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At least 1,603 records · Page 89
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NASA NTRS1986

The physics of the cometary contact surface

The contact surface, which separates outflowing cometary plasma from solar wind controlled cometary plasma, is explained in terms of a balance between the magnetic pressure gradient force and ion-neutral drag. Giotto data indicate that the plasma pressure inside the contact surface cannot balance the sum of the external plasma and magnetic pressures, and therefore a Venus-like ionopause is not present at the contact surface. An expression for the magnetic field strength as a function of cometocentric distance is derived from the momentum equation. The theoretical magnetic field profile agrees quite well with the profile measured by the Giotto magnetometer in the vicinity of the contact surface.

Cravens, Thomas E.↗
NASA NTRS1986

Three-dimensional structure of the heliosphere as inferred from observations with a Japanese Halley spacecraft

A sinusoidal neutral line with a pair of giant regions appeared on the sun about one year before the launch of 'Sakigake', the first of two Japanese Comet Halley spacecraft. The Sakigake magnetometer data during the early part of the mission (February-March 1985) are well interpreted by an eastward shift of the tilting neutral sheet. The shift is further explained by an effect of a new giant region appearing at about 10 deg heliolatitude and about 50 deg Carrington longitude in August 1984. The toward polarity ratio of IMF observed by Sakigake changed from about 22 percent in February to about 62 percent in early June and then increased rapidly up to 98 percent. This ratio is interpreted as a decrease of the tilt angle of the sheet down to only about 4 deg. It is the first spacecraft observation of 'the disappearing sector structure' with such small tilt angle.

Saito, T.↗
NASA NTRS1986

Balloon gondola diagnostics package

In order to define a new gondola structural specification and to quantify the balloon termination environment, NASA developed a balloon gondola diagnostics package (GDP). This addition to the balloon flight train is comprised of a large array of electronic sensors employed to define the forces and accelerations imposed on a gondola during the termination event. These sensors include the following: a load cell, a three-axis accelerometer, two three-axis rate gyros, two magnetometers, and a two axis inclinometer. A transceiver couple allows the data to be telemetered across any in-line rotator to the gondola-mounted memory system. The GDP is commanded 'ON' just prior to parachute deployment in order to record the entire event.

Cantor, K. M.↗
NASA NTRS1986

Persistence of solar wind features

Using data from the plasma and magnetometer experiments on board the Voyagers 1 and 2 during the approach to Jupiter, solar wind persistence is investigated over the period from January 1978 (Voyager 1 passing by Voyager 2) through February 1979. The trajectories of both spacecraft provided a unique opportunity to study the radial evolution and variation of the solar wind over about 3 AU, and to analyze the persistence of solar wind features along the radially increasing separation distance of both Voyagers. Some emphasis is placed on a period of DOY (day of year) 152 through 212, 1978, in which the observed propagation delay time of solar wind signatures between both Voyagers significantly deviates from the expected delay time. A decrease in the correlation coefficient of the corresponding Voyager 1 and 2 data profiles indicates a remarkable change of the solar wind flow. This period in question coincides to a great extent with the interval V of June-July 1978, selected by STIP (Study of Travelling Interplanetary Phenomena).

Rucker, H. O.↗
NASA NTRS1987

The ionotail of Venus - Its configuration and evidence for ion escape

The configuration and morphology of the plasma clouds in the ionotail of Venus (revealed by the Pioneer Venus Orbiter) are studied, and the rate of planetary ion escape, which may be associated with the dissipation and removal of the ionospheric plasma, is estimated. The data supplied by the Orbiter's instruments, the Orbiter electron temperature probe, the ion mass spectrometer, the neutral mass spectrometer, the magnetometer, and the plasma analyzer, are analyzed, and the results of the observations are discussed.

Brace, L. H.↗
NASA NTRS1987

Enhancements of energetic ions associated with travelling compression regions in the deep geomagnetic tail

This paper presents a representative example of an enhancement in energetic ion flux associated with the International Sun-Earth Explorer 3 (ISEE 3) spacecraft's encounter with a traveling compression region (TCR). Data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3 are studied, along with magnetic field data from the vector helium magnetometer. It is concluded that the ion enhancements seen are spatial in nature, thus supporting the idea that TCRs are the lobe signatures of plasmoids moving along the magnetotail, away from earth.

Murphy, N.↗
NASA NTRS1985

The Galileo attitude and articulation control system - A radiation-hard, high precision, state-of-the-art control system

The Galileo mission and spacecraft, consisting of a Jupiter-orbiter and an atmospheric entry probe, are discussed. Components will include: magnetometers and plasma-wave antennas on a boom, high-gain antenna, probe vehicle, two different bus electronics packages, and a radioisotope thermoelectric generator. Instruments, investigators and objectives are tabulated for both probe science and orbiter science investigations. Requirements in the design of the attitude and articulation control system are very stringent because of the complex dynamics, flexible body effects, the need for autonomy, and the severe radiation environment in the Jupiter nighborhood. Galileo was intended to be ready for launch via Space Shuttle in May of 1986.

Stephenson, R. Rhoads↗
NASA NTRS1987

Upper limit on the intrinsic magnetic field of Venus

A data set of 18,000 low-altitude nightside vector measurements from the Pioneer Venus Orbiter (PVO) magnetometer is used to establish a new upper limit on the intrinsic dipolar magnetic moment of Venus. Various techniques are used to search for coherence in the magnetic observations which might be attributable to such an intrinsic field, with negative results. A similar search is conducted for surface magnetism, that is, geographically correlated magnetic features, also with negative results. The present study represents the best limits that PVO observations can provide and is limited by the amount of low-altitude nightside data available. New upper limit to the magnetic moment is 8.4 x 10 to the 10th T/cu m or about 10 to the -5th of the terrestrial magnetic moment. This is a factor of four lower than the previous upper limit.

Phillips, J. L.↗
NASA NTRS1987

Do flux transfer events cause long-period micropulsations in the dayside magnetosphere?

Russell and Elphic (1979) suggested the possibility that flux transfer events (FTEs) generate long-period micropulsations in the outer, dayside magnetosphere. The reasons for this hypothesis are examined and results are presented for two pulsation surveys designed to test it. In the first survey, observations of micropulsations by GOES 2 are compared with ISEE 3 and IMP observations of interplanetary magnetic field (IMF) direction; since FTEs are highly correlated with southward IMF, a similar correlation between pulsations and southward IMF was sought. In the second survey, a catalog of ISEE 1 and 2 magnetopause crossings is compared with magnetometer readings from GOES 2 and 3 to determine whether Pc 4 and Pc 5 pulsations are more likely to occur after FTEs have taken place. Results indicate that there is a correlation between FTEs and transversely polarized Pc 4 pulsations with periods between 60 and 120 s, but not between FTEs and Pc pulsations with periods greater than 120 s. This suggests that the generating (or driving) frequency associated with FTEs lies between 60 and 120

Gillis, E. J.↗
NASA NTRS1987

Magnetic field draping in the Comet Halley coma - Comparison of Vega observations with computer simulations

During the Vega-1 encounter with Comet Halley, the magnetometer observed draping and compression of the interplanetary magnetic field. These are reproduced well by a three-dimensional MHD simulation of the cometary interaction. Rotations in the magnetic field similar to those at closest approach are also observed 2.75 hours earlier. It is suggested that both rotations correspond to the same IMF interval and that the spacecraft had overtaken the plasma and encountered 'older' magnetic field as it penetrated the coma. Analysis of the MHD model indicates that it should take about 3 to 5 hours for a solar wind parcel to pass from the unperturbed solar wind to Vega-1 at closest approach. A simulated magnetic field profile composed of nested sections for different IMF orientations closely resembles the observations. This result supports the hypothesis of layered magnetic orientations in the coma.

Schwingenschuh, K.↗
NASA NTRS1987

A one-dimensional time-dependent model of the magnetized ionosphere of Venus

The behavior and time evolution of the large-scale magnetic fields and plasma of the dayside Venus ionosphere are studied using a one-dimensional model. The coupled continuity, momentum, and Maxwell's equations are solved simultaneously for O(+), O2(+), and H(+), and the magnetic field. The calculated magnetic field profiles are in good agreement with Pioneer Venus orbiter magnetometer observations. The magnetic field structure is quasi-steady for slow changes of the solar wind dynamic pressure. The peak at 165 km is maintained by downward convection from higher altitudes. The time scale for the decay of the field by the pure one-dimensional vertical diffusion/convection process is several hours unless the flux is resupplied from the top of the ionosphere.

Shinagawa, H.↗
NASA NTRS1987

The Uranian bow shock - Voyager 2 inbound observations of a high Mach number shock

The Voyager 2 magnetometer and plasma detector measured a high Mach number, high beta bow shock on the dayside of the Uranian magnetosphere. Although the average conditions on either side of the shock are consistent with the Rankine-Hugoniot (MHD) relations for a stationary, quasi-perpendicular shock, the data revealed both detailed structure in the transition region as well as considerable variability in the downstream magnetosheath plasma. The bulk plasma parameters and the magnetic field exhibited some of the characteristics of a supercritical shock: an overshoot followed by damped oscillations downstream, consistent with recent theoretical models of high Mach number quasi-perpendicular shocks.

Bagenal, Fran↗
NASA NTRS1987

Detection of a hot plasma component within the core regions of Jupiter's distant magnetotail

Voyager-2 PLS low-energy plasma data and the magnetometer data are combined with the the LECP ion data (E greater than 28 keV) for the distant magnetotail observations (R = 5000-9000 Jupiter radii). A definite enhancement of LECP fluxes within the core regions (where the PLS densities and magnetic-field pressure are lower than in the surrounding regions) is shown, indicating that this hot tenuous plasma is present within the core regions. In general there is a strong anticorrelation between PLS density and LECP fluxes, while a less pronounced anticorrelation between magnetic-field pressure and LECP fluxes is observed. Estimates of LECP pressures suggest that this hot plasma can provide the previously described missing pressure in the core if heavy ions dominate the ion composition. The angular dependence of the LECP data indicates a flow of this hot plasma in the anti-Jupiter direction. This outflowing plasma could be the remnant of the magnetospheric wind observed near Jupiter by LECP. On the basis of this preliminary study, the core regions are found to have similarities to a plasma sheet.

Sittler, Edward C., Jr.↗
NASA NTRS1987

Spaceborne magnetometry

Low orbit (160 km altitude) satellite magnetometry is discussed. Magnetic field simulations and theoretical analyses were performed. They suggest that at 160 km, resolution value is 10, compared to 0.6 for Magsat, and that the satellite can reveal significant information on crustal tectonics. Vector and scalar magnetometers should be carried.

Taylor, P. T.↗
NASA NTRS1987

The comet rendezvous asteroid flyby mission

The Comet Rendezvous Asteroid Flyby Mission (CRAF) is described. After gravity assists from Venus and Earth, the spacecraft will fly by the asteroid 46 Hestia en route to a rendezvous with P/Tempel 2 in Nov. 1996, when the comet is near aphelion. The scientific experiments for the CRAF mission are: an imaging system; a visual and infrared mapping spectrometer; an infrared radiometer; a penetrator carrying a gamma-ray spectrometer, a scanning differential calorimeter and evolved gas analyzer, accelerometers, and temperature probes; a neutral gas and thermal ion mass spectrometer; a secondary ion mass spectrometer for analyzing dust, gas, and thermal ions; a scanning electron microscope and particle analyzer; an X-ray fluorescence and gas chromatographic analyzer for collected dust and ice samples; a dust counter and velocity analyzer; a retarding potential ion mass spectrometer; a supra-thermal ion mass spectrometer and electron analyzer; a magnetometer; and a coordinated radio, electron, and plasma wave analyzer.

Neugebauer, M.↗
NASA NTRS1987

Pioneer 8 traversal of the geomagnetic tail at 1600 R(E)

The Pioneer 8 spacecraft (launched on December 13, 1967) first crossed the geomagnetic tail at a downstream distance of 500-800 R(E) as it left the earth-moon system, and during a two-week period centered on January 23, 1968, the magnetometer, plasma probe and plasma wave instrument all detected specific tail-related phenomena. In April and May of 1985, Pioneer 8 traversed the expected position of the earth's tail once again, at an average downstream distance of 1650 R(E). This report contains a summary of the 1985 Pioneer 8 measurements, and a brief discussion of the limited information available on solar wind variations and terrestrial magnetic activity indicators.

Scarf, Frederick L.↗
NASA NTRS1987

The variation of protons, alpha particles, and the magnetic field across the bow shock of Comet Halley

Data from the Ion Mass Spectrometer and the magnetometer on the Giotto spacecraft are used to examine the structure of the inbound crossing of the Comet Halley bow shock on March 13, 1986. It is found that the velocity decrease, the field strength increase, and the heating of picked up cometary protons occurred over a broad region corresponding to several heavy-ion gyroradii. The solar-wind protons and alphas, on the other hand, were compressed and heated at a narrow structure on the leading edge of the broad shock region.

Neugebauer, M.↗
NASA NTRS1987

An eastward propagating compressional Pc 5 wave observed by AMPTE/CCE in the postmidnight sector

This paper presents a detailed analysis of a compressional Pc 5 wave observed in the postmidnight sector on July 21, 1986, using data from the magnetometer, the charge-energy-mass spectrometer, and the medium-energy particle analyzer aboard the AMPTE/Charge Composition Explorer (CCE) spacecraft. The Pc 5 wave exhibited harmonically related transverse and compressional magnetic oscillations, modulation of the flux of medium energy protons, and a large azimuthal wave number, i.e., properties that are similar to those of compressional Pc5 waves observed previously at geostationary orbit. The unique observations recorded by the AMPTE/CCE included the occurrence of the wave in the postmidnight sector, its sunward propagation with respect to the spacecraft, and the left-handed polarization of the perturbed magnetic field. In spite of the morphological uniqueness observed, the excitation of the July 21 event is considered to be due to the same type of instability as operates at geostationary orbit.

Takahashi, K.↗