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Several investigations based on data from the magnetometer on the Galileo Orbiter are described. From Galileo’s initial inbound pass, we learn that the magnetosphere can experience large changes in its magnetic configuration. Between 50 and 30 R,, the radial magnetic forces on the plasma were larger in 1995 than in 1973 when Pioneer 10 passed through the same region of the magnetosphere, implying that either external or internal plasma forces were also larger. Although there are several ways to interpret the change of the magnetic configuration, we suggest that the variations are governed principally by the solar wind dynamic pressure and that the dayside magnetosphere as far in as -30 RJ may be more strongly affected by the solar wind than has previously been recognized. Minor effects of higher mass loading may also be present. Data from the flyby of 10 show a large magnetic perturbation; we argue that it is plausible that 10 has an intrinsic magnetic field and that also contributions from plasma perturbations are significant. We find unexpected evidence that molecular ions are being picked up over a large spatial region in the vicinity of the moon. During the pass by Ganymede we observed a large magnetic perturbation consistent with an intrinsic dipole field. The multiple flybys of Ganymede scheduled for later portions of Galileo’s mission will allow us to test our understanding of the magnetic signature.

An overview of the Galileo magnetometer observations from the crossing of the tail magnetopause at an antisolar distance of close to 100 R(E) through exit into the solar wind on the dayside is presented. These measurements are linked with correlative data from ground stations and from IMP 8 which was ideally located to serve as a monitor of the solar wind upstream of the bow shock. A time line of the important geomagnetic events of the day that provides a framework for the full multiinstrument analysis of the flyby data is presented. The observations are used to investigate apsects of the relationship between magnetotail dynamics and the separate intensifications of a multiple onset substorm inferred from ground-based data. It is proposed that the signatures associated with individual substorm intensifications are localized in the dawn-to-dusk extent even at remote locations in the magnetotail, just as they are in the ionosphere, and that the tail disturbances associated with successive substorm intensifications step across the tail towards the dusk flank.

Comparisons of simultaneous Explorer 35 and Apollo 15 subsatellite magnetometer measurements indicate that the inductive response of the moon to solar wind transients is not detectable at subsatellite altitude (100 km) on the day side of the moon. Thus, within the limitations of the present study, it appears that day side confinement of the induced fields by a thin (≤ 100 km) current layer just above the lunar surface adequately describes global electromagnetic induction in the moon. This result is a direct experimental justification of a hitherto questioned assumption in the inversion of magnetic field data to lunar electrical conductivity profiles.

An approximate inversion, using a specific model, of Apollo subsatellite vector magnetometer data has provided estimates for the locations, scale sizes, and magnetization characteristics of strongly magnetized lunar crustal materials across a portion of the lunar far side. The nearly model-independent local bulk directions of magnetization, while otherwise random, exhibit a tendency to lie in or near planes perpendicular to the present lunar spin axis. The apparent existence of such a preferred plane of magnetization must be regarded as unfavorable for proposed local, presumably random sources of large-scale, coherent magnetization and implies that any assumed internal magnetizing dipole or external linear magnetizing field must also have been oriented mainly perpendicular to the present spin axis. The otherwise random orientations of these vectors when projected onto the equatorial plane suggests a simple explanation for the negligible lunar global dipole moment.

The existence of a day-side lunar cavity in the plasma sheet, showing some depletion of plasma, has been inferred from cavity-associated magnetic characteristics observed by orbital and surface lunar magnetometers. These characteristics include a day-side enhancement in the mean magnetic field and day-side levels of amplification of eddy current induced magnetic field fluctuations typical of cavity confinement.

A combination of orbital photographic, selenochemical, and magnetic surveys may elucidate the mechanism by which the lunar surface became magnetized and possibly yield an estimate of the intensity of the ancient magnetizing field and its time variation. The determination of the size and shape of the magnetized regions requires the measurement of the altitude dependence of field, especially at low altitudes (less than 100 km) and with a high enough sampling rate to resolve the profile at the edges of magnetized bodies. The planned Lunar Polar Orbiter may well provide the necessary data.

Data obtained by the Pioneer 10 vector helium magnetometer are presented along with models of the intrinsic magnetic field of Jupiter and its magnetosphere. Data acquired between 2.84 and 6.0 Jupiter radii, where the intensity of the planetary field ranged between 1900 and 18,400 gamma, were used to develop a six-parameter eccentric dipole model of the field. The dipole so derived has a moment of 4.0 G (R sub J) cubed and a tilt angle with respect to Jupiter's rotation axis of 11 deg. A model of the Jovian magnetosphere is presented in which the essential feature is an eastward current sheet that forms an annulus with Jupiter at the center. At large distances from the planet the current sheet is nearly parallel to Jupiter's equator but, in general, does not lie in it. The current sheet is warped, so that it is above the equator on one side and below it on the other. The current sheet rotates with the planet, more or less like a rigid body, this behavior causes an apparent up and down motion and periodic crossings of the current sheet by Pioneer.

Analysis of vector helium magnetometer measurements during the Pioneer 10 outbound pass through the Jovian magnetosphere reveals that the average location of the near-equatorial current sheet is a surface whose shape is determined by the velocity at which the magnetic perturbations produced by a rotating tilted dipole propagate to large radial distances. A model is presented which is a surface encircling the planet and rotating rigidly with it. Based on a linear fit to the times of the current sheet crossings, a formula is derived for the surface representing the average position of the near-equatorial current sheet in the Jovian magnetosphere.

Equations providing numerical values of the geomagnetic field spherical harmonic spatial power spectrum as defined by Lowes (1966, 1974) are obtained and this power spectrum is related to various other power spectra. Equations relating the spherical harmonic spatial power spectrum to average great circle power spectra for components of the vector magnetic field in the radial direction, along the great circle track and perpendicular to the first two directions are derived under the assumption that the sources of the field are internal. A statistical model for the crustal and core geomagnetic fields is proposed and used to derive equations for the expected main and crustal spherical harmonic power spectra. The model equations are then compared with observations to determine a scale factor which is then used to obtain an estimate for the core radius and a great circle power spectrum for the field component perpendicular to the great circle and radial directions which are in good agreement with observations. The predicted spherical harmonic power spectrum for the crustal field is found to be consistent with POGO satellite and aircraft data. Other possible models for the crustal and core geomagnetic fields are also briefly considered.

Effects of the time dependence of the sun's magnetic field upon the interplanetary field in the solar wind are considered. General expressions for the interplanetary field are developed under the assumptions that the radial component of the sun's field at some reference surface r/sub 0/ can be described in terms of spherical harmonics with coefficients which are functions of time represented by Fourier series and that the solar wind expands radially in a steady state flow. Some of the simplest field configurations are examined in additional detail. It is shown that the variation in the sun's field may produce elliptically polarized fluctuations in the interplanetary field; may generate field strengths in the theta direction, i.e., the north-south direction, that are comparable in magnitude to the spiral field; may produce B/sub theta/ with nonzero averages over significant periods of time; and may produce systematic deviations from the nominal steady state spiral angle over such periods. (AIP)

The Weber-Davis model of the solar wind is generalized to include the effects of latitude. The principal assumptions of perfect electrical conductivity, rotational symmetry, a polytropic relation between pressure and density, and a flow aligned magnetic field in a system rotating with the Sun, are retained. The original three dimensional magnetohydrodynamic flow problem is reduced to a two dimensional hydrodynamic flow problem. The solution at 1 Au is most sensitive to a latitudinal dependence in the coronal boundary temperature and least sensitive to a latitudinal dependence in the magnetic field magnitude. A solution obtained for an approximate dipolar variation in the coronal magnetic field magnitude predicts that the latitudinal flow is initially toward the equator due to magnetic channeling; however, this effect is rapidly overcome and the latitudinal flow at 1 Au is toward the pole and not significantly different from the solution for constant boundary conditions.

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.

Properties of lunar crustal magnetization thus far deduced from Apollo subsatellite magnetometer data are reviewed using two of the most accurate available magnetic anomaly maps, one covering a portion of the lunar near side and the other a part of the far side. The largest single anomaly found within the region of coverage on the near-side map correlates exactly with a conspicuous light-colored marking in western Oceanus Procellarum called Reiner Gamma. This feature is interpreted as an unusual deposit of ejecta from secondary craters of the large nearby primary impact crater Cavalerius. The mean altitude of the far-side anomaly gap is much higher than that of the near side map and the surface geology is more complex; individual anomaly sources have therefore not yet been identified. The mechanism of magnetization and the origin of the magnetizing field remain unresolved, but the uniformity with which the Reiner Gamma deposit is apparently magnetized, and the north-south depletion of magnetization intensity across a substantial portion of the far side, seem to require the existence of an ambient field, perhaps of global or larger extent.

For a low-level geomagnetic satellite survey, for which the motion of the satellite converts spatial variation into temporal variation, the limit on accuracy may well be background temporal fluctuations. The sources of the temporal fluctuations are current systems external to the Earth and include currents induced in the Earth due to these sources. The internal sources consist primarily of two components, the main geomagnetic field with sources in the Earth's core and a crustal geomagnetic field. Power spectra of the vertical geomagnetic field internal component that would be observed by a spacecraft in circular orbit at various altitudes, due to satellite motion through the spatially varying geomagnetic field, are compared to power spectra of the natural temporal fluctuations of the geomagnetic field vertical component (natural noise) and to the power spectrum for typical fluxgate magnetometer instrument noise. The natural noise is shown to be greater than this typical instrument noise over the entire frequency range for which useful measurements of the geomagnetic field may be made, for all geomagnetic latitudes and all times. Thus there would be little benefit in reducing the instrument noise below the typical value of 10−4 gamma2 Hz−1 plus a 1/f component of 10 milligamma rms decade−1. For a given satellite altitude, there is a maximum frequency above which the natural noise is greater than the power spectrum of the crustal geomagnetic field vertical component. Below this maximum frequency, the situation is reversed. This maximum frequency depends on geomagnetic latitude (and to a lesser extent on time of day and season of year), being lower in the auroral zone than at lower latitudes. The maximum frequency is also lower at higher satellite altitudes. The maximum frequency determines the spatial resolution obtainable on a magnetic field map. The spatial resolution (for impulses) obtainable at low latitudes for a 100-km satellite altitude (possibly achievable by tethering a small satellite at this altitude to a space vehicle at a higher altitude) is 60 km, while at the auroral zone the obtainable spatial resolution is 100 km. At the higher satellite altitude of 300 km the obtainable spatial resolution is 230 km at low latitudes and 530 km at the auroral zone. At 500-km satellite altitude, the obtainable spatial resolution is 500 km at low latitudes, while maps cannot be made at all for the auroral zone unless the data are selected for “quiet” days. For the lower satellite altitudes, greater spatial resolution can be obtained than at higher altitudes. Furthermore since the crustal geomagnetic field power spectrum is larger at lower altitudes, the relative error due to the natural noise is less than for higher altitudes.

Pioneer 10 magnetic field measurements, supplemented by previously published plasma data, have been used to identify shocks at 2.2 AU associated with the large solar flares of early August 1972. The first three flares, which gave rise to three forward shocks at Pioneer 9 and at earth, led to only a single forward shock at Pioneer 10. The plasma driver accompanying the shock has been tentatively identified. A local shock velocity at Pioneer 10 of 717 km/s has been estimated by assuming that the shock was propagating radially across the interplanetary magnetic field. This velocity and the rise time of ≃2 s imply a shock thickness of ∼1400 km, which appears to be large in comparison with the characteristic plasma lengths customarily used to account for the thickness of the earth's bow shock. This Pioneer 10 shock is identified with the second forward shock observed at Pioneer 9, which was then at 0.8 AU and radially aligned with Pioneer 10, since it was apparently the only Pioneer 9 shock that was also driven. The local velocity of the Pioneer 9 shock of 670 km/s, previously inferred by other authors, compares reasonably well with the local velocity at Pioneer 10, but both values are significantly smaller than the average value computed from the time interval required for the shock to propagate from the sun to Pioneer 9 (2220 km/s). The velocity implied by the time required to propagate from Pioneer 9 to Pioneer 10 (770 km/s) is in reasonable agreement with the local velocities. The fourth solar flare also gave rise to a forward shock at Pioneer 10 as well as at Pioneer 9. The local velocity at Pioneer 10, estimated on the basis of quasi-perpendicularity, is 660 km/s, a value which again agrees well with previously derived velocities for the Pioneer 9 shock of 670 km/s. The local velocities for this shock and the velocity between Pioneer 9 and Pioneer 10 (635 km/s) are also significantly less than the average velocity of propagation from the sun to Pioneer 9 (830 km/s). The general finding that the local velocities of both shocks are approximately equal at 0.8 and 2.2 AU but significantly slower than the average speeds nearer the sun is interpreted as evidence of a major deceleration of the shocks as they propagate outward from the sun that is essentially completed when the shocks reach 0.8 AU, there being little, if any, subsequent deceleration. This conclusion is qualitatively inconsistent with previous inferences of a deceleration of the shocks as they propagate from 0.8 to 2.2 AU. A third, reverse shock is also identified in the Pioneer 10 data which was not seen either at Pioneer 9 or at earth. The estimated speed of this shock is 530 km/s, and its estimated thickness is ≲500 km, which compares well with an anticipated proton inertial length of 500 km.

Measurements of the magnetic fields by the electron reflection method in the neighborhood of the long structural rille Rima Sirsalis show that a magnetic field with a strength of at least 100 nT (100 gammas) is present over a region of the order of 10 km in width and at least 300 km long. The center of the magnetized region closely parallels and is centered on the rille. The linear magnetization feature extends at least to latitude 8 deg S, 60 km beyond the place where the rille disappears at the edge of Oceanus Procellarum. This extension is coincident with the extrapolation of the rille based on photographs. However, the magnetization is much weaker or entirely absent at 5 deg S and has vanished at 0 deg latitude. These results suggest that the rille is indeed a structural feature and has associated with it magnetization, either in the form of intrusive magnetized rock or of a gap in an otherwise more or less uniformly magnetized layer of rock of large extent in two dimensions. Furthermore, the rille structure evidently is present for some distance beneath the lava flows of the Oceanus Procellarum basin.

The electric currents in the solar wind are described for the particular case of Parker's model, in which the sun's magnetic field is a dipole located at the center of the sun and inclined at an arbitrary angle to the sun's axis of rotation. The current consists of two components. The density of one depends only upon the sun's dipole moment, and that of the other depends upon the solar wind velocity and the sun's angular velocity as well as its dipole moment. The latter component flows in heliographic meridional planes. The electromagnetic forces of this component tend to accelerate the plasma in the leading half of a magnetic sector and to decelerate it in the following half. If the angle λ between the sun's spin axis and the dipole axis is different from either 0° or 90°, the electromagnetic force near the equatorial plane has a southward component in the positive sector and a northward component in the negative sector. The generalization to a more complicated solar magnetic field is discussed. The electromagnetic torque on the sun slows its rotation and for λ ≠ 0° or 90° also has oscillating components in the two directions perpendicular to the spin axis. These components tend to increase λ, indicating that the stable configuration is λ = 90°. For a given dipole moment, λ = 90° is also the orientation that maximizes the electromagnetic braking. Possible electromagnetic contributions to the sun's differential rotation are also considered.

Theta /north-south/ component in spherical polar coordinates of interplanetary magnetic field from Explorer 33 and 35 measurements

T characteristics of the flow of the solar wind, a tenuous, magnetized plasma emitted continuously by the sun, are known to be altered greatly when the plasma encounters an obstacle of dimensions in excess of 100,000 km. The magnetosphere of the Earth presents an obstacle of such size. In studies of magnetohydrodynamics, properties of the resulting interaction between the solar wind and the magnetosphere are of interest for a number of reasons. In connection with planetary exploration, the reason for much of our interest is a practical one, namely, that the interaction creates disturbances in a large volume surrounding the obstacle, thereby increasing the detectability of a planetary magnetic field. In the following, the observations of the properties of the interplanetary medium and the development of theoretical models for the solar wind will be summarized. Next, from the available measurements, parameters typical of a quiet solar wind will be calculated. The interaction of the solar wind with the Earth's dipolar magnetic field will be described, and the method employed in scaling the observed effects for interactions with dipolar fields of different moments will be given. Finally, some speculations concerning interactions of the solar wind with planetary bodies not shielded by magnetic fields will be summarized.

The total ionizing component of cosmic radiation was measured on October 11, 1958 by means of an ionization chamber mounted on the Pioneer I lunar probe vehicle. Data were taken over an altitude range of 3500 to 36,000 km and a latitude range of 35°N to 5°N. The calibration procedure and the analysis of the telemetered data are described.

Measurements of the magnetic fields in the distant geomagnetic cavity and in the region of interaction between the solar wind and the magnetosphere are described. These measurements were obtained on March 11, 1960, with instruments aboard the interplanetary probe Pioneer 5 in the region between 5.2 and 15.4 Re and between 1500 and 1700 local time. The observations obtained between 5.2 and 15.4 Re are found to be consistent with a geomagnetic boundary region that exhibits an enhanced field just inside the geomagnetic cavity, and beyond the cavity, exhibits a region of disordered fields in which both the mean field and the amplitude of the field variations decrease with increasing geocentric distance. Evidence for the extension of an interaction region to at least 25.6 Re along the spacecraft trajectory on the day of the launch is presented. The observations are compared with two models for this interaction region.

Power spectra and fluctuations of interplanetary magnetic field from Mariner 4 data for solar active and quiet days

In this note, evidence is presented to show that variations in the direction of V, the solarwind plasma velocity, produce a significant part of the activity that is measured by Kp. The activity produced by these variations in V/V may well be dominant during periods of low to moderate Kp. However, other solar-wind quantities may also be effective in producing geomagnetic activity. Most of the coupling mechanisms between the magnetosphere and the solar wind that have been proposed to date may be separated into two categories. In the first category are the mechanisms that produce the Kp component of geomagnetic activity in such a way that the level of activity depends upon the magnitude of some solar-wind quantity, or some function of more than one such quantity. Presumably, then, somewhere in the region of interaction between the solar wind and the magnetosphere, the transfer of the energy required for geomagnetic activity occurs at a rate determined by the magnitude of the controlling solar-wind quantity. Thus one would expect Kp and the magnitude of this controlling quantity, as measured in near-earth interplanetary space, to be correlated.

Electrostatic turbulence in bow shock magnetic structures observed by OGO 5, explaining turbulence as ion acoustic or Buneman mode due to two stream instability

In this paper we examine the changes in the magnetic field on the midilight meridian 15-8 R/sub E/ behind thc earth. These changes are conveniently divided into two main phases: a growth phase and an expansion phase. The beginnings of the growth phase coincide within experimental error to the arrival of a southward solar wind magnetic field at the dayside magnetopause and also to the beginning of the growth phase at synchronous orbit and ground observatories. During the growth phase the magnitude of the field increases in the lobe of the tail. In the cusp both the magnitude and the inclination of the field incrcase. Slow changes in the cross-tail field aecompany these effects. Nearly coincident with the onset of the polar substorm the lobe field magnitude begins to decrease, the cusp field magnitude and inclination decrease, and there are sudden changes in the cross-tail field. About this time the plasma sheet begins to expand. Inside the expanding plasma sheet and associated with the field rotation ara large fluctuations in the field. The response of the field at 8 R/sub E/ was simultaneous with the ground onset, whereas further back at 11.3 and 2.4 R/sub E/ above the expected neutralmore » sheet the largest changes were delayed by 15 min. (auth)« less

Geomagnetic variability relations to interplanetary magnetic field transverse fluctuations, discussing data from Mariner flights

During a period of intense solar activity between March 26 and April 6, 1960, measurements of the magnetic fields in interplanetary space were obtained with the space probe, Pioneer V. The measured component of the fields, associated with the solar plasma ejected during this period, attained strengths of 40 or more gammas (compared to an average value of 2.7 gammas during normal periods). It is argued that these comparatively high fields existed in interplanetary space for many hours. From estimates of the plasma velocity, the radial extent of these regions of high field is calculated to be 0.7 AU. The fields associated with these regions were observed to be quite regular. Similar results accompanied most of the more intense solar activity during the flight of Pioneer V. Possible implications of these results are discussed.

Interplanetary magnetic field power spectra from Mariner 4 spacecraft data used to determine cosmic ray diffusion tensor and variation in interplanetary space

Variations in polarity distribution of interplanetary magnetic field from observations by Mariner II and IV and IMP I

Interplanetary magnetic field measurements from Mariner and OGO satellites at various paths, regions and intervals, finding dominant polarity effect dependent on sun latitude

Charged particle behavior in idealized interplanetary magnetic field, discussing Parker spiral field model

Band limited micropulsations observed in space during magnetospheric substorm by fluxgate magnetometer on OGO 5

Satellite studies of magnetospheric substorms are necessarily statistical since there are few properly instrumented satellites in space at any one time. To carry out these studies, one selects an ensemble of similar substorms, determines an origin for substorm time, and performs an ensemble average to eliminate noise. Similarity between different substorms can be determined from contour maps of midlatitude magnetic perturbations as a function of longitude and universal time. The origin of substorm time can also be determined from midlatitude perturbations and is usually taken as the beginning of the positive bay near midnight. Use of these techniques has led to a phenomenological model of magnetospheric substorms that includes a growth phase. The controversy over the existence of the growth phase is at least partially a consequence of differing definitions of the origin of substorm time. Use of auroral zone magnetograms often gives an earlier onset than midlatitude magnetograms. However, midlatitude onsets do appear to order satellite data systematically. In this paper we illustrate the discrepancy between these two onset times and show how midlatitude onsets order satellite data in agreement with the growth phase model. Three events of increasing complexity are considered. Each has been previously reported in the literature and later criticized. We speculate that a polar magnetic substorm begins when a pair of X- and O-type neutral lines form on closed field lines within the plasma sheet. Subsequently, the midlatitude positive bay begins when magnetic merging first encounters open field lines in the lobe of the tail.

Quasi-sinusoidal fluctuations of magnetic field during geomagnetic storms measured by ATS 1 in synchronous equatorial orbit

The flights of Mariner 2, 4, and 5 yielded measurements of the interplanetary magnetic field between heliocentric ranges 0.65 and 1.5 AU and between heliographic latitudes 7.5° and −7.5°. Our analysis of these measurements indicates that for a given polarity the north-south component or more precisely the θ component in a spherical polar coordinate system (r, θ, ϕ), has in general a mean value that is significantly different from zero. For an interplanetary field directed outward from the sun, the mean value of Bθ is negative (northward) above the solar equatorial plane and positive (southward) below it and vice versa for an inward field. Thus, at a point off the equator, the component of B in the rθ plane is tilted away from the equatorial plane more than the radial from the sun through that point. The magnitude of the mean value probably depends on both r and θ. By assuming that the radial dependence is a power law in r and the θ dependence is a constant times the latitude β = [(π/2) - θ], we defined BθS as (〈Bθ+〉 - 〈Bθ−〉)/2, and from Mariner 4 data we obtained BθS ≃ −(0.09±0.03)β(r/r0)−(1.8±0.2), where Bθ is in units of 1 γ, β is in degrees, and r0=1.0 AU.

One class of waves observed in the interplanetary medium within several earth radii of the earth's bow shock consists of discrete wave packets with amplitudes that are a significant fraction of the background magnetic field. In the spacecraft frame, these wave packets have periods of about 2.5 sec, grow rapidly in time and decay more slowly, and are left-handed with respect to the magnetic field. By using the measured solar wind density and cold plasma dispersion theory, however, we show that these wave packets must be right-handed in the plasma frame at frequencies from about 2 to 4 times the proton gyrofrequency. The propagation vector of these waves is found to point away from the earth's bow shock and to lie between the solar wind flow direction and the average spiral field. The waves are being carried away from the sun by the solar wind, since their velocity is less than the solar wind speed, but they appear to be associated with the intersection of the field line with the earth's bow shock. Since they must be generated in the solar wind plasma, this generation may be caused by the presence of particles streaming upstream from the shock.

Magnetic field characteristics in the earth's magnetosheath as observed by the Mariner 4 magnetometer experiment have been described by Coleman et al. [1966] and Siscoe et al. [1967a]. This Letter presents further results of that experiment from the region where multiple shock transitions were observed. The data strongly suggest the existence of primarily torsional oscillations near the shock with periods between 10 and 40 sec in the spacecraft frame and planes of oscillation aligned roughly parallel to the expected shock front. Mariner 4 was launched November 28, 1964, and traversed the magnetosheath between 1 and 2 RE (earth radii) south of the ecliptic plane, essentially in the 0400 local time meridian plane. Geomagnetic conditions at this time were very quiet. Four vector field measurements were made in each 12.6-sec data frame with intermeasurement spacings of 1.5, 0.9, 2.4, and 7.8 sec. Data digitalization produced an uncertainty of ±0.35γ (1γ = 10-5 gauss) on each component. There is also an uncertainty that could be as large as 1γ, but which we estimate from the consistency with the spiral field model to be about 0.5γ, in the determination of each component of the spacecraft field of about 30γ at the magnetometer. This leaves the zero from which all interplanetary field values are measured correspondingly uncertain.

A preliminary analysis of the data from the UCLA magnetometer on board the Apollo 15 subsatellite indicates that remnant magnetization is a characteristic property of the Moon, that its distribution is such as to produce a rather complex pattern or fine structure, and that a detailed mapping of its distribution is feasible with the present experiment. The analysis also shows that lunar induction fields produced by transients in the interplanetary magnetic field are detectable at the satellite orbit so that in principle the magnetometer data can be used to determine the latitudinal and longitudinal as well as radial dependences of the distribution of electrical conductivity within the Moon. Finally, the analysis indicates that the plasma void or diamagnetic cavity which forms behind the Moon when the Moon is in the solar wind, is detectable at the satellite's orbit and that the flow of the solar wind near the limbs is usually rather strongly disturbed.

Substorm related magnetic field variations in near geomagnetic tail from OGO 5 inbound pass

Quiet day geomagnetic field measurements at synchronous orbit ATS 1, calculating equatorial component of interaction force between solar wind and earth

Mariner IV flight magnetometer data, determining interplanetary field and geomagnetic variability relationship

Magnetometer data obtained during the first four lunations after the deployment of the Apollo 15 subsatellite have been used to construct contour maps of the lunar magnetic field referred to 100 km altitude. These contour maps cover a relatively small band on the lunar surface. Within the region covered there is a marked near side-far side asymmetry. The near-side field is generally weaker and less structured than the far-side field. The strongest intrinsic lunar magnetic field detected is between the craters Van de Graaff and Aitken, centered at 20°S and 172°E. The variation in field strength with altitude for this feature suggests that its scale size is on the order of 80 km. A magnetization contrast between this region and its surroundings of the order of 6 × 10−5 emu-cm−3 is obtained assuming a 10-km thick slab. Preliminary Apollo 16 magnetometer data at extremely low altitude (0 to 10 km) show a very structured magnetic field with field strengths up to 56γ. Large compressions in the magnetic field magnitude, just above the lunar limb regions, are occasionally detected when the Moon is in the solar wind. The occurrence of limb compressions is strongly dependent on the selenographic coordinates of the lunar region on the solar wind terminator beneath the orbit of the sub-satellite. The discovery of remanent magnetization of varying strength over much of the lunar surface and its correlation with limb compression source regions supports the hypothesis that limb compressions are due to the deflection of the solar wind by regions of strong magnetization at the lunar limbs. If this hypothesis is correct, then the map of lunar regions associated with compressions indicates that the northerly equatorial region on the far side is less strongly magnetized than the southerly equatorial region on the far side.

Quasi-sinusoidal magnetic-field (micropulsations) are regularly observed at ATS 1 during geomagnetic storms. The wave events typically occur during the main phase and tend to be confined to the afternoon sector. All observed events have been closely correlated with magnetospheric substorm activity. Power spectral analysis shows that the observed oscillations are composed of harmonically related components. Twenty-six wave events were observed in 1967; the analysis of three typical Pc 5 events is presented here. The observations are compared with the predictions of theory for a hot inhomogeneous plasma. It is suggested that the observed micropulsations can be understood as the occurrence of either an Alfven wave instability, or a drift instability, of the enhanced storm-time ring current.

Mariner 2 magnetometer data are presented with a more thorough analysis and discussion than in a previously published preliminary report. No magnetic fields attributable to Venus were detected. An upper bound on the magnetic dipole moment of Venus Mv is estimated in several ways, subject to uncertainties regarding the dipole orientation and the nature of the interaction between the solar wind and the planetary field. On the basis of Mariner's not entering the Venus magnetosphere, the upper bound lies between 1/2 and 1/6 of ME, the earth's magnetic dipole moment. However, the absence of field fluctuations near Venus corresponding to the shell of disordered fields detected outside the earth's magnetosphere by satellites and space probes shows that Mv is less than ME by at least an order of magnitude and makes it probable that Mv is less than ME/20. This result is consistent with qualitative predictions based on the dynamo theory of planetary fields. The energy flux of cosmic radiation above the Venus atmosphere, except perhaps for cosmic rays of the lowest energies, should everywhere approximate the intensity in the earth's polar regions. Any radiation zone of trapped high-energy electrons is likely to be minor in comparison with the earth's.

The study of ULF waves in space has been in progress for about 12 years. However, because of numerous observational difficulties the properties of the waves in this frequency band (10-3 to 1 Hz) are poorly known. These difficulties include the nature of satellite orbits, telemetry limitations on magnetometer frequency response and compromises between dynamic range and resolution. Despite the paucity of information, there is increasing recognition of the importance of these measurements in magnetospheric processes. A number of recent theoretical papers point out the roles such waves play in the dynamic behavior of radiation belt particles.

Magnetic field measurements made with the Apollo 15 subsatellite established the existence of remanent magnetization over much of the Moon's surface. The major features of the structure of the measured remanent field are apparently associated with large craters, the most apparent detected to date being that in the vicinity of the Van de Graaff crater. A preliminary contour map of relative magnetic intensity was produced for a limited region of the Moon. A high resolution plot of the Van de Graaff region was constructed from vector data in an attempt to locate the source of the largest feature observed.

A study of several 10- to 45-sec wave events in the morning sector at Tungsten, Northwest Territories, Canada (approximately at the foot of the ATS 1 field line), reveals that at least two distinctly different types of wave phenomena occur. Digital power spectral and coherency analysis has been used to determine the average characteristics of the two types of waves. The first, identified as Pi 1, is characterized by a broad-band spectral enhancement with an average center frequency of ∼0.07 Hz (∼14 sec), nearly linear E-W polarization, and a close association with substorms. The second type, Pc 3, is characterized by narrow spectral lines at an average frequency of ∼0.04 Hz (∼27 sec), nearly linear N-S polarization, and no close association with substorms. Although these two types of wave activity occur in the same local time sector, sometimes simultaneously, there is no indication of any relationship between them.

Measurements of the interplanetary magnetic field taken with Imp 3, Pioneer 6, and Explorer 34 constitute a large portion of the data available at low and moderate solar activity and provide nearly continuous coverage from mid-1965 through 1966 without radial effects. Study of these observations provides further evidence for the following B sub theta effect initially discovered with Mariners 2, 4, and 5. At low or moderate solar activity, the mean value of B sub theta is negative (approximately northward in the observations) above the solar equatorial plane and positive below it for an interplanetary field directed outward from the sun, and vice versa for an inward field. Thus, for an outward field, the r-theta component of a line of magnetic force above or below the equatorial plane was skewed relative to the average value of r in the direction away from the equatorial plane. Comparisons between different spacecraft are discussed.