckling of Silicon Sheet

Buckling problems on the production of wide and thin silicon sheet are discussed. Buckling occurs in all of the processes which are investigated for their potential in mass producing wide silicon sheet for use in making solar cells. It is a fact that the processes which produce good ribbon 2 cm in width do not yield the same quality product at 10 cm in width. Buckling develops precisely because the sheets are wide and very very thin. The buckling of very thin cantilever plates due to temperature variations in the axial direction is considered. The temperature variation in the width direction was determined. Axial temperature variations which cause very thin plates to buckle in a torsional mode are demonstrated. It is assumed that the particular variation of the stress function in the width direction and thereafter the analysis is exact.

Dillon, O. W., Jr.↗
NASA NTRS1984

Comparison of an analytical approximation for particle motion in a current sheet with precise numerical calculations

Approximate analytic solutions exist for particle motion in a one-dimensional current sheet with a constant normal magnetic field component. These solutions are tested against precise numerical calculations, and a range of validity of the analytic solutions is inferred. For example, in the geomagnetic tail neutral sheet, for a dawn-dusk electric field of 0.1-1 mV/m, lobe field of 10-40 nT, and sheet thickness of 1000 km, the analytic solutions serve as a good predictor of particle motion when the normal magnetic field component is less than 3 or 4 nT. By using the analytic solutions, initial distribution functions are mapped into final (accelerated) distributions, and the analytic mappings are compared with numerical mappings.

Speiser, T. W.↗
NASA NTRS1984

Energetic (greater than 100 keV) O(+) ions in the plasma sheet

The first measurements of very energetic (112 - 157 keV) O(+) ions in the earth's magnetosphere are presented. The observations were made with the UMd/MPE ULECA sensor on ISEE-1 on 5 March 1981 at geocentric distances approximately 20 R(E) in the earth's magnetotail. During this time period an Energetic Storm Particle event was observed by the nearly identical sensor on the ISEE-3 spacecraft, located approximately 250 R(E) upstream of the earth's magnetosphere. The ISEE-1 sensor observed a similar temporal profile except for several sharp intensity enhancements, corresponding to substorm recoveries during which the plasma sheet engulfed the spacecraft. During these plasma sheet encounters we observe O(+)/H(+) abundance ratios, at approximately 130 kev, as large as 0.35. In between plasma sheet encounters the O(+)/H(+) ratio at this energy is consistent with zero.

Ipavich, F. M.↗
NASA NTRS1985

Differential measurement of cosmic-ray gradient with respect to interplanetary current sheet

Simultaneous magnetic field and charged particle measurements from the Voyager pacecraft at heliographic latitude separations from 10 deg to 21 deg are used to determine the latitude gradient of the galactic cosmic ray flux with respect to the interplanetary current sheet. By comparing the ratio of cosmic ray flux at Voyager 1 to that at Voyager 2 during periods when both spacecraft are first north and then south of the interplanetary current sheet, we find an estimate of the latitudinal gradient with respect to the current sheet of approximately -0.15 + or- 0.05%/deg under restricted interplanetary conditions.

Christon, S. P.↗
NASA NTRS1985

Cosmic ray intensity and the tilt of the neutral sheet

Recent publications have related long-term variations in cosmic ray intensity at the Earth with long term variations in the tilt of the neutral sheet in the inner heliosphere. The tilt of the neutral sheet from 1971 to 1974 is compared with the cosmic ray intensity at Earth, recorded by the Mt. Washington neutron monitor. The remarkable large decreases in cosmic ray intensity which occurred in 1973 and 1974 correlate well with excursions in the tilt of the neutral sheet which occurred earlier during these same two years.

Saito, T.↗
NASA NTRS1985

Differential measurement of cosmic-ray gradient with respect to interplanetary current sheet

Simultaneous magnetic field and charged particle measurements from the Voyager spacecraft at heliographic latitude separations from 10 deg. to 21 deg. are used to determine the latitude gradient of the galactic cosmic ray flux with respect to the interplanetary current sheet. By comparing the ratio of cosmic ray flux at Voyager 1 to that a Voyager 2 during periods when both spacecraft are first nort and then south of the interplanetary current sheet, we find an estimate of the latitudinal gradient with respect to the current sheet of approximately -0.15 + or 0.05% deg under restricted interplanetary conditions.

Christon, S. P.↗
NASA NTRS1985

Spatial variation of cosmic rays near the heliospheric current sheet

A quantitative comparison between theoretical predictions and observations of the intensity of galactic cosmic rays near the interplanetary current sheet is reported. Comparison of model calculations is made with a statistical analysis of observations of galactic cosmic rays at Earth and the simultaneous position of the current sheet. An ensemble of different current sheet inclinations is used, in order to make the analysis of the computations approximate the method used to analyses the data.

Jokipii, J. R.↗
NASA NTRS1985

Variation of cosmic rays and solar wind properties with respect to the heliospheric current sheet. I - Five-GeV protons and solar wind speed

One of the principal uncertainties in understanding the propagation of cosmic rays in the heliosphere is related to the role of latitudinal transport by gradient and curvature drifts. The steady state distribution of cosmic rays with respect to the heliospheric current sheet or in latitude may possibly provide some information regarding the existing problems. Newkirk and Lockwood (1981) have considered the current sheet with respect to its role as a natural surface of symmetry for the propagation of cosmic rays in the heliosphere. The present investigation represents an extension of this study. Central objectives are related to a determination of the gradient of 5-GeV cosmic rays with respect to the heliospheric current sheet at 1 AU, the possible effect of the solar cycle upon such gradients, and an evaluation of the ability of several theoretical models to explain the gradients.

Newkirk, G., Jr.↗
NASA NTRS1984

Magnetotail energy storage and the variability of the magnetotail current sheet

The antiparallel field configuration in the earth magnetotail is a prime location to search for the important astrophysical process known as magnetic reconnection. The magnetic evidence that reconnection occurs in the earth's magnetotail is that the energy of the tail invariably decreases at the time of global substorm onsets, and that an increased number of closed field lines are seen in the near-earth magnetotail after substorms. This increased northward flux is often convecting earthward from the expected site of the X line after the substorm onset, while southward flux is often convecting tailward. Prior to a substorm, energy accumulates in the tail as the field assumes a more taillike configuration with a smaller field component across the equatorial plane. The high-beta plasma sheet becomes thin and has been observed to be at least as thin as a few thousand km near substorm onset times. The small Bn component and thin plasma sheet are the conditions which favor the onset of the tearing-mode instability, which is thought to lead to neutral-line formation and the reconnection associated with substorm onset. Intervals of weak and highly variable fields in the plasma sheet provide evidence for the tearing-mode instability.

Fairfield, D. H.↗
NASA NTRS1985

Laboratory experiments on reconnection in current sheets

Insights obtained from laboratory experiments concerning the reconnection of current sheets are discussed. Consideration is given to: the nonlocal characteristics of reconnection; the build-up of pinch current sheets; and impulsive flux transfer events. The application of the experimental results to present models of current sheet reconnection during solar flares is also discussed.

Bratenahl, A.↗
NASA NTRS1985

Current sheets in solar flares

Numerical simulations of current sheets in solar flares are described, including new features such as the presence of a shock in Petschek's mechanism and impulsive burst-like reconnection due to secondary tearing and coalescence. The general properties of magnetic reconnection are discussed in connection with the basic requirements of numerical current sheet models. Emphasis is given to the need for realistic criteria for energy balance, the Lundquist number, and line tying in calculations of tearing and reconnection modes. The need for analytical models of current sheet processes to compare with the numerical simulations is also stressed.

Priest, E. R.↗
NASA NTRS1986

Variation of cosmic rays and solar wind properties with respect to the heliospheric current sheet. II - Rigidity dependence of the latitudinal gradient of cosmic rays at 1 AU

The role which empirical determinations of the latitudinal variation of cosmic rays with respect to the current sheet may have in illuminating the importance of the cross-field drift of particles in the large-scale heliospheric magnetic field is discussed. Using K coronameter observations and measured solar wind speeds, the latitudinal gradients have been determined with respect to the current sheet for cosmic rays in four rigidity ranges. Gradients vary between approximately -2 and -50 pct/AU. The rigidity dependence of the decrease of cosmic ray flux with distance from the current sheet lies between the -0.72 to -0.86 power of the rigidity, with the exact dependence being determined by the definition used for the median rigidity of each monitor.

Newkirk, G., Jr.↗
NASA NTRS1986

The effect of the heliospheric current sheet on cosmic ray intensities at solar maximum - Two alternative hypotheses

The capabilities of two alternative models for accounting for the reversal of the solar magnetic field polarity at the point of solar maximum and for the associated modulation of cosmic ray intensities at 1 AU are assessed. One model posits a continuous increase in the inclination of the heliospheric current sheet to a point of verticality at maximum, when it overturns and the reversal occurs. The alternative view is that the sun sheds the magnetic field of the previous cycle and generates a new field of opposite polarity. Some data do exist for a tilted current sheet which increases its tilt with proximity to the solar maximum. However, coronal data also support the presence of isolated regions of anomalous polarity which spread over the surface of the sun as maximum approaches, a condition commensurate with flux shedding. Both models predict heliospheric current sheet configurations which would produce some cosmic ray modulations observed over the course of the solar cycle.

Thomas, B. T.↗
NASA NTRS1986

Latitudinal extent of the heliospheric current sheet and modulation of galactic cosmic rays

Possible correlations were investigated between the modulation of galactic cosmic rays and changes in the maximum heliographic latitude of the heliospheric current sheet during solar maximum. Solar photospheric magnetic field data from 1976-82 were extrapolated by means of potential theory to obtain the maximum latitudinal extent of the current sheet in the northern and southern hemispheres. The latitudinal differences were compared with terrestrial neutron flux data and Pioneer 10 data on cosmic rays with energies below 200 MeV at heliocentric distances of 10-30 AU. Account was taken for the time delay for the solar wind to propagate out to the spacecraft location. The current sheet maxima were nearly symmetrical about the solar equator except during sunspot maxima, when the solar poles were reversing. Significant changes in the latitudinal symmetry were highly correlated with the neutron data and cosmic fluctuations at Pioneer 10. The sensitivity of cosmic rays to the polarity reversals is in accordance with predictions of the gradient drift model.

Smith, E. J.↗
NASA NTRS1986

A statistical study of plasma sheet dynamics using ISEE 1 and 2 energetic particle flux data

Plasma sheet dynamics during substorms are studied by analyzing 461 cases of transient dropout events of the 1.5 and 6-keV particle fluxes detected by ISEE 1 and 2 satellites. The instruments for detecting low- and high-energy particles are described. The spatial distribution of flux dropout events, and the events' relationship to magnetospheric activity level are examined. Substorm events without observed flux dropout events are investigated. The data reveal that the flux dropout distribution is isotropic, between 12-23 earth radii, and is present in the entire nightside plasma sheet; and the substorms without flux dropout are more frequent near earth and magnetospheric flanks. It is observed that tailward of 12 earth radii the flux dropout events and substorms without flux dropout are similar. The Chao et al. (1977) MHD rarefaction wave propagation model and the Hones (1973, 1980) near-tail, X-type magnetic neutral line formation model are discussed and compared to the experimental data. It is noted that neither model explains the plasma sheet dynamics observed.

Dandouras, J.↗