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At least 1,369 records · Page 76
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NASA NTRS1986

Joule heating and field-aligned currents: Preliminary results from DE-2

There are three main processes by which energy is transferred from the magnetosphere to the thermosphere: (1) charge exchange of the ring current particles; (2) precipitation of charged particles; and (3) joule dissipation by the magnetosphere-ionosphere current systems. The importance of this last process has been recognized and the rate of joule heating has been estimated by many workers. Observations of the electric (E) and magnetic (B) fields from Dynamics Explorer Satellite 2 are providing a new set of data on field-aligned currents. One of the remarkable features found in these observations is the high correlation between an orthogonal pair of the E and B field components. In recent years, observational data have accrued concerning the relationship between the interplanetary magnetic field and the size of the polar cap and also about the evolution of a substorm or a magnetic storm. It is suggested that these findings be incorporated in future model calculations.

Sugiura, M.↗
NASA NTRS1985

Current collection from the space plasma through defects in solar array insulation

Operating high-voltage solar arrays in the space environment can result in anomalously large currents being collected through small insulation defects. Tests simulating the electron collection have shown that there are two major collection modes. The first involves current enhancement by means of a surface phenomenon involving secondary electron emission from the surrounding insulator. In the second mode, the current collection is enhanced by vaporization and ionization of the insulator material, in addition to the surface enhancement of the first mode. The electron collection due to surface enhancement (first mode) has been modeled. Using this model, simple calculations yield realistic predictions.

Robinson, R. S.↗
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

Collisional tearing instability in the current sheet with a low magnetic Lundquist number

An MHD model is used to calculate the growth rate of the collisional tearing mode instability in a current layer with a Lundquist number in the range 3-100,000. The maximum growth rate is found to be a function of the Alfven transit time in the current sheet and the Lundquist number. When the Lundquist number is below 50, the growth rate asymptotically approaches a value of about -0.075/transit time. The discovery of the dependence of the asymptotic value is considered to be of use in studies of the current sheet in the diffusion region of a magnetic reconnection configuration, where the Lundquist number will be in the range 2-100.

Lee, L. C.↗
NASA NTRS1986

Dc and microwave characteristics of a high current double interface GaAs/InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistor

Extremely large current double interface GaAs/In(0.15)Ga(0.85)As/Al(0.15)Ga(0.85)As pseudomorphic modulation-doped field-effect transistors (MODFET's) grown by molecular beam epitaxy were achieved. The 1-micron gate devices studied have peak current levels (430 mA/mm at 300 K and 483mA/mm at 77 K) roughly one and a half to two times that found in single interface pseudomorphic MODFET's. These devices also retain high transconductances over a broad range of gate voltage, peaking at 312 mS/mm at 300 K and 362 mS/mm at 77 K. Excellent microwave performance is also obtained with a maximum frequency of oscillation (f-max) of 37 GHz and a current gain cut-off frequency of as high as 23 GHz at 300 K. An output power level of 14 dBm (1 dB gain compression) was obtained at 6 GHz for a 290-micron gate width. This double interface single quantum well MODFET may be of great importance in millimeter wave power amplifiers.

Henderson, T.↗
NASA NTRS1985

Thunderstorm currents and lightning charges at the NASA Kennedy Space Center

The NASA Kennedy Space Center, (KSC) and the U.S. Air Force are currently operating an extensive network of ground-based electric-field mills in order to detect atmospheric electrical hazards to ground operations, launches, and landings of spacecraft. Over the past 10 years, the University of Arizona has utilized data provided by this network, and other instrumentation, to investigate the electrical structure of thunderstorms and lightning in Florida. Recent analyses have included estimates of the Maxwell current density that thunderstorms produce at the ground and computations of the locations and magnitudes of lightning-caused changes in the cloud charge distribution. This work is reviewed, and an example of how thunderstorm current patterns and lightning charges develop in both space and time is presented.

Krider, E. P.↗
NASA NTRS1986

Preliminary measurements of very low dark currents in MLA/SWIR HgCdTe/DRO arrays

In infrared astronomy observations, one of the key detector performance parameters is the dark current which, together with the optics, establishes the fundamental limit of a space-based instrument. This technical note summarizes an effort to measure dark current performance of the MLA/Short Wave Infrared photovoltaic HgCdTe array (2.5 micron cutoff) at temperatures from 65 to 120 K. The preliminary results indicate that dark current decreased systematically to 100 K, then the value became measurement-limited at about 3.0 E-18 A/pixel, or 19 electrons/sec/pixel.

Yang, B. T.↗
NASA NTRS1986

Eddy currents in a conducting sphere

This report analyzes the eddy current induced in a solid conducting sphere by a sinusoidal current in a circular loop. Analytical expressions for the eddy currents are derived as a power series in the vectorial displacement of the center of the sphere from the axis of the loop. These are used for first order calculations of the power dissipated in the sphere and the force and torque exerted on the sphere by the electromagnetic field of the loop.

Bergman, John↗
NASA NTRS1986

The heliospheric current sheet - 3-dimensional structure and solar cycle changes

The reversal in polarity of the interplanetary current sheet/sector structure is investigated during the recent solar maximum. Multipoint observations by ISEE-3 and Pioneer 11 show that a simple two sector or occasional four sector structure persisted throughout the maximum and out to distances of 10 AU. The polarity reversal occurred between March 1979 and October 1980, without any indication of an abrupt transition that might permit a more precise timing. The reversal coincided approximately with the reversal in the sun's polar cap fields. The current sheet appeared to be highly inclined during the ascending and descending phases of the solar cycle but was apparently too complex to describe as a simple inclined current sheet during solar maximum.

Smith, E. J.↗
NASA NTRS1986

Solar wind speed azimuthal variation along the heliospheric current sheet

Analysis of the speeds measured by Voyager 1 and 2 while skimming along a horizontal (east-west) portion of the current sheet over several days in 1977 is reported. The results demonstrate that in this case speed variations exist and would be large enough to significantly deform the sheet within a few AU or less if the current sheet were anything but perfectly horizontal. The spatial scale of the speed variation ranges from the smallest measureable scale using one hour averaged data up to tens of degrees in longitude. A deformation example is given under the assumption that the observed velocity variation exists on a current sheet that is initially perpendicular to the heliographic equator.

Suess, S. T.↗
NASA NTRS1986

Response of an emerging flux tube to a current-driven instability

Emerging magnetic flux plays an important role in the development of active regions on the sun and, perhaps, in the subsequent activation of flares. However, the energy input that produces preflare brightenings and flares probably does not come from the flux emergence itself but from one or more associated energy-releasing processes - likely candidates include magnetic reconnection and various current-driven plasma micro-instabilities. Here the interplay between the changing physical characteristics of an emerging magnetic-flux tube and the onset and evolution of a representative 'bump-on-tail' plasma current-driven instability is investigated. The microinstability heats the ambient material, thus changing the macroscopic characteristics of the plasma in which the model and current-driven instability occurs.

Karpen, J. T.↗
NASA NTRS1985

Measurement and analysis of solar cell current-voltage characteristics

Approaches to measurement and analysis of solar cell current-voltage characteristics under dark and illuminated conditions are discussed. Measurements are taken with a computer based data acquisition system for temperatures in the range of -100 to +100 C. In the fitting procedure, the various I(oi) and C(i) as well as R(S) and R(SH) are determined. Application to current-voltage analyses of high efficiency silicon cells and Boeing CdS/CuInSe2 are discussed. In silicon MINP cells, it is found that at low voltages a tunneling mechanism is dominant, while at larger voltages the I-V characteristics are usually dominated by emitter recombination. In the case of Boeing cells, a current transport model based on a tunneling mechanism and interface recombination acting in series has been developed as a result of I-V analyses.

Olsen, Larry C.↗
NASA NTRS1986

Modeling substorm current systems using conductivity distributions inferred from DE auroral images

The first attempt to systematically use DE 1 auroral images to infer ionospheric conductivities is presented. These conductivities are then used to compute the distributions of ionospheric and field-aligned current patterns during auroral substorms. It is concluded that the western electrojet is, in general, collocated with the region of high auroral luminosity, while the region of relatively low luminosity in the evening sector is collocated with the eastward electrojet. The upward field-aligned currents exist in the brightest auroral region on the poleward side of the evening auroral oval and on the equatorward side of the morning oval. A significant amount of ionospheric currents can flow in regions where there are no bright auroral emissions.

Kamide, Y.↗
NASA NTRS1987

Radiation from large space structures in low earth orbit with induced ac currents

Large conducting space structures in low earth orbit will have a nonnegligible motionally induced potential across their structures. The induced current flow through the body and the ionosphere causes the radiation of Alfven and lower hybrid waves. This current flow is taken to be ac and the radiated power is studied as a function of the ac frequency. The current may be ac due either to inductive coupling from the power system on the structure or by active modulation. A Space Station-like structure and tether are studied. For the Space Station structure the radiation impedance is particularly high for frequencies in the tens of kilohertz range which suggests that the Space Station may be efficient source of lower hybrid waves. The tether is also shown to be a generator of VLF waves up to source ac frequencies in the megahertz range. The implications for these two structures are discussed.

Hastings, D. E.↗
NASA NTRS1986

Heliospheric current sheet and its solar cycle variations

The three-dimensional structure of the heliospheric current sheet is constructed for Carrington rotations 1647, 1654, 1661, 1667, 1693, 1699, and 1719. These rotations are chosen from different epochs during the sunspot cycle 21, so that geometrical changes of the current sheet during a sunspot cycle can be inferred. Only for relatively simple neutral lines on the source surface, do the structures of the current sheet approximate those envisaged by Svalgaard and Wilcox (1976) and Thomas and Smith (1981).

Akasofu, Syun-Ichi↗
NASA NTRS1987

The influence of eddy currents on magnetic actuator performance

The present investigation of the effects of eddy currents on EM actuators' transient performance notes that a transfer function representation encompassing a first-order model of the eddy current influence can be useful in control system analysis. The method can be extended to represent the higher-order effects of eddy currents for actuators that cannot be represented by semiinfinite planes.

Zmood, R. B.↗