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Acuna, M. H.

Publications and source records attributed to Acuna, M. H..

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Magnetic field studies by Voyager 1 - Preliminary results at Saturn

Confirmation and refinement of Saturnian magnetosphere features established by the Pioneer 11 emission are claimed for Voyager 1 magnetic field studies of the planet. The radius of the magnetopause at the subsolar point is 23 Saturn radii, and a magnetic tail of 80 Saturn radii diameter was discovered. The tail extends away from the sun and is similar to both type II comet tails and the terrestrial and Jovian magnetic tails. Data from Voyager's very close flyby of Titan, which is located within the Saturn magnetosphere, shows an absence of any substantial, intrinsic satellite magnetic field.

Ness, N. F.↗
NASA NTRS1980

Standing Alfven wave current system at Io: Voyager 1 observations

The enigmatic control of the occurrence frequency of Jupiter's decametric emissions by the satellite Io is explained theoretically on the basis of its strong electrodynamic interaction with the corotating Jovian magnetosphere leading to field aligned currents connecting Io with the Jovian ionosphere. Direct measurements of the perturbation magnetic fields due to this current system were obtained by the magnetic field experiment on Voyager 1 on 5 March 1979 when it passed within 20,500 km south of Io. An interpretation in the framework of Alfven waves radiated by Io leads to current estimates of 2.8 million amps. A mass density of 7400 to 13600 proton mass units per Cu cm is derived which compares very favorably with independent observations of the torus composition characterized by 7-9 proton mass units per electron for a local electron density of 1050 to 1500 per cu cm. The power dissipated in the current system may be important for heating the Io heavy ion torus, inner magnetosphere, Jovian ionosphere, and possibly the ionosphere or even the interior of Io.

Acuna, M. H.↗
NASA NTRS1980

The magnetic field of Saturn - Further studies of the Pioneer 11 observations

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.

Acuna, M. H.↗
NASA NTRS1980

The Magsat precision vector magnetometer

This paper examines the Magsat precision vector magnetometer which is designed to measure projections of the ambient field in three orthogonal directions. The system contains a highly stable and linear triaxial fluxgate magnetometer with a dynamic range of + or - 2000 nT (1 nT = 10 to the -9 weber per sq m). The magnetometer electronics, analog-to-digital converter, and digitally controlled current sources are implemented with redundant designs to avoid a loss of data in case of failures. Measurements are carried out with an accuracy of + or - 1 part in 64,000 in magnitude and 5 arcsec in orientation (1 arcsec = 0.00028 deg).

Acuna, M. H.↗
NASA NTRS1980

The magnetic field of Saturn - Pioneer 11 observations

The Pioneer 11 high-field fluxgate magnetometer experiment consists of two biaxial fluxgate sensors assemblies and an associated electronics system that is designed to measure fields up to 10 gauss along three orthogonal axes. It was used to provide a higher upper range than that provided by the helium vector magnetometer whose maximum measureable field is only 1.4 gauss. Observations of the intrinsic magnetic field of Saturn measured by the high-field fluxgate magnetometer were found to be much weaker than expected. An analysis of preliminary data combined with the preliminary trajectory yield a model for the main planetary field which is a simple centered dipole. It was determined that the polarity of Saturn is opposite that of Earth, and that the tilt is small, within 2 deg plus or minus 1 deg.

Acuna, M. H.↗
NASA NTRS1979

Magnetic field studies at Jupiter by Voyager 2 - Preliminary results

The Voyager 2 magnetic field experiment, for which the instrumentation is identical to that on Voyager 1, operated flawlessly throughout the second Jupiter encounter. The paper presents a brief overview of the results obtained to date on the Jovian magnetosphere, the bow shock, the magnetopause, and the extended magnetic tail. The results and the magnetic field geometry confirm the earlier conclusion from Voyager 1 that Jupiter has an enormous magnetic tail, approximately 300-400 Jupiter radii in diameter, trailing behind the planet with respect to the supersonic flow of the solar wind. Additional observations of the distortion of the inner magnetosphere by a concentrated plasma show a spatial merging of the equatorial magnetodisk current with the current sheet in the magnetic tail. Disturbances near Ganymede are discussed.

Ness, N. F.↗
NASA NTRS1979

Jupiter's magnetic tail

Voyager 1 observations of the Jovian magnetosphere are discussed which are most naturally interpreted in terms of a well-developed magnetic tail on the nightside of the planet. It is shown that this tail, with a 'neutral sheet' separating the upper and lower lobes of opposite field polarity, is formed and controlled by external forces associated with the solar wind. The inner magnetosphere's current tail is found to merge with the magnetotail's neutral sheet. It is concluded that this configuration leads to a strong local-time control of the outer Jovian magnetosphere rather than planetary control.

Ness, N. F.↗
NASA NTRS1979

Magnetic field studies at Jupiter by Voyager 2: Preliminary results

The Voyager 2 magnetic field experiment is described and compared to the Voyager 1 experiment and data. The magnetosphere, the bow shock, the magnetopause, and the extended magnetic tail of Jupiter are discussed. Two crossings of the near equatorial current sheet were observed in the magnetosphere and its tail every 10 hour rotation period of the planet. A definitive mapping of the geometry and character of these enhanced plasma and depressed magnetic field regions is discussed. The interaction of the satellite Ganymede with the Jovian magnetosphere, which leads to disturbances as the Jovian magnetosphere corotates with the planet past the satellite is analyzed.

Ness, N. F.↗
NASA NTRS1979

Jupiter's magnetic tail: Voyager 1

Magnetic field observations by the Voyager 1 spacecraft during the outbound traversal of the Jovian magnetosphere in March 1979 suggest the detection of an extended magnetic tail, which has been formed by the solar wind interaction with the planetary field. The apparent diameter of the tail is 300-400 times the radius of Jupiter but its length is not measured. When combined with the GSFC O4 model of the planetary field, this magnetosphere topology leads to polar cap auroral zones approximately 20 deg in diameter, considerably smaller than earth's. The northern zone is found to be highly eccentric, encircling neither the rotational pole nor the magnetic pole of Jupiter, and limited to System III (1965) longitudes approximately 133 deg to 190 deg and latitudes approximately 62 deg to 82 deg.

Ness, N. F.↗
NASA NTRS1979

Magnetic field studies at Jupiter by Voyager 1 - Preliminary results

Results obtained by the Goddard Space Flight Center magnetometers on Voyager 1 are described. These results concern the large-scale configuration of the Jovian bow shock and magnetopause, and the magnetic field in both the inner and outer magnetosphere. There is evidence that a magnetic tail extending away from the planet on the nightside is formed by the solar wind-Jovian field interaction. This is much like earth's magnetosphere but is a new configuration for Jupiter's magnetosphere not previously considered from earlier Pioneer data. The analysis and interpretation of magnetic field perturbations associated with intense electrical currents (approximately 5 million amperes) flowing near or in the magnetic flux tube linking Jupiter with the satellite Io and induced by the relative motion between Io and the corotating Jovian magnetosphere are reported. These currents may be an important source of heating the ionosphere and interior of Io through Joule dissipation.

Ness, N. F.↗
NASA NTRS1979

Magnetic field studies at Jupiter by Voyager 1: Preliminary results

Results obtained by the Goddard Space Flight Center magnetometers on Voyager 1 concerning the large scale configuration of the Jovian bow shock and magnetopause, and the magnetic field in both the inner and outer magnetosphere are highlighted. There is evidence that a magnetic tail extending away from the planet on the nightside is formed by the solar wind-Jovian field interaction. This is much like Earth's magnetosphere but is a new configuration for Jupiter's magnetosphere not previously considered from earlier Pioneer data. Magnetic field perturbations associated with intense electrical currents (approximately 5 x 10 to the 6th power amps) flowing near or in the magnetic flux tube linking Jupiter with the satellite Io and induced by the relative motion between Io and the co-rotating Jovian magnetosphere are analyzed and interpreted. These currents may be an important source of heating the ionosphere and interior of Io through Joule dissipation.

Ness, N. F.↗
NASA NTRS1979

Contributions to the Fourth Solar Wind Conference

Recent results in interplanetary physics are examined. These include observations of shock waves and post-shock magnetic fields made by Voyager 1, 2; observations of the electron temperature as a function of distance between 1.36 AU and 2.25 AU; and observations of the structure of sector boundaries observed by Helios 1. A theory of electron energy transport in the collisionless solar wind is presented, and compared with observations. Alfven waves and Alvenic fluctuations in the solar wind are also discussed.

Acuna, M. H.↗
NASA NTRS1978

The MAGSAT vector magnetometer: A precision fluxgate magnetometer for the measurement of the geomagnetic field

A description of the precision triaxial fluxgate magnetometer to be flown aboard the MAGSAT spacecraft is presented. The instrument covers the range of + or - 64,000 nT with a resolution of + or - 0.5 nT, an intrinsic accuracy of + or - 0.001% of full scale and an angular alignment stability of the order of 2 seconds of arc. It was developed at NASA's Goddard Space Flight Center and represents the state-of-the-art in precision vector magnetometers developed for spaceflight use.

Acuna, M. H.↗
NASA NTRS1977

Magnetic field experiment for Voyagers 1 and 2

The magnetic field experiments of the Voyager program involve studies of the planetary fields of Jupiter, Saturn, possibly Uranus, and several satellites; the solar wind and satellite interactions with the planetary fields, as well as large- and micro-scale features of the interplanetary magnetic field will also be investigated. Dual low field and high field magnetometer systems with dynamic ranges of + or - 0.5 G and + or - 20 G respectively provide high reliability for the missions and permit the separation of the spacecraft and ambient fields. Quantization uncertainty, rms noise levels and data compaction schemes of the magnetometer systems are also mentioned.

Behannon, K. W.↗
NASA NTRS1977

Jupiter's internal magnetic field geometry relevant to particle trapping

Some field-geometric features of relevance to particle trapping in the inner magnetosphere and polar-cap regions of Jupiter are described using an internal-field model that includes terms up to the order of n equals 3 (octupole). Adiabatic particle parameters for detectors on Pioneers 10 and 11 during their flights through Jupiter's inner magnetosphere are determined along with the configuration of the intersections of particle drift shells with the planetary ionosphere. Possible correlations between drift-shell contours and the planet-locked characteristics of Jovian decametric radio emission are investigated, and longitudinal asymmetries of the ionospheric plasma source function are analyzed. It is shown that if the ionosphere is the main source of magnetospheric plasma, the latitude dependence of the plasma source function should have a considerable effect on the longitudinal asymmetry of the corotating plasma.

Roederer, J. G.↗
NASA NTRS1976

Analog-to-binary conversion of video data

Accurate and controllable technique for converting television information to binary form has been developed for systems requiring video signals to be used with automatic data-processing equipment. High-speed comparator circuit ignores out-of-focus features and is insensitive to overall brightness changes in picture.

Acuna, M. H.↗
NASA NTRS1976

The main magnetic field of Jupiter

The main magnetic field of Jupiter has been measured by the Goddard Space Flight Center flux gate magnetometer on Pioneer 11. Analysis of the data yields a more detailed model than that obtained from Pioneer 10 results. In a spherical harmonic octupole representation the dipole term (with opposite polarity to earth's) has a magnitude of 4.28 G times the radial distance cubed at a tilt angle of 9.6 deg and a system 111 longitude of 232 deg. The quadrupole and octupole moments are 24% and 21% of the dipole, respectively. This leads to a significant deviation of the planetary magnetic field from a simple offset dipole topology at distances of less than three times the radial distance. The north polar field strength is 14 G, and in the Northern Hemisphere the 'footprint' of the Io associated flux tube traverses the magnetic polar region. Associated L shell splitting in the radiation belts, warping of the charged particle equatorial planes, and enhanced absorption effects due to the satellites Amalthea and Io are expected as a result of the field complexity.

Acuna, M. H.↗