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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.
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The reference citatons are grouped by experiment. Experiments include: the charged particle instrument experiment; the ultraviolet photometer experiment; imaging experiments; and magnetometer experiments. The asteroid detector experiment and the plasma analyser experiment are also included.
The mean value of the correlation between local magnetic field and vector potential, known as the magnetic helicity, is a measure of the lack of mirror reflection symmetry of magnetic covariances in a turbulent medium. A method is presented for extraction of helicity spectra from magnetometer data, and applied to an evaluation of the magnetic helicity of interplanetary magnetic fluctuations.
The estimation of planetary magnetic fields from observations of the magnetic field gathered along a spacecraft flyby trajectory is examined with the aid of generalized inverse techniques, with application to the internal magnetic field of Jupiter. Model nonuniqueness resulting from the limited spatial extent of the observations and noise on the data is explored and quantitative estimates of the model parameter resolution are found. The presence of a substantial magnetic field of external origin due to the currents flowing in the Jovian magnetodisc is found to be an important source of error in estimates of the internal Jovian field, and new models explicitly incorporating these currents are proposed. New internal field models are derived using the vector helium magnetometer observations and the high field fluxgate observations of Pioneer 11, and knowledge of the external current system gained from the Pioneer 10 and Voyagers 1 and 2 encounters.
Available data on imaging, infrared spectroscopy and radiometry, triaxial fluxgate magnetometers, the multifilter photopolarimeter, planetary radio astronomy, and radio science is described. The Voyager experiments are listed, and it is indicated on which experiments data is available.
Although the dayside ionosphere of Venus is often field-free except for fine-scale features, large-scale steady ionospheric magnetic fields with magnitudes sometimes exceeding 100 gammas are occasionally observed by the Pioneer Venus Orbiter magnetometer. These fields are mainly horizontal and can assume any angle in the horizontal plane. The orientation of the field may change along the spacecraft trajectory. The field magnitude in the upper ionosphere usually shows a distinct minimum near approximately 200 km altitude, but the altitude profile is otherwise arbitrary. With few exceptions, the observations of these large scale fields occur when periapsis is at solar zenith angles less than 50 deg. The occurrence of large-scale fields is often coincident with the observation of high solar wind dynamic pressures by the Pioneer Venus Orbiter plasma analyzer closely following the ionosphere encounter. However, the detection of this phenomenon even during some orbits for which the dynamic pressure is not extraordinarily high suggests that other factors, such as hysteresis effects, must also play a role in determining the occurrence frequency of large-scale magnetic fields in the dayside Venus ionosphere.
The paper deals with the Pioneer 11 vector helium magnetometer observations of Saturn's planetary magnetic field, magnetosphere, magnetopause, and bow shock. Models based on spherical harmonic analyses of measurements inside 8 Saturn radii show that the planetary field has a high degree of symmetry about the rotation axis. The vector dipole moment has a tilt angle of less than 1 deg and is offset along the polar axis by 0.04 plus or minus 0.02 Saturn radius. Equatorial offsets derived from the models show pronounced variability and could be consistent with a very small offset. Large impulsive field compressions are observed in the magnetosheath near noon. Multiple crossings of the bow shock are observed, and the absence of significant changes in field direction indicates that it is quasi-perpendicular.
Analysis of magnetic field observations by the Goddard Space Flight Center high-field flux gate magnetometer on the Pioneer 11 spacecraft during Saturn encounter yields estimates of the planetary field. The field is mainly dipolar but rather weaker than expected, with a moment equal to 0.20 G cubic Saturn radii or 4.3 x 10 to the 28th G cu cm, opposite in polarity to earth's. Surprisingly, the field appears to be axially symmetric but with a small (0.04 Saturn radii) offset to the north so that N (S) polar field intensities are 0.6 (0.4) G, respectively. The deduced polar offset appears not to be an artifact of the limited spatial extent of the observations or the presence of fields of external origin. The average stand-off distance of the magnetopause is expected to be 20 Saturn radii, i.e., at the orbit of Titan, so that this largest of solar system satellites is immersed not only in the Saturnian magnetosphere but also at times in its magnetosheath and sometimes even in the interplanetary medium.
The interaction of Io with its plasma torus and the Jovian magnetic field is described and examined in the context of several currently popular models. Three specific matters are addressed. First, features implied by sub-Alfvenic flow which must be common to all models are discussed. Next, the magnetic signature observed near Io by the Goddard Space Flight Center Voyager 1 magnetometer is examined and it is pointed out that the preliminary estimate of 5,000,000 A current may be an overestimate. Good fits are obtained with alternative current distributions which yield currents as small as 700,000 A through Io. The best fits are obtained for an Alfven Mach number of 0.15, but good fits are also found for Alfven Mach numbers between 0.1 and 0.25. Lastly, the crucial role of charged particle data for probing the near Io interaction is pointed out. Published data from the low energy charged particle (LECP) detector are made to suggest that Io has an intrinsic magnetic field of magnitude comparable with earlier estimates. Predictions are made which can test this picture as further data become available.
This paper describes the application of a single-chip microcomputer in a high-altitude balloon instrumentation system. The system, consisting of a magnetometer, a stepping motor, a microcomputer and a gray code shaft encoder, is used to provide an orientation reference to point a scientific instrument at an object in space. The single-chip microcomputer, Intel's 8748, consisting of a CPU, program memory, data memory and I/O ports, is used to control the orientation of the system.
The 2605 usable orbits of Apollo 15 and 16 subsatellite magnetometer data have been reexamined for intervals suitable for analysis of crustal magnetic anomalies. To minimize plasma-related disturbances, segments from 274 of these orbits were selected from times when the moon was either in a lobe of the geomagnetic tail or in the solar wind with the subsatellites in the lunar wake. External field contributions which remained in the selected intervals were minimized by (1) quadratic detrending of individual orbit segments with lengths much greater than anomaly wavelengths and (2) two-dimensional filtering with minimum passed wavelengths less than or equal to anomaly wavelengths. Improvements in coverage, accuracy, and resolution of previously published anomaly maps produced from these data are obtained. In addition to improved maps of the Reiner Gamma and Van de Graaff-Aitken anomalies studied previously, a third region of relatively high-amplitude anomalies centered near the crater Gerasimovich on the southeastern far side has been mapped. Both the Van de Graaff-Aitken region and the Gerasimovich region are marked by the general occurrence of extensive groups of Reiner Gamma-type swirls.
For three orbit paths of the Pioneer Venus orbiter the interaction between the solar wind and the Venusian ionosphere has been studied. Results of the retarding potential analyzer and the magnetometer are described for the boundary region between the solar wind and the planetary ionosphere. These are the first measurements that show that a transition region exists between the two plasmas of different origin. The observed magnetic field and current system producing it appear strong enough to stop the solar wind flow in front of the ionosphere and to separate the shocked solar wind from the ionosphere. The transition region between the ionosheath and the ionosphere is called the 'mantle'. The observed mantle electron energy spectra close to the ionopause show ionospheric character. With increasing height the number of electrons that have ionospheric energies decreases, and the number of electrons that have solar wind energies gradually increases toward the ionosheath boundary, where only solar wind energy spectra are observed. The mantle surrounds the frontside of the ionosphere and extends probably more than eight Venus radii downstream.
Dayside ion composition measurements made by the orbiter ion mass spectrometer and the orbiter electron temperature probe on the Pioneer Venus orbiter are used to infer the dominant processes involved in the dynamic response of the Venus ionosphere to the solar wind. The analysis is confined to the topside ionosphere in the vicinity of the subsolar point, where the ionosphere-solar wind interaction is expected to be maximized. Height profiles of the ion composition and plasma temperatures in the main body of the topside ionosphere, lying between the ionopause and chemical equilibrium regions, reveal that the ionosphere is in a compressed state. This region of the ionosphere is interpreted in terms of a stationary equilibrium where the compression is derived from the ponderomotive force j x B. The estimated magnitude of this force is confirmed by the magnetic field measurements made by the orbiter magnetometer.
This paper examines the possible occurrence of tearing modes in the dayside magnetopause. First, the expected magnetic signature of tearing, as obtained from existing theory, is reviewed. Magnetometer data from one terrestrial magnetopause crossing of Jupiter's magnetopause are then examined in detail. Magnetic field oscillations are found in three subsegments of the terrestrial crossing at a frequency of 0.1-0.2 Hz and with peak amplitudes of 5-10 nanotesla (nT), and in one segment of the Jovian crossing, at 0.05-0.1 Hz and with 2-nT amplitude. The frequency range, as well as the orientation of the magnetic field perturbation vectors, agrees with a model in which tearing-produced magnetic islands are convected past the satellite with the plasma flow in the current layer. In both cases the magnetopause structure was of the rotational discontinuity type with a nonvanishing normal magnetic field component. Hence, if the tearing structures were active, i.e., growing, at the observation site, ion tearing (or perhaps resistive tearing, with the resistivity provided by microturbulence) must be invoked. But it is also possible that the structures were passive, consisting of 'debris' from active tearing elswhere on the magnetopause surface, this debris being convected along the magnetopause past the observation site.
The latitude and magnetic local time distributions of Pi 3 pulsations relative to polarization and the position of maximum intensity are presented using data from the high-latitude North American International Magnetospheric Study magnetometer network. These distributions are compared with those of Pc 5 and Pi 2 pulsations. The locus of Pi 3 maximum amplitude and the corresponding curve for Pc 5 are similar in general shape, but the former is located at considerably lower geomagnetic latitudes than the latter. For Pi 2 pulsations, the latitude of maximum amplitude is found to agree well with the auroral oval, whereas the locus of Pi 3 amplitude maximum deviates from the auroral oval appreciably in the dawn and dusk regions and is near the poleward border of the auroral oval in the premidnight region where the equatorward boundary of the oval is lowest in latitude. It is concluded that dayside Pi 3 pulsations may have a different generation mechanism from that for nightside Pi 3 events.
To facilitate geologic interpretation of satellite elevation potential field data, analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation. A powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion. Linear transformations of the equivalent source field lead to corresponding geoidal anomalies, pseudo-anomalies, vector anomaly components, spatial derivatives, continuations, and differential magnetic pole reductions. A number of examples using 1 deg-averaged surface free-air gravity anomalies of POGO satellite magnetometer data for the United States, Mexico, and Central America illustrate the capabilities of the method.
Pioneer Venus Orbiter Electron Temperature Probe measurements of hundreds of bow shock and ionopause crossings are employed in describing the configuration of these two boundaries and their variations in response to changes in solar wind pressure. The average bow shock configuration is found to be well represented by an Archimedian hyperboloid whose altitude at the subsolar point is 0.46 Venus radii, a value slightly greater than that derived from Pioneer Venus magnetometer data using a fit to a general conic section. It is noted that the average bow shock configuration exhibits a high degree of azimuthal symmetry near the terminator. The orbit to orbit variability of the shock location is unexpectedly large, the standard deviation being about 10%. A tendency is noted for the bow shock and the ionopause to expand and contract simultaneously, but the weakness of their orbit by orbit correlation suggests that the ionopause of Venus is not the only obstacle to the solar wind. It is thought that such processes as photoion pickup and charge exchange with neutrals may be important in diverting the solar wind plasma around the planet.
A comparative study of the direction of linear polarization of Jovian decimetric (synchrotron) radiation as measured astronomically and as determined from a model of the inner Jovian magnetosphere is discussed. It is noted that the model depicts the radiation as coming from rings of relativistic electrons in the Jovimagnetic equator at varying radial distances from the center of the planet. The equator is determined through each of two magnetic representations - the O4 model of Acuna and Ness (1976) and the P10-11 model of Smith et al. (1976) - derived from in situ Pioneer magnetometer measurements. Deviations from a (planar) dipole equator are found to occur at nearly all longitudes in both models; no evidence is found for a longitudinally localized magnetic anomaly.
Initial observations of the dayside magnetopause thickness using magnetometers on board ISEE 1 (International Sun-Earth Explorer) and ISEE 2 indicate that this boundary may only be a few ion gyro-radii thick during periods of observed southward interplanetary magnetic field (IMF). Such a thin current layer can destabilize the collisionless tearing mode, a probable first step in the reconnection sequence. The linear growth rate is calculated assuming typical magnetopause parameters and it is found that it is a sensitive function of IMF orientation, magnetopause thickness, and electron number density. It is also shown that the finite size of the dayside magnetopause necessitates the existence of a two-dimensional tearing mode wave vector spectrum, a consequence of the inability of a thermal electron to maintain Landau resonance with the wave for a growth period. Implications regarding reconnection are discussed.