Average and substorm conditions in the lobe and plasma sheet regions of the earth's magnetotail are studied as a function of downstream distance and east‐west location using ISEE 3 magnetometer and plasma analyzer measurements. On the basis of 756 magnetopause crossings a low‐latitude magnetotail diameter of 60±5 RE at |X| = 130 ‐ 225 RE is determined. The strength of the lobe magnetic field from |X| = 20 to 130 RE is shown to fall off as X−0.53±0.05. Flaring ceases on average at |X| = 120 ± 10 RE with a relatively constant BL = 9.2 nT beyond that distance. The ratios |By/Bx| and |Bz/Bx| in the translunar tail lobes are small and relatively constant with mean values of 0.10 and 0.06, respectively. These results are shown to be in good agreement with the Coroniti‐Kennel flaring tail models of lobe magnetic field configuration with slightly enhanced By due to Maxwell stresses exerted at the magnetopause by the solar wind. The plasma parameters Vx, ne, βe, and MA in the lobes all increase with distance down the tail while Te decreases. The mean values of these lobe quantities at |X| = 200 ‐ 220 RE are Vx = −200 ‐ 250 km/s, n = 0.1 ‐ 0.2 cm−3, βe = 0.02 ‐ 0.05, MA = 0.3 ‐ 0.4, and Te = 5 ‐ 8×105 °K. Strong density and weak velocity and temperature gradients are observed as ISEE 3 moves from the center of the lobes out toward the magnetopause. In particular, factor of 3–6 increases in plasma density are observed as the spacecraft moves from the center of the tail at |Y′| < 10 RE (Y′ refers to the aberrated GSM system) toward the dawn and dusk portions of lobes at |Y′| > 20 RE. Good agreement is found between the leaky magnetopause model of Pilipp and Morfill (1978) and the strong density/weak velocity gradients observed in the lobes. Substorm activity, as measured by AE(9), is only weakly correlated with magnetic field strength, electron beta, or Alfvénic Mach number in the lobes at |X| > 200 RE. The plasma sheet magnetic field intensity and electron temperature decrease with increasing downstream distance, while flow speed, density, and Alfvénic Mach number all increase. Average plasma sheet parameters at |X| = 200 ‐ 220 RE are B = 4.0 nT, Vx = −500 km/s, ne = 0.3 cm−3, Te = 1.2×106 °K, and MA = 2.7. Electron beta is independent of downstream distance with a mean value of approximately 0.7. On the basis of pressure balance arguments the estimated total plasma beta in the |X| > 60 RE plasma sheet is 4.5, and the Ti/Te ratio is 5.5. With respect to reconnection, the most significant results are the correlations between Bz, Vx, and AE(9) in the plasma sheet, the variation in these parameters with X and ± Y, and their implications for the location of the distant neutral line. The highest tailward flow speeds are found to be proportional to the magnitude of the embedded southward Bz. Furthermore, both tailward Vx and southward Bz are shown to be well correlated with AE(9). Earthward of |X| = 100 RE the average Bz is northward and the flow is on average sub‐Alfvénic. Between |X| = 100 and 180 RE the flow becomes predominan...

The Pioneer Saturn vector helium magnetometer has detected a bow shock and magnetopause at Saturn and has provided an accurate characterization of the planetary field. The equatorial surface field is 0.20 gauss, a factor of 3 to 5 times smaller than anticipated on the basis of attempted scalings from Earth and Jupiter. The tilt angle between the magnetic dipole axis and Saturn's rotation axis is < 1°, a surprisingly small value. Spherical harmonic analysis of the measurements shows that the ratio of quadrupole to dipole moments is < 10 percent, indicating that the field is more uniform than those of the Earth or Jupiter and consistent with Saturn having a relatively small core. The field in the outer magnetosphere shows systematic departures from the dipole field, principally a compression of the field near noon and an equatorial orientation associated with a current sheet near dawn. A hydromagnetic wake resulting from the interaction of Titan with the rotating magnetosphere appears to have been observed.

Evidence of a north-south asymmetry in the global heliosphere, Ðrst inferred from Ulysses cosmic-ray observations, is investigated using simultaneous Ulysses and W ind magnetic Ðeld observations. Such an asymmetry, presumably associated with a southward displacement of the heliospheric current sheet (HCS), is expected to produce signiÐcantly di †erent magnitudes of the radial Ðeld component, in o B R o , the SunÏs north and south magnetic hemispheres or, alternatively, in the positive and negative magnetic sectors. Ulysses, while at high latitudes, spends time predominantly in Ðrst one and then the other hemisphere. As a consequence, measurements in the positive sector are obtained several months later than measurements made in the negative sector, making comparisons susceptible to temporal changes. To address this ambiguity, the Ðelds in both sectors observed by the W ind spacecraft in the ecliptic were compared. A large di †erence in of B30% was observed at W ind between 1994 December and 1995 o B R o April, with larger in the south than in the north. Subsequent measurements show a gradual o B R o increase in the north (outward) radial component and a decrease in the south (inward) component, ending in only a small di †erence by 1995 June. Thus, the W ind observations are consistent with a southward displacement of the HCS of B10¡ and with the energetic particle observations. The secular time variation, which occurred as the spacecraft transited from the south to the north hemisphere, explains why a signiÐcant north-south di †erence in was not evident in the Ulysses measurements. o B R o The current sheet conÐguration and various questions and implications associated with these results are also discussed.

The Pioneer 11 vector helium magnetometer provided precise, contititious measurements of the magnetic fields in interplanetary space, inside Jupiter's magnetosphere, and in the near vicinity of Jupiter. As with the Pioneer 10 data, evidence was seen of the dynanmic interaction of Jupiter with the solar wind which leads to a variety of phenomena (bow shock, upstream waves, nonlinear magnetosheath impulses) and to changes in the dimension of the dayside magnetosphere by as much as a factor of 2. The magnetosphere clearly appears to be blunt, not disk-shaped, with a well-defined outer boundary. In the outer magnetosphere, the magnetic field is irregular but exhibits a persistent southward component indicative of a closed magnetosphere. The data contain the first clear evidence in the dayside magnetosphere of the current sheet, apparently associated with centrifugal forces, that was a donminatnt feature of the outbound Pionieer 10 data. A modest westward spiraling of the field was again evident inbound but not outbound at higher latitudes and nearer the Sun-Jupiter direction. Measurements near periapsis, which were nearer the planet and provide better latitude and longitude coverage than Pioneer 10, have revealed a 5 percent discrepancy with the Pioneer 10 offset dipole mnodel (D(2)). A revised offset dipole (6-parameter fit) is presented as well as the results of a spherical harmonic analysis (23 parameters) consisting of an interior dipole, quadrupole, and octopole and an external dipole and quadrupole. The dipole moment and the composite field appear moderately larger than inferred from Pioneer 10. Maximum surface fields of 14 and 11 gauss in the northern and southern hemispheres are inferred. Jupiter's planetary field is found to be slightly more irregular than that of Earth.

The fundamental aim of the Ulysses space mission is to extend our understanding of the heliosphere into three dimensions. By April 1994, the spacecraft had reached a heliographic latitude of 60°S. Hourly averages of the Ulysses heliospheric magnetic field observations have been analyzed to determine to what extent the underlying field direction within 60° of the heliographic equator can be described by the Parker spiral model. At all latitudes from near the ecliptic southward to 60°S, the most probable value of the azimuthal orientation of the field lines remained in approximate agreement with the Parker model. Once Ulysses passed southward of the maximum latitude of the heliospheric current sheet at about 30°S it became possible to study the distribution of the azimuth angle in purely inward polarity southern hemisphere fields without the confusion between the northern and southern hemisphere sectors. This distribution is revealed to be highly asymmetric with a greater probability of observing field lines with an azimuth angle less tightly wound than the most probable angle. Comparison with near‐ecliptic data showed a similar asymmetry in inward polarity field sectors while in outward polarity sectors there was an asymmetry in the opposite sense. We suggest that the asymmetry in the >30°S azimuth angle distribution arises due to the presence of long‐period radially outward propagating Alfvén waves in the solar wind flows originating from the southern polar coronal hole. In addition, from studying the meridional (north‐south) orientation of the field lines we find that at south heliographic latitudes within about 25° of the equator there is a tendency in the Ulysses data set for the field lines to be on average deflected equatorward of their expected direction, most likely due to flow deflections associated with the interaction of high‐ and low‐speed solar wind streams.

A variance analysis of high frequency, inertial range fluctuations in the polar magnetic field as recorded by the Ulysses spacecraft indicates that they are largely transverse to the ambient magnetic field direction and highly anisotropic. Despite large changes in the mean field direction caused by the presence of large amplitude Alfvén waves, alignment of minimum variance vectors of 5 minute intervals of linearly detrended data with the mean field direction was good, with around 75% of minimum variance vectors lying within 26° of the mean field direction during a typical 5 day interval. Such fluctuations transverse to the mean field have anisotropies—the ratio of power transverse to the field to that parallel—of around 30. We find a small tendency for the field‐transverse fluctuations to become more isotropic with distance. Intervals whose minimum variance directions are not field‐aligned appear to be indicative of a significantly different type of fluctuations, with higher total variances and lower anisotropy.

A detailed observational treatment of bidirectional electrons (~ 50 to 500 eV) in the distant magnetotail (r • 100 Rg) is presented. It is found that electrons in this energy range commonly exhibit strong, field-aligned anisotropies in the tail lobes. Because of large tail motions, the ISEE 3 data provide extensive sampling of both the north and south lobes in rapid succession. These data demonstrate directly the strong asymmetries that exist between the north and south lobes at any one time. The bidirectional fluxes are found to occur predominantly in the lobe directly connected to the sunward interplanetary magnetic field in the open magnetosphere model (north lobe for away sectors and south lobe for toward sectors). Electron anisotropy and magnetic field data are presented which show the transition from unidirectional (sheath) electron populations to bidirectional (lobe) populations. Thus we demonstrate the open nature of the distant magnetopause and show that the source of the higher-energy, bidirectional lobe electrons is the tailward directed electron heat flux population in the distant magnetosheath. Taken together, the present evidence suggests that the bidirectional electrons that we observe in the distant tail are closely related to the polar rain electrons observed previously at lower altitudes. Furthermore, these data provide strong evidence that the distant tail is composed largely of open magnetic field lines in contradistinction to some recently advanced models. INTRODUCTION The ISEE 3 deep-tail mission has provided new opportunities for exploring the geomagnetic tail and has already produced an extensive literature demonstrating many discoveries and synthetical understandings (e.g., Geophysical Research Letters, volume 11, special issue, October 1984). During its deep-tail excursions (October 1982 to Octob er 1983), ISEE 3 revealed that most features of the near tail were still identifiable [Bame et al., 1983; Slavin et al., 1983] but many new features were also seen. Particularly important have been studies demonstrating the "connectivity" between near-earth magnetospheric processes and the distant tail [e.g., Scholer et al., 1984a, b; Gosling et al., 1984; Baker et al., 1984; Slavin et al., 1984]. The distant tail during quiet conditions consists of a plasma sheet which separates two lobes of relatively low-• plasma [Tsurutani et al., 1984; Slavin eta!., 1985]. It is found, however, that lobe densities can be quite high [Gosling et al., 1984, 1985] and thus the distinction between the plasma (or "current") sheet and tail lobes often becomes more subtle than in the near tail [see Zwickl et al., 1984]. Superimposed on the "quiet" structure of the distant tail are dynamical features causod by the interaction between the magnetosphere and the solar wind. Of course, flapping motion in the variably directed solar wind flow plays a role [Bame et al., 1983], Moreover, the tail grows and diminishes in size in concert with the near-earth substorm growth and expansive phase onsets [Baker et al., 1984]. It ...

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