Engineering topics

Mcnutt, R. L., Jr.

Publications and source records attributed to Mcnutt, R. L., Jr..

At least 19 records

NASA NTRS1995

The distance to the heliospheric VLF emission region

Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983/84 and 1992/3. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approximately 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approximately 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approximately 115 AU.

Mcnutt, R. L., Jr.↗

NASA NTRS1993

Large-scale density structures in the outer heliosphere

The Plasma Science experiment on the Voyager 2 spacecraft has measured the solar wind density from 1 to 38 AU. Over this distance, the solar wind density decreases as the inverse square of the heliocentric distance. However, there are large variations in this density at a given radius. Such changes in density are the dominant cause of changes in the solar wind ram pressure in the outer heliosphere and can cause large perturbations in the location of the termination shock of the solar wind. Following a simple model suggested by Suess, we study the non-equilibrium, dynamic location of the termination shock as it responds to these pressure changes. The results of this study suggest that the termination shock is rarely if ever at its equilibrium distance and may depart from that distance by as much as 50 AU at times.

Belcher, J. W.↗

NASA NTRS1993

A low-mass faraday cup experiment for the solar wind

Faraday cups have proven to be very reliable and accurate instruments capable of making 3-D velocity distribution measurements on spinning or 3-axis stabilized spacecraft. Faraday cup instrumentation continues to be appropriate for heliospheric missions. As an example, the reductions in mass possible relative to the solar wind detection system about to be flown on the WIND spacecraft were estimated. Through the use of technology developed or used at the MIT Center for Space Research but were not able to utilize for WIND: surface-mount packaging, field-programmable gate arrays, an optically-switched high voltage supply, and an integrated-circuit power converter, it was estimated that the mass of the Faraday Cup system could be reduced from 5 kg to 1.8 kg. Further redesign of the electronics incorporating hybrid integrated circuits as well as a decrease in the sensor size, with a corresponding increase in measurement cycle time, could lead to a significantly lower mass for other mission applications. Reduction in mass of the entire spacecraft-experiment system is critically dependent on early and continual collaborative efforts between the spacecraft engineers and the experimenters. Those efforts concern a range of issues from spacecraft structure to data systems to the spacecraft power voltage levels. Requirements for flight qualification affect use of newer, lighter electronics packaging and its implementation; the issue of quality assurance needs to be specifically addressed. Lower cost and reduced mass can best be achieved through the efforts of a relatively small group dedicated to the success of the mission. Such a group needs a fixed budget and greater control over quality assurance requirements, together with a reasonable oversight mechanism.

Lazarus, A. J.↗

NASA NTRS1992

Plasma observations near Neptune - Results from Voyager 2

Results from observations made by the Plasma Science experiment on Voyager 2 at Neptune are reviewed. The magnetosphere of Neptune is filled with a tenuous plasma, which consists of at least two components: a light ion, probably H(+) and a heavy ion, probably N(+). Triton's atmosphere or ionosphere is thought to be the source of both heavy and light ions. Much of the low energy plasma in the inner magnetosphere is concentrated near the magnetic equator and near closest approach to the planet. The large tilt of the magnetic dipole axis from the rotation axis produces a dynamic magnetosphere which goes from an earthlike configuration to a pole-on configuration and back every 16 hours. The polar cusp regions change location and size as the planet rotates; at the time of the inbound magnetopause crossing, the phase of Neptune's rotation was such that the spacecraft entered the magnetosphere through the southern polar cusp region. Outbound from Neptune observations made in the magnetosheath show a possible signature of diurnal oscillation of the plasma mantle that grows, shrinks, and rocks, in a diurnal cycle. After the encounter with Neptune's magnetosphere, an upstream wave event was observed when the interplanetary magnetic field connected to the bow shock. The low frequency waves observed appear to be a mixture of Alfvenic and/or fast mode waves propagating away from the planet.

Zhang, Ming↗

NASA NTRS1992

The abundance of O(2+) in the Jovian magnetosphere

From a synthesis of data from the Plasma-Science and Ultraviolet-Science instruments on Voyager 1 a radial profile is presented of O(2+) abundance between 4.9 and 42 Jovian radii. A sharp rise is noted in O(2+) mixing ratio near 7.5 Jovian radii, coincident with a sharp rise in effective electron temperature at the outer boundary of the Io plasma torus. Beyond 8.5 Jovian radii the O(2+) mixing ratio is found to be roughly constant which indicates freezing of the ionization prevailing at the outer edge of the hot torus.

Bagenal, F.↗

NASA NTRS1992

Global properties of the plasma in the outer heliosphere. I - Large-scale structure and evolution

Pioneers 10 and 11, and Voyager 2, have active plasma analyzers as they proceed through heliocentric distances of the order of 30-50 AU, facilitating comparative studies of the global character of the outer solar wind and its variation over the solar cycle. Careful study of these data show that wind ion temperature remains constant beyond 15 AU, and that there may be large-scale variations of temperature with celestial longitude and heliographic latitude. There has thus far been no indication of a heliospheric terminal shock.

Barnes, A.↗

NASA NTRS1991

The plasma environment of Uranus

An overview of the observational results on the plasma environment at Uranus is given, and the implications of these observations for magnetospheric physics at Uranus are discussed. During the Voyager 2 encounter with Uranus, an extended magnetosphere filled with a tenuous plasma was detected. This low-energy plasma was found to consist of protons and electrons, with no significant heavy ion contribution, and with a density in the regions sampled by the spacecraft of at most three electrons per cubic centimeter. The plasma electrons and ions exhibit both a thermal component (with temperatures of tens of eV) and a hot component (with temperatures of a few keV). The thermal ion component is observed both inside and outside an L-shell value near 5, whereas the hot ion and electron component is excluded from the region inside of that L-shell. The source of the thermal component of the plasma is either the planetary ionosphere or the neutral hydrogen corona surrounding Uranus, whereas the hot component is convected in from the magnetotail, with probably an ionospheric source.

Belcher, J. W.↗

NASA NTRS1990

Thermal plasma in outer planet magnetospheres

The plasma environments of the outer planets are a study in contrasts. The magnetosphere of Jupiter is dominated by the prodigious plasma output of Io, with losses due to diffusion driven by mass loading. At Saturn, the small icy satellites are the major sources of plasma for the inner magnetosphere. The low mass loading rates there imply that the densities of the plasma tori are limited by dissociative recombination, rather than diffusive transport. At Uranus, the icy satellites are negligible plasma sources compared to the input from the extended neutral hydrogen cloud and the ionosphere. Convection driven by the solar wind penetrates deep into the inner magnetosphere because of the unique orientation of the rotation axis of Uranus. The expected magnetosphere of Neptune is similar to that of Saturn and Jupiter, with Triton, the ring arcs, and the planet as possible plasma sources. The Voyager 2 encounter with Neptune holds out the hope of a passage through a nonterrestrial auroral region, a unique event in planetary exploration.

Belcher, J. W.↗

NASA NTRS1989

Plasma observations near Neptune - Initial results from Voyager 2

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptune's magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptune's magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Triton's atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptune's magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

Belcher, J. W.↗

NASA NTRS1989

Remote sensing of the termination of the solar wind via in situ plasma measurements

Since 1983 the Plasma Wave experiments on the two Voyager spacecraft have detected low frequency radio noise in the outer heliosphere which has been postulated to emanate from the terminal shock of the solar wind or, possibly, from the heliopause itself. The solar wind data from the Plasma Science experiment on the Voyager spacecraft are studied to search for correlations with these radio emissions. It is found that two anomalous high speed streams passed Voyager 2 in late 1982 and early 1983 and it is suggested that the interaction of the streams with the heliospheric terminal shock is responsible for the generation of the most intense radio noise observed later in the same year. If the stream speeds did not decrease in traveling to the interaction region, that region is about 135 AU from the sun. This is consistent with previous estimates of the distance to the inner heliospheric shock.

Mcnutt, R. L., Jr.↗

NASA NTRS1988

Meridional plasma flow in the outer heliosphere

Voyager 2 observations made in the outer heliosphere near 25 AU and within 2 deg of the heliographic equatorial plane show periodic variations in the meridional (North/South) flow velocities that are much more prominent than the East/West variations. An autocorrelation analysis shows that the flow variation has a period of about 25.5 days in the latter half of 1986, in approximate agreement with the solar rotation period. The results suggest that increased pressure in interaction regions remains the best candidate for the driver of the nonradial flows.

Lazarus, A. J.↗

NASA NTRS1988

Possible explanations of north-south plasma flow in the outer heliosphere and meridional transport of magnetic flux

Observed meridional plasma flow and its connection with other plasma parameters in the outer heliosphere are discussed. The dynamics of the flow are examined locally and compared with observed plasma parameters and a global flow model which predicts such flows in a steady solar wind. The observational evidence supports stream dynamics and associated pressure gradients as responsible for driving the flow. Such a meridional flow may result in a net transport of magnetic flux from regions near the heliographic equator. The amplitude of the observed meridional component of solar wind flow is consistent with observed magnetic flux deficits in the outer heliosphere. The limited coverage of heliographic latitude by Voyager 2 precludes a direct measurement of the full flow pattern; however, the magnitude of reported magnetic flux deficits and the unambiguous, regular variations in the meridional flow suggest that the stream interactions do produce a net movement of magnetic flux away from the heliographic equator.

Mcnutt, R. L., Jr.↗

NASA NTRS1987

The low energy plasma in the Uranian magnetosphere

The Plasma Science experiment on Voyager 2 detected a magnetosphere filled with a tenuous plasma, rotating with the planet. Temperatures of the plasma, composed of protons and electrons, ranged from 10 eV to about 1 keV. The sources of these protons and electrons are probably the ionosphere of Uranus or the extended neutral hydrogen cloud surrounding the planet. As at earth, Jupiter, and Saturn, there is an extended magnetotail with a central plasma sheet. Although similar in global structure to the magnetospheres of these planets, the large angle between the rotation and magnetic axes of the planet and the orientation of the rotation axis with respect to the solar wind flow make the Uranian magnetosphere unique.

Mcnutt, R. L., Jr.↗

NASA NTRS1986

Plasma observations near Uranus - Initial results from Voyager 2

The results of observations of the spatial distribution and physical properties of the space plasma near Uranus with instrumentation on board Voyager 2 are described. The data revealed the existence of a magnetosphere that held a warm component with a temperature of 4-50 eV and a peak density of 2 protons/cu cm and a hot component with a temperature of a few electron volts and a density of about 0.1 proton/cu cm. Only the warm component was observed within the L shell. The numerous crossings made of the plasma sheet in the magnetotail were at locations which suggested that the magnetotail has a geometric structure similar to that of the earth magnetotail. Finally, possible sources of the magnetospheric plasma particles are discussed.

Bridge, H. S.↗

NASA NTRS1985

Revised ion temperatures for Voyager plasma measurements in the Io plasma torus

A calculation error in previous computations of ion temperatures in the Io plasma torus of the Jovian magnetosphere from Voyager plasma-science-experiment measurements is reported, and its effects on subsequently published studies are evaluated. It is found that the temperatures reported by Bagenal et al. (1980) and Bagenal and Sullivan (1981) for Jupiter and by Bridge et al. (1981) for Saturn are half the correct values, with major effects on ionic-species scale heights, plasma-density maps, and flux-tube content estimations. The temperatures given by Bridge et al. (1979) and McNutt et al. (1981) are not affected by the error. A corrected isodensity contour map is presented, and uncertainties in the measurement of ion temperatures are discussed.

Bagenal, F.↗

NASA NTRS1983

Low-energy plasma ion observations in Saturn's magnetosphere

Attention is given to observational data gathered by Voyager plasma experiments in Saturn's magnetosphere which indicate that the number density and temperature of the plasma in the outer magnetosphere are very variable. The H(+) and either O(+) or N(+) ionic components resolved do not have the same thermal speeds or temperatures. There is some evidence for inward and outward radial flow, in addition to the azimuthal motion, in approximately one-third of the Voyager 1 cold ion spectra. The sources of the plasma are still undetermined. The plasma sheet becomes well established within 16 Saturn radii from the planet in the Voyager 1 data, and within 10 radii in the Voyager 2 data. In general, the plasma and magnetic field appear to be in dynamical equilibrium. In comparing the Saturn magnetosphere to that of Jupiter, much less acceleration of low energy plasma to high energies is found in Saturn.

Lazarus, A. J.↗

NASA NTRS1983

Heavy ions in the outer Kronian magnetosphere

The possible sources of the cold plasma observed in the outer magnetosphere of Saturn are analyzed. On the basis of the O(+)-H charge exchange species-specific loss mechanism, as well as abundance and rate considerations, it is concluded that the dominant heavy ion populating the equatorial outer magnetosphere is that of atomic nitrogen. Possible sources of hot plasma are also discussed, as are the inhibition of corotation by mass loading and the radial variation of composition. It is found that the observed deviations from corotation and current mass loading estimates indicate either a somewhat higher ionospheric conductance than is implied by the UVS and RSS measurements, or an overestimate of mass loading. It is suggested that the plasma gap observed by Voyager 1 outbound may be associated with a composition change.

Eviatar, A.↗

NASA NTRS1983

Force balance in the magnetospheres of Jupiter and Saturn

Spacecraft measurements of the plasma populations and magnetic fields near Jupiter and Saturn have revealed that large magnetospheres surround both planets. Magnetic field measurements have indicated closed field line topologies in the dayside magnetospheres of both planets while plasma instruments have shown these regions to be populated by both hot and cold plasma components convected azimuthally in the sense of planetary rotation. By using published data from the Voyager Plasma Science (PLS), Low Energy Charged Particle (LECP), and Magnetometer (MAG) instruments, it is possible to investigate the validity of the time stationary MHD momentum equation in the middle magnetospheres of Jupiter and Saturn. At Saturn, the hot plasma population is negligible in the dynamic sense and the centrifugal force of the cold rotating plasma appears to balance the Lorentz force. At Jupiter, the centrifugal force balances about 25 percent of the Lorentz force. The remaining inward Lorentz force is balanced by pessure gradients in the hot, high-beta plasma of the Jovian magnetodisk.

Mcnutt, R. L., Jr.↗