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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Chatanika radar observations of ionospheric and field-aligned currents

A method for estimating field-aligned current densities from ground-based measurements of the horizontal ionospheric currents at two or more latitudes using the Chatanika incoherent scatter radar is presented. It is assumed that the one-dimensional divergence of the horizontal currents approximates the current density of a field-aligned current sheet. The field-aligned current densities found via radar data agree with those given by simultaneous magnetic observations by the Triad satellite. It is noted that downward or upward field-aligned currents having densities of 10 to the -6th to 10 to the -5th A/sq m are achieved in the evening or morning sector. The densities are further increased during periods of increased geomagnetic activity and when current directions are reversed. This reversal generally occurs during auroral oval expansion.

Kamide, Y.↗
NASA NTRS1977

Lagrangian drifter design for the determination of surface currents by remote sensing

In estuaries, the interaction of wind, tidal current, and mixing of fresh and saline water produces a variable depth profile of current, with foam lines and convergence zones between water types. Careful measurement of surface currents via Lagrangian drifters requires a drifter design appropriate to both the depth of current to be measured and the tide and wind conditions of interest. The use of remote sensing to track drifters contributes additional constraints on drifter design. Several designs of biodegradable drifters which emit uranine dye plumes, resolvable in aerial imagery to 1:60,000 scale, were tested for wind drag in field conditions against data from calibrated current meters. A 20 cm-vaned wooden drifter and a window shade drifter set to 1.5 m depth had negligible wind drag in winds to 8 m/sec. Prediction of oil slick trajectories using surface current data and a wind factor should be approached cautiously, as surface current data may be wind-contaminated, while the usual 3.5% wind factor is appropriate only for currents measured at depth.

Gordon, H. H.↗
NASA NTRS1979

Induced electric currents in the Alaska oil pipeline measured by gradient, fluxgate, and SQUID magnetometers

The field gradient method for observing the electric currents in the Alaska pipeline provided consistent values for both the fluxgate and SQUID method of observation. These currents were linearly related to the regularly measured electric and magnetic field changes. Determinations of pipeline current were consistent with values obtained by a direct connection, current shunt technique at a pipeline site about 9.6 km away. The gradient method has the distinct advantage of portability and buried- pipe capability. Field gradients due to the pipe magnetization, geological features, or ionospheric source currents do not seem to contribute a measurable error to such pipe current determination. The SQUID gradiometer is inherently sensitive enough to detect very small currents in a linear conductor at 10 meters, or conversely, to detect small currents of one amphere or more at relatively great distances. It is fairly straightforward to achieve imbalance less than one part in ten thousand, and with extreme care, one part in one million or better.

Campbell, W. H.↗
NASA NTRS1981

Equatorial disc and dawn-dusk currents in the frontside magnetosphere of Jupiter - Pioneer 10 and 11

Observations by Pioneer 10 and 11 show that the strongest azimuthal fields are observed near the dawn meridian (Pioneer 10) while the weakest occur near the noon meridian (Pioneer 11), suggesting a strong local time dependence for the corresponding radial current system. Modeling studies of the radial component of the field observed by both spacecraft suggest that the corresponding azimuthal current system must also be a strong function of local time. Both the azimuthal and the radial field component signatures exhibit sharp dips and reversals, requiring thin radial and azimuthal current systems. There is also a suggestion that these two current systems either are interacting or are due, at least in part, to the same current. It is suggested that a plausible current model consists of the superposition of a thin, local-time-independent azimuthal current system plus the equatorial portion of a tail-like current system that extends into the dayside magnetosphere.

Jones, D. E.↗
NASA NTRS1982

Relations between the Birkeland currents, the auroral electrojet indices and high latitude Joule heating

Field-aligned currents were postulated by Birkeland (1908) to explain the magnetic perturbations in the auroral zone. Theoretical models have been developed to examine the effect of these currents on the ionosphere. These models, in general, involve very extensive computer programs, and it is difficult to see how their very complicated boundary conditions and assumptions affect the relationships between the Birkeland currents and magnetic activity. In the present investigation, a simplified analysis is used to study the average behavior of the large-scale ionospheric current paths and to examine the interrelationships of various parameters. The relationship of the parameters of the current deposition regions to the magnetic indices is investigated along with the polar cap potential. Attention is given to the experimental values of coefficients, and relations between the Birkeland current densities, current intensities, currents, and the AL, AU, AE indices are discussed.

Nisbet, J. S.↗
NASA NTRS1982

A new theory of sources of Birkeland currents

An approach to collisionless plasma shows the existence of current orthogonal to B along the low latitude boundary layer of the magnetosphere driven by electric field which is orthogonal to both B and the layer. In this case relationship P perpendicular + (sq B/8 pi) - (sq epsilon E perpendicular/8 pi) = constant holds on a line orthogonal to B and the layer, where epsilon is the dielectric constant of the plasma for electric fields orthogonal to B. Across the geomagnetic tail there flows a current in the direction of the dawn-dusk electric field, and in this case a relationship P perpendicular + (sq B/8 pi) + (sq epsilon E perpendicular/8 pi = constant, holds aong a line orthogonal to E and B. Divergence of both these currents is shown to be a source of Birkeland currents. Some of the boundary layer current is continuous with current across the tail. Electric currents of physically similar origin flow in interplanetary space, and when the magnetosphere interrupts them, additional Birkeland currents are driven.

Cole, K. D.↗
NASA NTRS1983

Relationships between field-aligned currents, electric fields and particle precipitation as observed by dynamics Explorer-2

The relationships between field-aligned currents, electric fields, and particle fluxes are determined using observations from the polar orbiting low-altitude satellite Dynamics Explorer-2. It is shown that the north-south electric field and the east-west magnetic field components are usually highly correlated in the field-aligned current regions. This proportionality observationally proves that the field-aligned current equals the divergence of the height-integrated ionospheric Pedersen current in the meridional plane to a high degree of approximation. As a general rule, in the evening sector the upward field-aligned currents flow in the boundary plasma sheet region and the downward currents flow in the central plasma sheet region. The current densities determined independently from the plasma and magnetic field measurements are compared. Although the current densities deduced from the two methods are in general agreement, the degree and extent of the agreement vary in individual cases.

Sugiura, M.↗
NASA NTRS1984

Modeling Jupiter's magnetospheric currents using Pioneer data - Evidence for a low-latitude cusp

The Jovian magnetospheric field measured by Pioneer 10 and 11 can be well modeled by a combination of current systems composing an azimuthally symmetric current disk, a dusk-dawn current sheet in both the dayside and the nightside magnetosphere, and an image dipole to represent the effects of currents on the magnetopause. The inclusion of a dusk-dawn current sheet in the dayside magnetosphere allows observations obtained both inbound and outbound to be simultaneously fit by an azimuthally symmetric current disk (i.e., without the need for local time dependent current densities). Similar disk current intensities are found to describe both Pioneer 10 and Pioneer 11 encounters. During the Pioneer 10 inbound passage the magnetopause was rapidly pushed inside the spacecraft position by a solar wind compression event. The changes that occurred in the magnetospheric field at this time can be described by relatively simple changes in the model parameters. The most striking feature of the models is that they suggest that the Jovian cusp is at much lower latitudes than is the case with the earth's magnetosphere.

Thomas, B. T.↗
NASA NTRS1984

The evolution of arguments regarding the existence of field-aligned currents

The present understanding of Birkeland (magnetically-field-aligned) currents was not obtained by a direct, logical course. The story is rather more complex. Starting at the end of the 19th century, the Norwegian scientist Kristian Birkeland laid out a compelling case, supported by both theory and experiment, for the existence of field-aligned currents that cause both the aurora and polar geomagnetic disturbances. Sydney Chapman, the British geophysicist, became the acknowledged leader and opinion maker in the field in the decades following Birkeland's death. Chapman proposed, in contradistinction to Birkeland's ideas, equivalent currents that were restricted to flow in the ionosphere with no vertical or field-aligned components. Birkeland's ideas may have faded completely if it had not been for Hannes Alfven, who became involved well after Chapman's ideas gained predominance. Alfven kept insisting that Birkeland's current system made more sense because field-aligned currents were required to drive most of the ionospheric currents. The author became personally involved when Zmuda et al. (1966) submitted to the Journal of Geophysical Research a paper reporting satellite data showing magnetic disturbances above the ionosphere that were consistent with field-aligned Birkeland currents, but which they did not interpret as being due to such currents.

Dessler, A. J.↗
NASA NTRS1984

Relationships between field-aligned currents, electric fields, and particle precipitation as observed by Dynamics Explorer-2

The relationships between field-aligned currents, electric fields, and particle fluxes are determined using observations from the polar orbiting low-altitude satellite Dynamics Explorer-2. It is shown that the north-south electric field and the east-west magnetic field components are usually highly correlated in the field-aligned current regions. This proportionality observationally proves that the field-aligned current equals the divergence of the height-integrated ionospheric Pedersen current in the meridional plane to a high degree of approximation. As a general rule, in the evening sector the upward field-aligned currents flow in the boundary plasma sheet region and the downward currents flow in the central plasma sheet region. The current densities determined independently from the plasma and magnetic field measurements are compared. Although the current densities deduced from the two methods are in general agreement, the degree and extent of the agreement vary in individual cases.

Sugiura, M.↗
NASA NTRS1984

Currents in the earth's magnetotail

Currents in the earth's magnetotail are detected with the plasma instrumentation on board the ISEE-1 spacecraft. Field-aligned currents directed into and out of the ionosphere are found in the boundary layer of the plasma sheet. Typical current densities are in the range of 5 x 10-9 to 5 x 10-8 A/m2. These currents are associated with the Region 1 current system that is observed previously at low altitudes. An intense current sheet is shown to exist at a discontinuity in convection electric fields during a period of great magnetic activity. Electron acceleration, similar to that for electron inverted-V precipitation regions at low altitudes, occurs in this current sheet. Examination of the plasma velocity distributions at a neutral-sheet crossing reveals that the neutral sheet current is carried by electrons and protons. The relative directions for the bulk flows of the proton and electron plasmas indicate that the first adiabatic invariant is not conserved for the protons, and quite possibly for the electrons also. These letter findings are in substantial agreement with a neutral sheet model for acceleration of charged particles in the presence of a weak electric field.

Frank, L. A.↗
NASA NTRS1984

A new theory of sources of Birkeland currents

An approach to collisionless plasma shows the existence of current orthogonal to B along the low latitude boundary layer of the magnetosphere driven by electric field which is orthogonal to both B and the layer. In this case relationship P perpendicular + (sq B/8 pi) - (sq epsilon E perpendicular/8 pi) = constant holds on a line orthogonal to B and the layer, where epsilon is the dielectric constant of the plasma for electric fields orthogonal to B. Across the geomagnetic tail there flows a current in the direction of the dawn-dusk electric field, and in this case a relationship P perpendicular + (sq B/8 pi) + (sq epsilon E perpendicular/8 pi) = constant, holds along a line orthogonal to E and B. Divergence of both these currents is shown to be a source of Birkeland currents. Some of the boundary layer current is continuous with current across the tail. Electric currents of physically similar origin flow in interplanetary space, and when the magnetosphere interrupts them, additional Birkeland currents are driven.

Cole, K. D.↗
NASA NTRS1986

Three-dimensional observations of Birkeland currents

The dominant coupling between the ionosphere and magnetosphere is now understood to be Birkeland currents. During substorms the Birkeland current system in the midnight sector is generally interpreted as a cross-tail current interruption and diversion along magnetic field lines to the conducting ionosphere, which carries the intensified auroral electrojet. In the present study, the equatorial current diversion, the Birkeland current intensification, and the auroral electrojet increase in activity were observed to be initiated simultaneously at the onset of substorms. The principal finding is that the field-aligned currents of the components of the current wedge have been observed simultaneously on the ground, at low-altitude satellite orbits, and at geosynchronous satellite orbits. Recent dynamic modeling work by Chen et al. (1982) appears to support our findings. These observations contribute significantly toward the confirmation of the substorm current wedge model and complement and extend the results obtained by Nagai (1982).

Barfield, J. N.↗
NASA NTRS1986

A three-dimensional MHD simulation of the interaction of the solar wind with the earth's magnetosphere - The generation of field-aligned currents

The time-dependent interaction of the solar wind with the earth's magnetosphere is simulated using a three-dimensional MHD model. The bow shock, magnetopause, magnetotail, and plasma sheet of the magnetosphere and Birkeland field-aligned currents that are dependent on the polarity of the z component of the IMF are produced. Twin convection cells and a dawn to dusk electric potential of 30-100 kV are detected at the equator in the magnetosphere. Four types of field-aligned currents are observed: region 1, region 2, dayside magnetopause currents in the dayside cusp region, and the dayside cusp currents for southward IMF. Region 1 and 2 field-aligned currents generated for all IMF conditions are 0.6-1.0 x 10 to the 6th A and 0.15-0.61 x 10 to the 6th A, respectively. The relationship between region 1 currents and field-aligned vorticity, and region 2 currents and pressure gradients are studied. The simulated data are compared with a theoretical analysis of the field-aligned currents and good correlation is observed.

Ogino, T.↗
NASA NTRS1986

Calculations of electric currents in Europa

Electrical currents should flow in the Galilean satellite, Europa, because it is located in Jupiter's corotating magnetosphere. The possible magnitudes of these currents are calculated by assuming that Europa is a differentiated body consisting of an outer H2O layer and a silicate core. Two types of models are considered here: one in which the water is completely frozen and a second in which there is an intermediate liquid layer. For the transverse electric mode (eddy currents), the calculated current density in a liquid layer is approximately 10 to the -5/Am. For the transverse magnetic mode (unipolar generator), the calculated current density in the liquid is severely constrained by the ice layer to a range of only 10 to the -10 to -11th power/ Am, for a total H2O thickness of 100 km, provided that neither layer is less than 4 km thick. The current density is less for a completely frozen H2O layer. If transient cracks were to appear in the ice layer, thereby exposing liquid, the calculated current density could rise to a range of 10 to the -6 to 10 to the -5/Am, depending on layer thicknesses, which would require an exposed area of 10 to the -9 to 10 to the -8 of the Europa surface. The corresponding total current of 2.3x10 to the 5th power A could in 1 yr. electrolyze 7x10 to the 5th power kg of water (and more if the cells were in series), and thereby store up to 10 the 8th power J of energy, but it is not clear how electrolysis can take place in the absence of suitable electrodes. Electrical heating would be significant only if the ice-layer thickness were on the order of 1 m, such as might occur if an exposed liquid surface were to freeze over; the heating under this condition could hinder the thickening of the ice layer.

Colburn, D. S.↗
NASA NTRS1989

Closure of field-aligned currents carried by super-Alfvenic auroral electrons

A linear analysis is carried out to investigate how the return current is generated for the field-aligned current carried by a super-Alfvenic auroral electron beam. If the electron beam is a thin sheet with thickness less than or equal to the electron inertia length, the negative space charge of the beam is neutralized by the ion polarization current produced by the beam. The return current in this case is generated on the two outsides of the beam connected to the polarization current to form a closed current loop. This current structure can explain the thin (a few km) current sheets in the auroral arcs observed by satellites.

Nakamura, Tadas↗
NASA NTRS1990

Modeling of induced currents from electrodynamic tethers in a laboratory plasma

The presently accepted picture of the current path for electrodynamic tethers envisions a quasi-dc current flow in a 'phantom loop' consisting of the tether, two field-aligned current channels into the ionosphere and a cross-field closing current in the E-layer. Predictions are made on the establishment and maintenance of a current loop in space based on observations of time-dependent currents between tethered electrodes in a large laboratory magnetoplasma. In addition to radiation from the contactors ('whistler wings'), the insulated tether is observed to emit waves (a 'whistler wedge'). The 'wedge' provides closure during loop formation by carrying cross-field polarization currents. Whistler spread within the ray cone leads to overlapping of the current wings not far from the tether hence minimizing the role of the ionospheric closure. Maintenance of the loop requires the continuous emission of whistler waves by the entire tether thereby providing severe radiation losses.

Urrutia, J. M.↗
NASA NTRS1990

Laboratory experiments on current flow between stationary and moving electrodes in magnetoplasmas

Laboratory experiments were performed in order to investigate the basic physics of current flow between tethered electrodes in magnetoplasmas. The major findings are summarized. The experiments are performed in an effectively very large laboratory plasma in which not only the nonlinear current collection is addressed but also the propagation and spread of currents, the formation of current wings by moving electrodes, the current closure, and radiation from transmission lines. The laboratory plasma consists of a pulsed dc discharge whose Maxwellian afterglow provides a quiescent, current-free uniform background plasma. Electrodes consisting of collectors and electron emitters are inserted into the plasma and a pulsed voltage is applied between two floating electrodes via insulated transmission lines. Besides the applied current in the wire, the total current density in the plasma is obtained from space and time resolved magnetic probe measurements via Maxwell's law. Langmuir probes yield the plasma parameters.

Stenzel, Reiner L.↗