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At least 163 records · Page 9
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NASA NTRS1976

Solar wind structure at large heliocentric distances - An interpretation of Pioneer 10 observations

Examination of hourly values of the solar wind speed observed by the Pioneer 10 spacecraft beyond a heliocentric distance of 4 AU reveals (1) a prevalent 'sawtoothlike' speed-time profile, most speed fluctuations displaying a rapid rise and a much slower decline, and (2) the nearly universal appearance of abrupt (on the 1-hour time resolution of these data) changes in the speed on the rising portions of the speed fluctuations. These previously unreported characteristics, as well as the rate of decay of stream amplitudes derived earlier by Collard and Wolfe, are in general agreement with the predictions of stream propagation models that neglect any conversion of kinetic energy to thermal energy outside of shock fronts. Thus the Pioneer 10 observations give the first confirmation of the general concept of solar wind stream evolution employed in these models, i.e., that solar wind speed inhomogeneities appear to steepen to form shock waves and that the 'wave amplitudes' decay slowly as the shock waves propagate outward from the sun.

Hundhausen, A. J.↗
NASA NTRS1975

Comparisons of ground-based monitor data with Pioneers 8, 9, 10 and 11 observations in 1968-1974

The paper examines ground-based neutron monitor data and the counting rates in different energy channels on the Pioneer 8, 9, 10 and 11 spacecraft to determine the scale size of the transient cosmic-ray variations (Forbush decreases) and the relationship of these variations to changes in the interplanetary magnetic field. Since the Pioneer spacecraft sampled the inner modulating region extensively, the spatial extent of Forbush decreases (Fd's) can be determined. In some cases the Fd modulating region was co-rotating and in others radially expanding. The azimuthal extent of the Fd region was variable but was typically about 45 deg in extent. These observations will be interpreted in terms of models proposed for Fds.

Lockwood, J. A.↗
NASA NTRS1976

Measurements of electron density irregularities in the ionosphere of Jupiter by Pioneer 10

It is demonstrated that when the frequency spectrum of log amplitude fluctuations is used, the radio-occultation experiment is a powerful tool for detecting, identifying, and studying ionospheric irregularities. Analysis of Pioneer 10 radio-occultation measurements reveals that the Jovian ionosphere possesses electron-density irregularities which are very similar to those found in the earth's ionosphere. This is the first time such irregularities have been found in a planetary ionosphere other than that of the earth. The Pioneer 10 results indicate that the spatial wave-number spectrum of the electron-density irregularities is close to the Kolmogorov spectrum and that the outer scale size is greater than the Fresnel size (6.15 km). This type of spectrum suggests that the irregularities are probably produced by the turbulent dissipation of irregularities larger than the outer scale size.

Woo, R.↗
NASA NTRS1976

Maneuver sequence design for the post-Jupiter leg of Pioneer Saturn

After passing the planet Jupiter in December 1974, Pioneer 11 is on a flight path on which it will encounter Saturn in late 1979. Following an uncorrected trajectory, the spacecraft would pass 2 million km behind Saturn. A sequence of midcourse maneuvers for modifying the Pioneer trajectory is discussed. The corrected flight path is to bring the spacecraft within 500,000 km of Saturn's satellite Titan. Attention is given to maneuver capabilities and constraints, the maneuver design concept, questions related to the selection of an interim aimpoint, and aspects of maneuver implementation.

Frauenholz, R. B.↗
NASA NTRS1976

Pioneer mission support

Status of the current Pioneer missions and initial operations planning for the Pioneer Venus mission is given.

Adamski, T. P.↗
NASA NTRS1976

Structure of the atmosphere of Jupiter from Pioneer 10 and 11 radio occultation measurements

A technique for incorporating effects of the oblateness of Jupiter's atmosphere into the data analysis procedure for radio-occultation measurements is described which makes use of a spherical harmonic representation of the gravity field to compute the shape of the planet. With this technique, the center of refraction is located by the radius of curvature and the normal direction at the closest approach point of the ray. The present technique, a subsequent approximation involving the use of a constant center of refraction, and the Abel integral transform inversion method for spherical planets are applied to Pioneer 10 and 11 data. All the intermediate results obtained show a temperature inversion between 10 and 100 mb, with temperatures from 130 to 170 K at 10 mb and from 80 to 120 K at 100 mb. Comparison of the radio-occultation profiles with radiative-convective equilibrium models and the temperature profile based on Pioneer 10 IR radiometer data indicates good agreement between the models and the occultation results.

Kliore, A. J.↗
NASA NTRS1976

The Pioneer 11 radio occultation measurements of the Jovian ionosphere

Radio occultation data obtained with the Pioneer 11 spacecraft are utilized to study Jupiter's ionosphere. The ingress measurements, which were conducted by using a stable earth-based frequency reference for the tracking link, yielded ionospheric data near the morning terminator at about 79 deg south latitude. Data were also taken during egress on the evening side near 20 deg north latitude. The latter measurements were conducted in the one-way mode; i.e., an on-board crystal oscillator was employed as a frequency reference for the downlink (spacecraft-to-earth) signal. These data confirm previous results obtained with Pioneer 10 and show that Jupiter has a multilayered ionosphere extending over an altitude range of more than 3000 km. The topside scale height near 79 deg south latitude was 540 + or - 60 km. Assuming a topside electron, H(+) distribution controlled by diffusion yields a plasma temperature of 850 + or - 100 K in this region. The radio data indicate that the upper atmosphere is either warmer or more dissociated into atomic hydrogen than previously anticipated.

Fjeldbo, G.↗
NASA NTRS1976

Pioneer 10 ultraviolet photometer observations of Jupiter - The helium to hydrogen ratio

Pioneer 10 UV photometric observations of solar He I 584-A emissions diffusely reflected from the atmosphere of Jupiter are discussed which establish the expected presence of He in the planet. The intensity and profile of the solar He line are remeasured and found to be more intense than originally supposed, implying a lower value for the diffuse 584-A reflectivity of Jupiter and less He than previously estimated. Other measurements of the solar He line are reviewed, resonance scattering by He is analyzed, and radiative-transfer calculations for Jovian model atmospheres are improved by including the anisotropic phase function for helium resonance scattering. The assumption of coherent scattering in the formation of the Jovian airglow line is shown to be an accurate approximation, and present estimates of the atmospheric temperature and eddy diffusion coefficient are examined. An atmospheric He/H abundance ratio greater than zero but less than approximately 28% is determined by assuming an isothermal atmosphere with a temperature of 150 K and the range of eddy-diffusion values given by Pioneer 10 Ly-alpha observations. The effect of higher temperatures on the He/H abundance is briefly evaluated.

Carlson, R. W.↗
NASA NTRS1976

Results from the GSFC fluxgate magnetometer on Pioneer 11

A high-field triaxial fluxgate magnetometer was mounted on Pioneer 11 to measure the main magnetic field of Jupiter. It is found that this planetary magnetic field is more complex than that indicated by the results of the Pioneer 10 vector helium magnetometer. At distances less than 3 Jupiter radii, the magnetic field is observed to increase more rapidly than an inverse-cubed distance law associated with any simple dipole model. Contributions from higher-order multipoles are significant, with the quadrupole and octupole being 24 and 21 percent of the dipole moment, respectively. Implications of the results for the study of trapped particles, planetary radio emission, and planetary interiors are discussed. Major conclusions are that the deviation of the main planetary magnetic field from a simple dipole leads to distortion of the L shells of the charged particles and to warping of the magnetic equator. Enhanced absorption effects associated with Amalthea and Io are predicted.

Acuna, M. H.↗
NASA NTRS1976

Field-aligned currents in the Jovian magnetosphere - Pioneer 10 and 11

Field-aligned currents, which are known to play an important role in the dynamics of the earth's magnetosphere, would also be expected to flow in the Jovian magnetosphere during magnetically active periods. Such currents are difficult to observe largely because of their limited spatial extent and intermittent nature, but on the Pioneer 11 inbound pass to Jupiter on day 336, 1800 GRT, near the L shell of Ganymede, the helium vector magnetometer recorded a structured perturbation with the characteristic signature of a field-aligned current. No analogous structure was found on a similar Pioneer 10 inbound pass (day 337, 1200 GRT). The perturbation could have been caused by turbulence with the wake of Ganymede through which the spacecraft was moving or the perturbation could represent the first observation of a Jovian substorm. The observations are discussed in terms of both models, and difficulties are examined.

Kivelson, M. G.↗
NASA NTRS1976

A review of the Jovian magnetosphere based upon Pioneer 10 and 11

A review and analysis are presented of data derived from the Pioneer 10 and Pioneer 11 flybys of Jupiter on the Jovian plasma, magnetic field, and energetic particles in circumjovian space. The design of the space probes is described along with the principal experiments flown. Overall features of the Jovian magnetosphere are drawn and contrasted with the earth's magnetosphere. The trajectories of the two space probes are contrasted and their data on Jupiter's plasmasphere are correlated. Isointensity contours and count rates of energetic particles are plotted, flux tubes within the Jovian magnetosphere are mapped, the ring current (plasma torus) encircling the planet is described, and possible effects of solar wind and of the moons immersed in Jupiter's magnetosphere are considered.

Trainor, J. H.↗
NASA NTRS1977

The Pioneer 11 imaging experiment of Jupiter

Pioneer 11 flew by Jupiter in December 1974 and obtained several hundred images of the planet. It is presently targeted for encounter with Saturn in 1979. The imaging photopolarimeter recorded spin-scan images with high photometric accuracy. It also recorded polarimetric and zodiacal-light data. Careful design of the instrument resulted in excellent performance in the Jovian radiation environment. Imaging data were displayed to maintain the proper shape of the planet. Color images were made by synthesizing green data from red and blue data. Pictures created from Pioneer 11 imaging data show complex detail within the Red Spot is well as indications of flow around it. Bright spots with trailing plumes are seen in the Equatorial Zone. The North Polar Region is devoid of belt structure, but numerous irregular cells are seen in red light. The Galilean satellites were imaged with a resolution of several hundred kilometers.

Swindell, W.↗
NASA NTRS1978

On the observation of a flare-generated shock wave at 9.7 AU by Pioneer 10

An apparent solar-flare-generated shock wave detected by Pioneer-10 at 9.7 AU on April 9, 1976 is discussed. The shock wave may be correlated with a radio emission burst from Jupiter not associated with Io (March 30). The fact that solar flares observed on March 20 were at the central meridian with respect to Jupiter and Pioneer-10 and the fact that solar activity was very low before March 20 contribute to the argument that a shock wave had propagated to the region of the spacecraft.

Dryer, M.↗
NASA NTRS1978

Pioneer Venus encounter will occur in December

The encounter time line and mission profile are presented for the Pioneer Venus 1 Spacecraft and for the four probes and transponder bus that comprise Pioneer Venus 2. Known facts about Venus are reviewed and the history of discoveries about the planet is related. Operations of the orbiter and multiprobe are described as well as the 30 instruments being carried and provisions for data transmission.

Source record↗
NASA NTRS1978

Pioneer 10 and 11 mission support

Estimates of the Deep Space Network performance capability in tracking Pioneer 10 to the telecommunications limit and for Pioneer 11 Saturn encounter are discussed. Graphs of downlink performance estimates giving geocentric range, antenna elevation, and time are included.

Miller, R. B.↗
NASA NTRS1979

Pioneer to encounter Saturn on September 1

The encounter of the Pioneer 11 Spacecraft with Saturn, designed to provide information on the evolution of the Sun and its planets, is described. Photographs and measurements of Saturn, its rings, and several of its 10 satellites, including Titan, to be taken by Pioneer instruments, are emphasized. The encounter sequence and spacecraft trajectory are discussed. A description of Saturn and its atmosphere is included. Onboard instruments and experiments are also described.

Source record↗
NASA NTRS1978

Measurements of the magnetic field orientation in the Jovian ionosphere deduced from Pioneer 10 and 11 scintillation observations

In this paper the S band scintillations observed during the Pioneer 10 and 11 occultation measurements of Jupiter are analyzed. It is found that while the electron density irregularities are isotropic in the collision-dominated lower ionosphere, they are anisotropic in the upper ionosphere because of alignment along the magnetic field. By using Rytov's approximation the frequency spectrum of the log-amplitude scintillations is derived for a wave propagating in an anisotropic turbulent medium. It is shown that the spectrum depends to a large extent on the direction of anisotropy and is therefore useful for measuring the orientation of the magnetic field in regions that have not yet been probed by direct measurements. This new technique is applied to the Pioneer 10 and 11 observations, and is used to deduce the first measurements of magnetic field orientation in the ionosphere of Jupiter.

Woo, R.↗
NASA NTRS1979

Retarding potential analyzer for the Pioneer-Venus Orbiter Mission

The retarding potential analyzer on the Pioneer-Venus Orbiter Mission has been designed to measure most of the thermal plasma parameters within and near the Venusian ionosphere. Parameters include total ion concentration, concentrations of the more abundant ions, ion temperatures, ion drift velocity, electron temperature, and low-energy (0-50 eV) electron distribution function. To accomplish these measurements on a spinning vehicle with a small telemetry bit rate, several functions, including decision functions not previously used in RPA's, have been developed and incorporated into this instrument. The more significant functions include automatic electrometer ranging with background current compensation; digital, quadratic retarding potential step generation for the ion and low-energy electron scans; a current sampling interval of 2 ms throughout all scans; digital logic inflection point detection and data selection; and automatic ram direction detection. Extensive numerical simulation and plasma chamber tests have been conducted to verify adequacy of the design for the Pioneer Mission.

Knudsen, W. C.↗