K. W. Behannon

Magnetic Field Observations near Venus: Preliminary Results from Mariner 10

The NASA-GSFC magnetic field experiment on Mariner 10 is the first flight of a dual magnetometer system conceived to permit accurate measurements of weak magnetic fields in space in the presence of a significant and variable spacecraft magnetic field. Results from a preliminary analysis of a limted data set are summarized in this report, which is restricted primarily to Venus encounter. A detached bow shock wave that develops as the super Alfv�nic solar wind interacts with the Venusian atmosphere has been observed. However, the unique coincidence of trajectory position and interplanetary field orientation at the time of bow shock crossing led to a very disturbed shock profile with considerably enhanced upstream magnetic fluctuations. At present it is not possible to ascertain the nature and characteristics of the obstacle responsible for deflecting the solar wind flow. Far downstream disturbances associated with the solar wind wake have been observed.

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 Earths magnetosphere but is a new configuration for Jupiters magnetosphere not previously considered from earlier Pioneer data. We report on the analysis and interpretation of magnetic field perturbations associated with intense electrical currents (approximately 5 x 106 amperes) flowing near or in the magnetic flux tube linking Jupiter with the satellite Jo and induced by the relative motion between Io and the corotating Jovian magnetosphere. These currents may be an important source of heating the ionosphere and interior of Io through Joule dissipation.

Magnetic field experiment for Voyagers 1 and 2

The magnetic field experiment to be carried on the Voyager 1 and 2 missions consists of dual low field (LFM) and high field magnetometer (HFM) systems. The dual systems provide greater reliability and, in the case of the LFMs, permit the separation of spacecraft magnetic fields from the ambient fields. Additional reliability is achieved through electronics redundancy. The wide dynamic ranges of ± 0.5 G for the LFMs and ± 20 G for the HFMs, low quantization uncertainty of ± 0.002 γ (γ = 10−5 G) in the most sensitive (± 8 γ) LFM range, low sensor RMS noise level of 0.006 γ, and use of data compaction schemes to optimize the experiment information rate all combine to permit the study of a broad spectrum of phenomena during the mission. Objectives include the study of planetary fields at Jupiter, Saturn, and possibly Uranus; satellites of these planets; solar wind and satellite interactions with the planetary fields; and the large-scale structure and microscale characteristics of the interplanetary magnetic, field. The interstellar field may also be measured.

Jupiter's magnetic field and magnetosphere

Among the planets of the solar system, Jupiter is unique in connection with its size and its large magnetic moment, second only to the suns. The Jovian magnetic field was first detected indirectly by radio astronomers who postulated its existence to explain observations of nonthermal radio emissions from Jupiter at decimetric and decametric wavelengths. Since the early radio astronomical studies of the Jovian magnetosphere, four spacecraft have flown by the planet at close distances and have provided in situ information about the geometry of the magnetic field and its strength. The Jovian magnetosphere is described in terms of three principal regions. The inner magnetosphere is the region where the magnetic field created by sources internal to the planet dominates. The region in which the equatorial currents flow is denoted as the middle magnetosphere. In the outer magnetosphere, the field has a large southward component and exhibits large temporal and/or spatial variations in magnitude and direction in response to changes in solar wind pressure.

Observations of Mercury's magnetic field

This paper presents a study of magnetic field data obtained by Mariner 10 during the third and final encounter with the planet Mercury on 16 March 1975. A well developed bow shock and modest magnetosphere, previously observed at first encounter on 29 March 1974, were again observed. In addition, a much stronger magnetic field near closest approach, 400γ versus 98γ, was observed at an altitude of 327 km and approximately 68° north Mercurian latitude. Spherical harmonic analysis of the data provides an estimate of the centered planetary magnetic dipole of 5.0 × 1022 gauss-cm3 with the axis tilted 12° to the rotation axis and in the same sense as Earths. The interplanetary field was sufficiently different between first and third encounters that in addition to the very large field magnitude observed it argues strongly against a complex induction process generating the observed planetary field. While a possibility exists that Mercury possesses a remanent field due to magnetization early in its formation, a present day active dynamo seems to be a more likely candidate for its origin. The existence of such a dynamo argues for a mature planetary interior with a well-developed core.

Spatial variations of the magnetosheath magnetic field

Abstract Measurements by Explorers 28, 33, 34 and 35 have been used to study the spatial characteristics of the magnetic field in the magnetosheath to a distance of 70 R E behind the Earth. During 1966–1967 there were 1661 hr when one spacecraft of this multi-satellite system was monitoring the interplanetary medium while at least one other satellite was making magnetosheath measurements. This has made possible a separation of time and space variations and has permitted a study of the direction and magnitude of the magnetosheath field as a function of position and interplanetary field orientation. Results indicate that the mag-netosheath field is several times the strength of the simultaneously measured interplanetary field in the sunward magnetosheath. This magnetosheath to interplanetary magnitude ratio decreases with distance from the subsolar point to values which are frequently less than unity at distances beyond 30 R E and away from the bow shock. This ratio also displays a dawn-dusk asymmetry which is dependent on the interplanetary field orientation. Interplanetary field lines perpendicular to the Earth-Sun line are associated with symetrically distorted magnetosheath field lines in the dawn and dusk hemispheres and are consistent with the draping of field lines around the magnetosheath. When the interplanetary field is aligned near the spiral angle, fields measured in the dusk hemisphere are much more ordered than those measured in the dawn hemisphere behind the Earth. These experimental results are in general agreement with the theoretical predictions of the gasdynamic model.

Satellite Studies of the Earth’s Magnetic Tail

One consequence of the interaction between the solar wind and the geomagnetic field is a deformation of the magnetosphere such as to produce a magnetospheric tail that extends away from the sun. The existence of the Earth’s magnetic tail was anticipated theoretically by Piddington (1960) and by Dungey (1961, 1963). Early measurements by Explorer 10 (Heppner et al., 1963) and Explorer 14 (Cahill, 1964) revealed the distortion of the geomagnetic field on the nighttime side of the earth. Measurements performed more recently by a number of high apogee satellites and deep space probes have verified that the magnetic tail is a permanent extension of the geomagnetosphere. Figure 1 summarizes the trajectories of some of these spacecraft which have investigated the antisolar region of the magnetosphere since 1963.

Configuration of Jupiter's magnetic tail and equatorial current sheet

Recent research reports by Behannon et al. (1981) and Connerney et al. (1981) are summarized. It is noted that the analysis made of the detailed neutral sheet crossings by the minimum variance method shows a consistent result with regard to the orientation of the neutral sheet in the magnetic tail as a two-dimensional surface rocking back and forth about the Jupiter sun-line as the rotation of the planet leads to a precession of the tilted dipole magnetic axis. The occurrence of neutral sheet crossings is found not to be consistent with any of the axially symmetric theoretical models proposed earlier on the basis of the 1974 Pioneer 10 observations. It is noted that a simple nonaxially symmetric model has been developed on the basis of the Voyager results which indicates the strong control upon orientation by the interaction of the solar wind with the Jovian magnetosphere. The model is described as simple because it improves the fit of theory to observation but uses fewer parameters. A quantitative model of the magnetodisc equatorial current sheet has been developed for the inner magnetosphere region which matches well the in-situ magnetic field observations.

Magnetic Fields in the Earth’s Tail

The initial mapping of the magnetic field in the geomagnetic tail out to a distance of 31 RE by IMP 1 established the basic structure of the tail. In addition to the generally steady well collimated nature of the field in the tail, a radius of approximately 20 RE out to a geocentric distance of 30 RE in the antisolar direction was deduced and the existence of a relatively thin (≼ 1 RE) neutral sheet separating oppositely directed fields was established (Ness, 1965; Speiser and Ness, 1967). Explorer 14 measurements showed a broad region of low field magnitude surrounding the field reversal region at a distance of 10 to 12 RE (Cahill, 1966). Observations of electrons and protons with E>100eV by the Vela satellites at geocentric distances between 15.5 and 20.5 RE showed that the plasma sheet surrounding the neutral sheet is often 4 to 6 RE thick near the midnight meridian, increasing to approximately twice that thickness near the dusk and dawn boundaries (Bame et al., 1967).