SEARCH · Search NASA

Results for “current”

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.

SourceResearch connectionFrom yearThrough year

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

At least 433 records · Page 24

All sources NASA NTRS DOE OSTI

NASA NTRS1971

Ionospheric effects of Birkeland currents.

Since Birkeland's observations of high-latitude magnetic perturbations led him to postulate the existence of geomagnetically aligned electric currents, theoretical studies have led to a general understanding of the mechanisms that are capable of driving such currents, and of the processes associated with formation and control of current configurations. Recent advances in experimental techniques have yielded information on the spatial and temporal behavior of such field-aligned Birkeland currents and have established their association to an individual auroral arc and energetic precipitating particles. This paper gives a brief summary of ionospheric effects related to production, maintenance, and control of Birkeland current systems. Available experimental data are discussed in relation to some of these effects.

Cloutier, P. A.↗

NASA NTRS1972

Storm time instabilities of the ring current.

Demonstration that the resonant cyclotron instability between radiation belt protons and ion-cyclotron waves can account for many observed features of the magnetospheric ring current. Analogous results apply to the electron-whistler interaction although the former are not considered to be important for ring current energetics. Particles with energy exceeding 100 keV are readily destabilized throughout the entire magnetosphere and their fluxes are consequently limited to the stably trapped levels described by Kennel and Petschek (1966). However, at the dominant ring current energies between 10 and 100 keV, a broad zone of stability exists in the region just outside the plasmapause. Particles injected into this outer zone during substorms may remain trapped for many drift orbits allowing an intense ring current to develop. In direct contrast, this stable ring current zone is bounded by regions of rapid precipitational loss within the plasmasphere and at large radial distance in the auroral zone.

Thorne, R. M.↗

NASA NTRS1972

Lightning criteria relative to space shuttles: Currents and electric field intensity in Florida lightning

The measured electric field intensities of 161 lightning strokes in 39 flashes which occurred between 1 and 35 km from an observation point at Kennedy Space Center, Florida during June and July of 1971 have been analyzed to determine the lightning channel currents which produced the fields. In addition, typical channel currents are derived and from these typical electric fields at distances between 0.5 and 100 km are computed and presented. On the basis of the results recommendations are made for changes in the specification of lightning properties relative to space vehicle design as given in NASA TMX-64589 (Daniels, 1971). The small sample of lightning analyzed yielded several peak currents in the 100 kA range. Several current rise-times from zero to peak of 0.5 microsec or faster were found; and the fastest observed current rate-of-rise was near 200 kA/microsec. The various sources of error are discussed.

Uman, M. A.↗

NASA NTRS1973

A possible current system associated with the Sqp variation.

It is suggested that the quiet day daily magnetic variation in the polar cap region, Sqp, results partly from the short-circuit effect of the magnetotail current by the polar ionosphere. This implies that there is an inward field-aligned current from the dawnside magnetopause to the forenoon sector of the auroral oval (positively charged) and an outward field-aligned current to the duskside magnetopause from the afternoon sector of the oval (negatively charged), together with the ionospheric (Pedersen and Hall) currents. The distribution of the magnetic field vectors of both combined current systems agrees with the observed Sqp vector distribution. The space charges provide an electric field distribution which is similar to that which has been observed by polar orbiting satellites.

Kawasaki, K.↗

NASA NTRS1973

A numerical study of polar ionospheric currents.

Numerical calculations for the electric current in the polar ionosphere have been made by assuming some realistic distributions of the electric field and conductivity. Two dynamo actions are taken into account; one of which is induced by ionospheric winds and the other by the solar wind. For the solar wind dynamo action, it is found that the secondary polarization field caused by nonuniform distribution of ionospheric conductivity is much larger than the primary field induced by the solar wind, suggesting its important effect on charged particles in the magnetosphere, and that the irrotational current having a source and sink is of the same order of magnitude as the solenoidal current closing its circuit in the ionosphere. It is also found that the solar wind is, in general, more effective than the ionospheric winds in producing polar current systems such as DP 1 and 2, but in some cases the ionospheric winds have a significant effect on the current distribution.

Maeda, H.↗

NASA NTRS1974

Thermally-restorable optical degradation and the mechanism of current transport in Cu2S-CdS photovoltaic cells

The photovoltaic properties of single-crystal Cu2S-CdS heterojunctions have been investigated as a function of heat treatment by detailed measurements of the dependence of short-circuit current on photon energy, temperature, and the state of optical degradation or enhancement. A coherent picture is formulated for the relationship between enhancement and optical degradation, and their effect on the transport of short-circuit photoexcited current and dark, forward-bias current in the cell. Optical degradation in the Cu2S-CdS cell is shown to be closely identical to optical degradation of lifetime in a homogeneous CdS:Cd:Cu crystal, indicating that the CdS:Cu layer near the junction interface controls carrier transport in the cell. It is proposed that both the photoexcited short-circuit current and the dark, forward-bias current are controlled by a tunneling-recombination process through interface states.

Fahrenbruch, A. L.↗

NASA NTRS1975

Observations of Birkeland currents at auroral latitudes

After many years of languishing interest, field-aligned currents (Birkeland currents) are now regarded as an important link between the magnetosphere and the ionosphere. We describe techniques that are used to measure them and then discuss in some detail measurements that have been interpreted as indicating the presence of Birkeland currents. We conclude that measurements substantiate the theory of large-scale flow from the magnetosphere to the polar caps in a dawn-dusk direction with opposite flow equatorward of the auroral zone connecting to the Alfven layer. Sheet current flow in premidnight discrete aurora seems experimentally established also, but its connection with auroral acceleration and with the larger-scale flow is not yet understood. The currents flowing during a substorm are not yet fully defined either.

Anderson, H. R.↗

NASA NTRS1975

Low-heat-leak current leads for intermittent use

The paper reports on the development of current leads for an orbiting superconducting magnetic spectrometer that were required to achieve a zero-current heat leak of 0.03 W with a total coolant flow rate as low as 0.011 g/sec cooling only the coldest third of the length of a lead. Analysis of heat flow phenomena in an insulated current lead in thermal contact with cryogenic gas flow is given for prediction of lead performance. A basic lead design was then tested and modified by such techniques as tapering, adding superconducting sections, and operating above the optimum current, until a lead satisfying the low-leak requirements was obtained. Operating the insulated leads at well above their optimum current for short periods at high mass flow is possible, and instability behavior is predictable.

Smoot, G. F.↗

NASA NTRS1976

Net field-aligned currents observed by Triad

The existence of a net current flowing into or away from the ionosphere in the current layer is inferred from the Triad-satellite magnetometer observation of a steplike level shift in the east-west component of the magnetic field at 800 km altitude. The current direction is toward the ionosphere on the morning side and away from it on the afternoon side. The current layer is identified as the same layer that was observed by the Ogo 5 satellite at much higher altitudes. Thus, the field-aligned currents observed by Triad must be considered to be an important element in the electrodynamical coupling between the distant magnetosphere and the ionosphere.

Sugiura, M.↗

NASA NTRS1976

Ring current proton decay by charge exchange

Explorer 45 (S3-A) measurements were made during the recovery phase of the moderate magnetic storm of February 24, 1972, in which a symmetric ring current had developed and effects due to asymmetric ring current losses could be eliminated. It was found that after the initial rapid decay of the proton flux, which is a consequence of the dissipation of the asymmetric ring current, the equatorially mirroring protons in the energy range 5-30 keV decayed throughout the L value range of 3.5-5.0 at the charge exchange decay rate calculated by Liemohn (1961). After several days of decay, the proton fluxes reached a lower limit where an apparent equilibrium was maintained, between weak particle source mechanisms and the loss mechanisms, until fresh protons were injected into the ring current region during substorms. While other proton loss mechanisms may also be operating, the results indicate that charge exchange is more than sufficient as a particle loss mechanism for the storm time proton ring current decay.

Smith, P. H.↗

NASA NTRS1976

Radiation from a current filament driven by a traveling wave

Solutions are presented for the electromagnetic fields radiated by an arbitrarily oriented current filament located above a perfectly conducting ground plane and excited by a traveling current wave. Both an approximate solution, valid in the fraunhofer region of the filament and predicting the radiation terms in the fields, and an exact solution, which predicts both near and far field components of the electromagnetic fields, are presented. Both solutions apply to current waveforms which propagate along the channel but are valid regardless of the actual waveshape. The exact solution is valid only for waves which propagate at the speed of light, and the approximate solution is formulated for arbitrary velocity of propagation. The spectrum-magnitude of the fourier transform-of the radiated fields is computed by assuming a compound exponential model for the current waveform. The effects of channel orientation and length, as well as velocity of propagation of the current waveform and location of the observer, are discussed. It is shown that both velocity of propagation and an effective channel length are important in determining the shape of the spectrum.

Levine, D. M.↗

NASA NTRS1977

Current-limited electron beam injection

The injection of an electron beam into a weakly collisional, magnetized background plasma was investigated experimentally. The injected beam was energetic and cold, the background plasma was initially isothermal. Beam and plasma dimensions were so large that the system was considered unbounded. The temporal and spatial evolution of the beam-plasma system was dominated by collective effects. High-frequency electrostatic instabilities rapidly thermalized the beam and heated the background electrons. The injected beam current was balanced by a return current consisting of background electrons drifting toward the beam source. The drift between electrons and ions gave rise to an ion acoustic instability which developed into strong three-dimensional turbulence. It was shown that the injected beam current was limited by the return current which is approximately given by the electron saturation current. Non-Maxwellian electron distribution functions were observed.

Stenzel, R. L.↗

NASA NTRS1977

Observation of a structured auroral field-aligned current system

A rocketborne experiment package was launched over a structured auroral arc from Poker Flat, Alaska, at 0722 UT on February 25, 1972. The results of the flight vector-magnetometer data have been detailed by Casserly and Cloutier (1975), who report the detection of a pair of oppositely directed Birkeland current sheets associated with the auroral band. This association has now been extensively investigated, and it has been shown that some observed structures in the arc due to the presence of a fold are also evident in the Birkeland current system. In particular, the field-aligned current to the east of the fold is at a higher geomagnetic latitude than the current to the west of the fold. Variations in other model current parameters are also shown to be well correlated with changes in the visual arc system.

Casserly, R. T., Jr.↗

NASA NTRS1977

Induced charging of shuttle orbiter by high electron-beam currents

Emission of high-current electron beams that was proposed for some Spacelab payloads required substantial return currents to the orbiter skin in order to neutralize the beam charge. Since the outer skin of the vehicle was covered with approximately 1200 sq m of thermal insulation which has the dielectric quality of air and an electrical conductivity that was estimated by NASA at 10 to the -9 power to 10 to the -10 power mhos/m, considerable transient charging and local potential differences were anticipated across the insulation. The theory for induced charging of spacecraft due to operation of electron guns was only developed for spherical metal vehicles and constant emission currents, which were not directly applicable to the orbiter situation. Field-aligned collection of electron return current from the ambient ionosphere at orbiter altitudes provides up to approximately 150 mA on the conducting surfaces and approximately 2.4 A on the dielectric thermal insulation. Local ionization of the neutral atmosphere by energetic electron bombardment or electrical breakdown may provide somewhat more return current.

Liemohn, H. B.↗

NASA NTRS1978

Structure of current sheets in magnetic holes at 1 AU

Current density profiles in several types of interplanetary magnetic holes were calculated assuming that the currents flow in planar sheets and that the magnetic field varies only in the direction normal to the sheet. The planarity was verified in four holes which were observed by two suitably spaced spacecraft. The structure of the current sheets ranges from very simple in some holes to very complex in others. The observed structures are found to be qualitatively consistent with models based on self-consistent solutions of Vlasov's equation and Maxwell's equations. Examples of complex, irregular magnetic holes are also presented, and they are shown to contain multiple, current sheets in which currents flow parallel to one another.

Fitzenreiter, R. J.↗

NASA NTRS1977

Reverse current in solar flares

We examine the proposal that impulsive X-ray bursts are produced by high-energy electrons streaming from the corona to the chromosphere. It is known that the currents associated with these streams are so high that either the streams do not exist or their current is neutralized by a reverse current. Analysis of a simple model in which the reverse current is stable indicates that the primary electron stream leads to the development of an electric field in the ambient corona which (a) decelerates the primary beam and (b) produces a neutralizing reverse current. It appears that, in some circumstances, this electric field could prevent the primary beam from reaching the chromosphere. In any case, the electric field acts as an energy exchange mechanism, extracting kinetic energy from the primary beam and using it to heat the ambient plasma. This heating is typically so rapid that it must be expected to have important dynamical consequences.

Knight, J. W.↗

NASA NTRS1978

Direct measurements of the atmospheric conduction current

A method of measuring the atmospheric conduction current above the ground has been employed to obtain data for 12 weeks during the first half of 1974. The instrument consists of a split aluminum sphere suspended by insulated wires to a wooden frame. The measuring electronics and the transmitter are enclosed within the spherical structure. The interaction of the instrument with its atmospheric electrical environment is analyzed, and it is shown that in steady state conditions, predictable differences in the instrumentally measured currents and the atmospheric conduction current will be less than 5% and in the nonsteady state situations the difference is less than 20%. Diurnal variations, a probable winter-summer variation, sunrise, and fog effects were observed for the data obtained during fair-weather conditions. Disturbed weather data are interpreted for the effects of low clouds on the atmospheric current. The charge concentrations within overcast clouds sufficient to produce the observed reversed atmospheric currents are estimated to be small in relation to values in thunderclouds.

Burke, H. K.↗

NASA NTRS1980

Calibration tests on magnetic tape lightning current detectors

The low cost, passive, peak lightning current detector (LCD) invented at the NASA/Kennedy Space Center, uses magnetic audio recording tape to sense the magnitude of the peak magnetic field around a conductor carrying lightning currents. Test results show that the length of audio tape erased was linearly related to the peak simulated lightning currents in a round conductor. Accuracies of + or - 10% were shown for measurements made using a stopwatch readout technique to determine the amount of tape erased by the lightning current. The stopwatch technique is a simple, low cost means of obtaining LCD readouts and can be used in the field to obtain immediate results. Where more accurate data are desired, the tape is played and the output recorded on a strip chart, oscilloscope, or some other means so that measurements can be made on that recording. Conductor dimensions, tape holder dimensions, and tape formulation must also be considered to obtain a more accurate result. If the shape of the conductor is other than circular (i.e., angle, channel, H-beam), an analysis of the magnetic field is required to use an LCD, especially at low current levels.

Crouch, K. E.↗