Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “currents”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

DE 1 observations of return current regions in the nightside auroral oval

Data from the DE-1 altitude plasma instrument and magnetometer were used to investigate the Birkeland current patterns in the premidnight auroral region and the charge carriers, as well as their relationships with the hot-plasma populations. As expected, upward currents were found to predominate throughout the body of the boundary plasma sheet (BPS). The charge carriers of the upward and the downward currents were found to be precopitating the auroral electrons and the upward accelerated ionospheric electron beams, respectively, throughout the auroral regions. However, in several cases, inverted-V structures interior to the BPS were observed, which were not associated with any detectable upward Birkeland current. It is suggested that, in these circumstances, localized field-aligned current systems may have closed at altitudes below the typical DE-1 altitudes of 10,000-20,000 km, or that field-aligned potential differences may have developed in regions without significant Birkeland currents.

Marshall, J. A.↗
NASA NTRS1990

Welding-Current Indicator

Light flashes on to indicate high current. Simple, inexpensive display circuit indicates when 3,000-A welding current flows in welding gun. Onset of welding current induces voltage and current in 1,000-turn, 28-gauge copper-wire coil. Single-transistor amplifier amplifies induced current, energizing light-emitting diode (LED) connected to collector of transistor. Light from LED gives simple, direct indication of welding current.

Hensley, Milton C.↗
NASA NTRS1990

Currents between tethered electrodes in a magnetized laboratory plasma

This paper describes the experimental setup and measurement techniques used to investigate currents between tethered electodes in a magnetized laboratory plasma. Experimental results include information on current propagation, the formation of wave wings, the limits of current collection, nonlinear effects and instabilities, charging phenomena, and the characteristics of transmission lines in plasma. The results were found to support certain predictions on tethers in space (e.g., the MHD far zone or the motional emf) while contradicting others (e.g., distant current closure), and revealed such new phenomena as current disruptions, current-neutralized beams, and wing spread.

Stenzel, R. L.↗
NASA NTRS1990

Extreme energetic particle decreases near geostationary orbit - A manifestation of current diversion within the inner plasma sheet

A qualitative model of magnetic field reconfiguration as might result from neutral line formation in the central plasma sheet late in a substorm growth phase is considered. It is suggested that magnetic reconnection probably begins before the substorm expansion phase and that cross-tail current is enhanced across the plasma sheet both earthward and tailward of a limited region near the neutral line. Such an enhanced cross-tail current earthward of the original X line region may contribute to thinning the plasma sheet substantially, and this would in turn affect the drift currents in that location, thus enhancing the current even closer toward the earth. In this way a redistribution and progressive diversion of normal cross-tail current throughout much of the inner portion of the plasma sheet could occur. The resulting intensified current, localized at the inner edge of the plasma sheet, would lead to a very thin plasma confinement region. This would explain the very taillike field and extreme particle dropouts often seen late in substorm growth phases.

Baker, D. N.↗
NASA NTRS1990

SPEAR-1: An experiment to measure current collection in the ionosphere by high voltage biased conductors

An experiment is described in which a high electrical potential difference, up to 45 kV, was applied between deployed conducting spheres and a sounding rocket in the ionosphere. Measurements were made of the applied voltage and the resulting currents for each of 24 applications of different high potentials. In addition, diagnostic measurements of optical emissions in the vicinity of the spheres, energetic particle flow to the sounding rocket, dc electric field and wave data were made. The ambient plasma and neutral environments were measured by a Langmuir probe and a cold cathode neutral ionization gauge, respectively. The payload is described and examples of the measured current and voltage characteristics are presented. The characteristics of the measured currents are discussed in terms of the diagnostic measurements and the in-situ measurements of the vehicle environment. In general, it was found that the currents observed were at a level typical of magnetically limited currents from the ionospheric plasma for potentials less than 12 kV, and slightly higher for larger potentials. However, due to the failure to expose the plasma contactor, the vehicle sheath modified the sphere sheaths and made comparisons with the analytic models of Langmuir-Blodgett and Parker-Murphy less meaningful. Examples of localized enhancements of ambient gas density resulting from the operation of the attitude control system thrusters (cold nitrogen) were obtained. Current measurements and optical data indicated localized discharges due to enhanced gas density that reduced the vehicle-ionosphere impedance.

Raitt, W. John↗
NASA NTRS1990

Current limiting mechanisms in electron and ion beam experiments

The emission and collection of current from satellites or rockets in the ionosphere is a process which, at equilibrium, requires a balance between inward and outward currents. In most active experiments in the ionosphere and magnetosphere, the emitted current exceeds the integrated thermal current by one or more orders of magnitude. The system response is typically for the emitted current to be limited by processes such as differential charging of insulating surfaces, interactions between an emitted beam and the local plasma, and interactions between the beam and local neutral gas. These current limiting mechanisms have been illustrated for 20 years in sounding rocket and satellite experiments, which are reviewed here. Detailed presentations of the Spacecraft Charging at High Altitude (SCATHA) electron and ion gun experiments are used to demonstrate the general range of observed phenomena.

Olsen, R. C.↗
NASA NTRS1990

Effects of neutral gas release on current collection during the CHARGE-2 rocket experiment

Observations of current collection enhancements due to cold nitrogen gas control jet emissions from a highly charged rocket payload in the ionosphere are reported. These observations were made during the second cooperative high altitude rocket gun experiment (CHARGE-2) which was an electrically tethered mother/daughter payload system. The current collection enhancement was observed at the daughter payload located 100 to 400 m away from the mother which was firing an energetic electron beam. The authors interpret these results in terms of an electrical discharge forming in close proximity to the daughter during the short periods of gas emission. The results indicate that it is possible to enhance the electron current collection capability of positively charged vehicles by means of deliberate neutral gas releases into an otherwise undisturbed space plasma. These results can also be compared with recent laboratory observations of hollow cathode plasma contactors operating in the ignited mode. Experimental observations of current collection enhancements due to cold nitrogen gas control jet emissions from a highly charged, isolated daughter payload in the nighttime ionosphere were made. These observations were derived from the second cooperative high altitude rocket gun experiment (CHARGE-2) which was an electrically tethered mother-daughter payload system. The rocket flew from White Sands Missile Range (WSMR) in December, 1985. The rocket achieved an altitude of 261 km and carried a 1 keV electron beam emitting up to 48 mA of current (Myers, et al., 1989a). The mother payload, carried the electron beam source, while the daughter acted as a remote current collection and observation platform and reached a distance of 426 m away from the main payload. Gas emissions at the daughter were due to periodic thruster jet firings to maintain separation velocity between the two payloads.

Gilchrist, B. E.↗
NASA NTRS1989

Microwave measurements of ocean currents for the SAXON experiment

Measurements of ocean surface currents carried out during the SAXON experiment using the University of Massachusetts stepped-frequency Delta-K radar (SFDKR) are presented. The measurement site was the Chesapeake light tower situated in the north Atlantic Ocean, 15 km off the coast of Virginia Beach, Virginia. The radar was placed on a platform 26 m above mean sea level. Measurements were made at grazing angles between 3 and 10 deg. A nearly uninterrupted record of ocean current velocity was obtained for a 26-day period. The measurement dependence on various system and hydrodynamic parameters and a comparison with an independent in-situ current measurement (EM current meter) are presented. Experimental results show that the Delta K spectrum with a distinguishable resonant line can be measured under all weather conditions, from calm seas with large swells to the fully developed seas occurring under winds as high as 20 ms-1. The only parameter that needs to be adjusted in real time is the frequency separation, Delta f, which has to be modified according to the varying shape of the ocean wave spectrum. Currents could not be accurately measured for very low wind conditions, even though resonant lines were present in the Delta K spectrum. The SFDKR can accurately measure wind-driven surface currents for wide range of wind conditions.

Popstefanija, Ivan↗
NASA NTRS1991

Generation of field-aligned current in the auroral zone

Generation of a magnetic field-aligned current in the auroral zone connecting the magnetospheric and ionospheric plasmas has been studied by means of a three dimensional particle simulation model. The model is of a magnetostatic variety appropriate for a low beta plasma in which the high frequency transverse displacement current has been eliminated. The simulation model is highly elongated along the magnetic field lines in order to model a highly elongated flux tube in the auroral zone. An enhanced field-aligned current was generated by injection of a magnetospheric plasma across the auroral zone magnetic field at the center of the model. Such a plasma injection may correspond to a plasmoid injection at the geomagnetic tail associated with magnetic reconnection during a substorm or a transverse plasma flow along the low latitude magnetopause boundary layer. The results of the simulations show that the field-aligned current can be enhanced over the thermal current by a factor of 5 - 10 via such injection. Associated with the enhanced current are the electrostatic ion cyclotron waves and shear Alfven waves excited in the auroral zone.

Okuda, Hideo↗
NASA NTRS1992

Laboratory study of the temporal evolution of the current-voltage characteristic of a probe in the wake of an object immersed in a pulsed flowing plasma

Measurements of the current-voltage characteristics of a Langmuir probe in the near wake of a disk immersed in a pulsed flowing plasma were made. A 1 cm diameter biasable sphere was placed in the ion-free near wake region of a 10 cm diameter disk immersed in a Mach 8 pulsed flowing plasma. The current-voltage characteristic of the sphere was observed as a function of time as the sphere bias was scanned from -5000 V to +1000 V. The collected current is found to be monotonically increasing with increasing positive bias voltage but exhibits a threshold voltage for current collection as the bias voltage becomes more negative. Potential measurements in the wake region were made for a sphere bias voltages below, at, and above the current collection threshold for a number of times during the wake formation period. The time evolution of the potential profile is shown to change as the sheath around the biased sphere is established. Predictions from the particle trajectory code SIMION are compared with data, showing excellent agreement in the prediction of the current collection threshold.

Meassick, S.↗
NASA NTRS1991

Field-aligned current associated with a distorted two-cell convection pattern during northward interplanetary magnetic field

The influence of the ionospheric conductance on the field-aligned current associated with a distorted two-cell convection pattern during northward IMF was investigated using the Heppner-Maynard (1987) convection model and the Utah State University conductivity model described by Rasmussen and Schunk (1987). Results show that the variation of the ionospheric conductivity distribution can significantly affect the features of the field-aligned current for northward IMF, where matching or mismatching between the conductance gradient and the convection electric field plays a key role. It was found that the increase of the field-aligned current in the polar cap observed during summer is mainly due to the increasing contribution from the Pedersen current, and that the increase of the field-aligned current in both the oval region and the evening-midnight sector during the active aurora period is mainly due to the increasing contribution from the Hall current.

Zhu, L.↗
NASA NTRS1992

Current disruptions in the near-earth neutral sheet region

Current disruption events observed by the Charge Composition Explorer during 1985 and 1986 are examined. Occurrence of current disruption was accompanied by large magnetic field turbulence and frequently with reversal in the sign of the field component normal to the neutral sheet. Current disruptions in the near-earth region are found to be typically shortlived (about 1-5 min), and their onsets coincide well with the ground onsets of substorm expansion or intensification in the local time sector of the footpoint of the spacecraft. These events are found almost exclusively close to the field reversal plane of the neutral sheet (within about 0.5 RE). Prior to current disruption the field strength can be reduced to as low as one seventh of the dipole field value and can recover to nearly the dipole value after disruption. The temporal evolution of particle pressure in the near-earth neutral sheet during the onset of current disruption indicates that the current buildup during the substorm growth phase is associated with enhancement in the particle pressure at the neutral sheet.

Lui, A. T. Y.↗
NASA NTRS1992

Study of high pressure and high current arcs in the cathode region of an arc heater

This paper investigates the behavior of high pressure, high current arcs in the downstream electrode region (cathode) of an arc heater. Computations are done for operating pressures in the range 50 atm to 200 atm and load currents in the range 6000 amp to 18,000 amp. At high pressures, due to the high current carrying capability of the dense gas, spot size remains reasonably small even for high load currents. Its value is 1.1 mm for a 50 atm arc with a load current of 18,000 amp. By increasing pressure it can be reduced further. Smaller spot size may allow us to design thinner electrode rings. Spot temperatures are high enough to cause ablation of the electrode material. In order to reduce material ablation at very high currents, the operating pressure must also be high and the arc should be rotated.

Durgapal, Prabha↗
NASA NTRS1990

DE-2 observations of filamentary currents at ionospheric altitudes

Conjunctive measurements made by the Dynamics Explorer 1 and 2 spacecraft on October 22, 1981, under conditions of southward IMF, suggest the existence of a cusp ion injection from a region at the magnetopause with a scale size of about 1/2 to 1 earth radii. Current signatures observed by the LAPI and MAGB instruments on board DE-2 indicate the existence of a rotation in the magnetic field that is consistent with a filamentary current system. The observed current structure can be interpreted as the ionospheric signature of a flux transfer event (FTE). In addition to this large-scale current structure there exist three small-scale filamentary current pairs. These current pairs close locally and thus, if the present interpretation of this event as an FTE is correct, represent the first reported observations of FTE interior structure at low-altitudes.

Smith, M. F.↗
NASA NTRS1992

Distribution of Reynolds stress carried by mesoscale variability in the Antarctic Circumpolar Current

Satellite altimeter data collected by the Geosat Exact Repeat Mission were used to investigate turbulent stress resulting from the variability of surface geostrophic currents in the Antarctic Circumpolar Current. The altimeter measured sea level along the subsatellite track. The variability of the along-track slope of sea level is directly proportional to the variability of surface geostrophic currents in the cross-track direction. Because the grid of crossover points is dense at high latitudes, the satellite data could be used for mapping the temporal and spatial variability of the current. Two and a half years of data were used to compute the statistical structure of the variability. The statistics included the probability distribution functions for each component of the current, the time-lagged autocorrelation functions of the variability, and the Reynolds stress produced by the variability. The results demonstrate that stress is correlated with bathymetry. In some areas the distribution of negative stress indicate that eddies contribute to an acceleration of the mean flow, strengthening the hypothesis that baroclinic instability makes important contributions to strong oceanic currents.

Johnson, Thomas J.↗
NASA NTRS1992

A modeling study of the time-averaged electric currents in the vicinity of isolated thunderstorms

A thorough examination of the results of a time-dependent computer model of a dipole thunderstorm revealed that there are numerous similarities between the time-averaged electrical properties and the steady-state properties of an active thunderstorm. Thus, the electrical behavior of the atmosphere in the vicinity of a thunderstorm can be determined with a formulation similar to what was first described by Holzer and Saxon (1952). From the Maxwell continuity equation of electric current, a simple analytical equation was derived that expresses a thunderstorm's average current contribution to the global electric circuit in terms of the generator current within the thundercloud, the intracloud lightning current, the cloud-to-ground lightning current, the altitudes of the charge centers, and the conductivity profile of the atmosphere. This equation was found to be nearly as accurate as the more computationally expensive numerical model, even when it is applied to a thunderstorm with a reduced conductivity thundercloud, a time-varying generator current, a varying flash rate, and a changing lightning mix.

Driscoll, Kevin T.↗
NASA NTRS1993

Numerical simulation of the subsolar magnetopause current layer in the sun-earth meridian plane

The formation and stability of the magnetopause current layer near the subsolar point in the sun-earth meridian plane are examined using a 2D electromagnetic particle simulation. For the case of zero IMF, the simulation results show that the current layer remains stable and is essentially the same as in the 1D simulation. The width of the current layer is given by the electron-ion hybrid gyroradius which is much smaller than the ion gyroradius. The current layer is found to remain stable for the northward IMF as well. As in the 1D simulation, the jump of the magnetic field at the current layer for the northward IMF remains small. For the southward IMF, collisionless magnetic reconnection is found to develop, leading to the formation of magnetic islands and density peaking within the current layer.

Okuda, H.↗
NASA NTRS1993

The equatorial electrojet and associated currents as seen in Magsat data

In view of the disagreement between reports of Langel and Estes (1985) and those of other investigators using Nagsat data sets, concerning the evidence for equatorial electrojet and meridional currents in the dawn Magsat data, this paper reexamines the Magsat data for the presence of equatorial electrojet and meridional current in both dawn and dusk local times, using a specially designed method for isolating field variations organized by dip-latitude. It is shown that such fields, if any, at dawn are very weak and do not result in a persistent pattern from longitude to longitude or a pattern consistent with that expected from an electrojet current. On the other hand, fields organized by dip-latitude are clearly present in the dusk data and it is possible to isolate longitudinal and seasonal variations in such fields. Current densities are computed for both the equatorial electrojet and meridional currents and are compared to the current densities and distributions found with recent models of the ionospheric dynamo and equatorial electrojet.

Langel, R. A.↗