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At least 1,441 records · Page 80

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NASA NTRS1991

The plasma environment, charge state, and currents of Saturn's C and D rings

The charge state and associated currents of Saturn's C an D rings are studied by modeling the flow of ionospheric plasma from the mid- to low-latitude ionosphere to the vicinity of the rings. It is found that the plasma density near the C and D rings, at a given radial location, will experience a one to two order of magnitude diurnal variation. The surface charge density (SCD) of these rings can show significant radial and azimuthal variations due mainly to variation in the plasma density. The SCD also depends on structural features of the rings such as thickness and the nature of the particle size distribution. The associated azimuthal currents carried by these rings also show large diurnal variations resulting in field-aligned currents which close in the ionosphere. The resulting ionospheric electric field will probably not produce a significant amount of plasma convection in the topside ionosphere and inner plasmasphere.

Wilson, G. R.↗

NASA NTRS1992

Differential Current Source

Circuit acts as voltage-controlled current source having high-impedance floating differential output. Maintains commanded current in load, regardless of changes in load impedance and regardless of any common-mode voltages on load. Provides constant excitation current to germanium resistance thermometer or other transducer, output voltage of which sensed by differential voltage amplifier.

Sutton, John F.↗

NASA NTRS1991

Critical currents and high-temperature superconductors

Experimental information on critical currents and epitaxial thin films of high temperature superconductors is summarized. Experiments performed to measure critical currents across single grain boundaries are described. A variety of mechanisms responsible for limiting currents in films as well as across grain boundaries are presented and their predictions compared with experimental data.

Chaudhari, P.↗

NASA NTRS1992

Study of eddy current probes

The recognition of materials properties still presents a number of problems for nondestructive testing in aerospace systems. This project attempts to utilize current capabilities in eddy current instrumentation, artificial intelligence, and robotics in order to provide insight into defining geometrical aspects of flaws in composite materials which are capable of being evaluated using eddy current inspection techniques.

Workman, Gary L.↗

NASA NTRS1991

On the limit of field-aligned current intensity in the polar magnetosphere

Field-aligned current (FAC) is here defined by 4 pi j = alpha B, where alpha is constant along a magnetic field line. The upper limit value of alpha in the polar magnetosphere, possible source regions of the strongest FAC and the relationship of them to some auroral and ionospheric irregularity cross-field scale sizes are discussed. Cross-field dimensions of the strongest FAC are related to the gyroradii of source particles (O(+), He(2+), He(+), H(+), e) in the current-generating region. It is suggested that experimental determination, and mapping of the values of alpha, may assist with the search for the generators of such currents in near-earth space including in the nearby solar wind. The upper limit of alpha is associated with the breakup of FAC systems.

Cole, Keith D.↗

NASA NTRS1991

DMSP F7 observations of a substorm field-aligned current

Observations are described of a substorm field-aligned current (FAC) system traversed by the DMSP F7 spacecraft just after 0300 UT on April 25, 1985. It is shown that the substorm FAC portion of the current system was located equatorward of the boundary between open and closed field lines. The equatorward boundary of the substorm FAC into the magnetotail was mapped using the Tsyganenko (1987) model, showing that the boundary corresponds to 6.9 earth radii. The result is consistent with the suggestion of Akasofu (1972) and Lopez and Lui (1990) that the region of substorm initiation lies relatively close to the earth and the concept that an essential feature of substorms is the disruption and diversion of the near-earth current sheet.

Lopez, R. E.↗

NASA NTRS1991

On dipolarization and its relation to the substorm current wedge

The near-earth properties of magnetospheric substorms are investigated using the Hesse and Birn (1991) self-consistent 3D MHD model of magnetotail activity. The simulation shows that an increase of Bz, a decrease of the cross-tail current, and an increase of region-1 field-aligned currents occurs roughly simultaneously, consistent with the empirical picture described by McPherron et al. (1973). It was also found that the region where the cross-tail current is reduced and diverted is located tailward from the dipolarization region. The results indicate that the apparent tailward propagation of the dipolarization effects does not support an earlier suggestion that the underlying process is initiated further earthward.

Hesse, Michael↗

NASA NTRS1991

Submicrosecond characteristics of lightning return-stroke currents

The authors describe the experimental results obtained during 1987 and 1988 triggered-lightning experiments in Florida. Seventy-four simultaneous submicrosecond time-resolved measurements of triggered return-stroke current (I) and current derivative (dI/dt) were made in Florida in 1987 and 1988. Peak currents ranged from about 5 to 76 kA, peak dI/dt amplitude from 13 to 411 kA/microsec and rise time from 90 to 1000 ns. The mean peak dI/dt values of 110 kA/microsec were 2-3 times higher than data from instrumented towers and peak I and dI/dt appear to be positively correlated. These data confirm previous experiments and conclusions supported by forty measurements. They are important in order to define, for example, standards for lightning protection. Present standards give a dI/dt maximum of 140 kA/microsec.

Leteinturier, Christiane↗

NASA NTRS1992

Weighted current sheets supported in normal and inverse configurations - A model for prominence observations

A technique which incorporates both photospheric and prominence magnetic field observations is used to analyze the magnetic support of solar prominences in two dimensions. The prominence is modeled by a mass-loaded current sheet which is supported against gravity by magnetic fields from a bipolar source in the photosphere and a massless line current in the corona. It is found that prominence support can be achieved in three different kinds of configurations: an arcade topology with a normal polarity; a helical topology with a normal polarity; and a helical topology with an inverse polarity. In all cases the important parameter is the variation of the horizontal component of the prominence field with height. Adding a line current external to the prominence eliminates the nonsupport problem which plagues virtually all previous prominence models with inverse polarity.

Demoulin, P.↗

NASA NTRS1992

Radial expansion of the tail current disruption during substorms - A new approach to the substorm onset region

A new method is used to examine the radial expansion of the tail current disruption and the substorm onset region. The expansion of the disruption region is specified by examining the time sequence (phase relationship) between the north-south component and the sun-earth component. This method is tested by applying it to the March 6, 1979, event. The phase relationship indicates that the current disruption started on the earthward side of the spacecraft, and expanded tailward past the spacecraft. The method was used for 13 events selected from the ISEE magnetometer data. The results indicate that the current disruption usually starts in the near-earth magnetotail and often within 15 RE from the earth.

Ohtani, S.↗

NASA NTRS1992

Extrapolation of electrical breakdown currents from the laboratory to Space Station

Recent experiments conducted in a plasma chamber at NASA/MSFC on anodized aluminum coatings representative of Space Station Freedom design show that if the aluminum used as a thermal control coating is biased more than 80 V negative with respect to the plasma, the anodization will experience dielectric breakdown. As the thin anodization layer creates a capacitive charge buildup, large currents are observed during the arc. How plasma generation at the arc site can support large currents and discharge the surface charge layer is investigated. The importance for Space Station Freedom is that currents similar to those observed in the laboratory can be observed on orbit.

Vaughn, Jason A.↗

NASA NTRS1990

Linear evolution of current sheets in sheared force-free magnetic fields with discontinuous connectivity

Thin current sheets arising in tenuous, magnetized solar coronal plasmas may constitute an important mechanism for energy buildups and subsequent energy releases; they could arise from the continuous-and-random motion of magnetic footprints associated with photospheric velocity fields. A model is presented for study of the quasi-static evolution of current sheets due to shearing of the footpoints, in a highly idealized geometry that incorporates an abrupt jump in field-line connectivity. The model highlights that formation of thin current layers and allows large shearing motions prior to violation of the linear approximation. Excess energy comparable to that released by solar flares can be stored in a sheared field.

Wolfson, Richard↗

NASA NTRS1992

The effects of pressure anisotropy on Birkeland currents in dipole and stretched magnetospheres

Attention is given to two effects which modify the rate of generation of Birkeland currents from the values given by the Vasyliunas (1970) formula in a dipole, namely, nonisotropic plasma pressure and the radial distention of magnetic field lines. The parallel current at any given point is the integrated effect of the diversion of perpendicular currents along the length of the flux tube from the equator. The result for j-parallel in I is fully nonlinear. In a dipole field the effect of anisotropy is modest: j-parallel at the ionosphere is, irrespective of the r0 value, about factor of 2.4 larger for a large P-parallel anisotropy (r = 0.1) than for the isotropic case and factor of 0.2 smaller for r = 10. In the stretched field the comparable values are factor of 10 and factor of 0.06 for a field line intersecting the ionosphere at a dipole colatitude of 16.4 deg and crossing the equator at r0 of 20. The results exhibit differences in plasma density and plasma pressure along field lines between the stretched and dipole models.

Birmingham, Thomas J.↗

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

Observed currents on the earth's high-latitude magnetopause

A survey of electrical currents of the earth's magnetosphere, principally at high latitudes, as inferred from magnetic vector data acquired by the Hawkeye 1 satellite, is reported. A total of 536 candidate crossings of the magnetopause were examined. A reduced data set of 139 selected cases was analyzed in detail though solar wind dynamic pressure data were available for only 117 of these cases. Inferred values of the lineal current densities on the magnetopause are in the range 5.5 to 157.5 mA/m over a wide range of solar wind dynamic pressure from 1.17 to 16.1 nPa. The apparent normal thickness of the magnetopause current sheet ranges from 30 to 850 km with mean and median values of 185 and 158 km, respectively. It is argued that the radial rate of motion of the magnetopause is of the order of 2 km/s and hence that its true thickness is of similar magnitude. The relationship of these results to models of the geomagnetic field and to other related work is discussed.

Van Allen, J. A.↗

NASA NTRS1992

Current saturation in submillimeter wave varactors

In semiconductor devices the speed of electrons cannot exceed certain limits. This phenomenon will affect varactor multipliers as well as other high frequency devices where the RF current through the active part of the device is primarily displacement current. Hence, 'saturation' of the varactor output power is expected at some point. This phenomenon is discussed in some detail and shown to severely deteriorate the multiplier performance at higher frequencies. Single barrier varactors should have an advantage over GaAs Schottky diode varactors because they can be fabricated on InAs and stacked in a series array, allowing for lower current densities and higher power handling.

Kollberg, Erik L.↗

NASA NTRS1992

3-dimensional current collection model

A three-dimensional, time dependent current collection model of a satellite has been developed for the TSS-1 system. The system has been simulated particularly for the Research of Plasma Electrodynamics (ROPE) experiment. The Maxwellian distributed particles with the geomagnetic field effects are applied in this numerical simulation. The preliminary results indicate that a ring current is observed surrounding the satellite in the equatorial plane. This ring current is found between the plasma sheath and the satellite surface and is oscillating with a time scale of approximately 1 microsec. This is equivalent to the electron plasma frequency. An hour glass shape of electron distribution was observed when the viewing direction is perpendicular to the equatorial plane. This result is consistent with previous findings from Linson (1969) and Antoniades et al. (1990). Electrons that are absorbed by the satellite are limited from the background ionosphere as indicated by Parker and Murphy (1967).

Hwang, Kai-Shen↗

NASA NTRS1992

Characterization of YBa2Cu3O7, including critical current density Jc, by trapped magnetic field

Spatial distributions of persistent magnetic field trapped by sintered and melt-textured ceramic-type high-temperature superconductor (HTS) samples have been studied. The trapped field can be reproduced by a model of the current consisting of two components: (1) a surface current Js and (2) a uniform volume current Jv. This Js + Jv model gives a satisfactory account of the spatial distribution of the magnetic field trapped by different types of HTS samples. The magnetic moment can be calculated, based on the Js + Jv model, and the result agrees well with that measured by standard vibrating sample magnetometer (VSM). As a consequence, Jc predicted by VSM methods agrees with Jc predicted from the Js + Jv model. The field mapping method described is also useful to reveal the granular structure of large HTS samples and regions of weak links.

Chen, In-Gann↗