Mercury Transiting the Sun

Short (<1 min) and long time (> 5 min) variations of the plasmasheet magnetic field have been examined during all intervals when ISEE-3 was at distances x < −200 Re. It is determined that short period magnetic turbulence increases by a factor of ∼3 with increasing geomagnetic activity, as indicated by AE. In contrast, long period field variations with North-then-South signatures at plasmasheet entry occur ∼2.5 times more frequently than South-then-North signatures. This result, combined with other previous ISEE-3 results, is in agreement with the interpretation that the North-South plasmasheet features are plasmoids propagating tailward. However, a statistical examination of the geomagnetic activity relationship indicates that there does not appear to be any substorm dependence on these North-South events.

This review presents a summary of past work on the ISEE-3 distant tail magnetic field observations. An attempt has been made to bring the many results together as a coherent whole, in the hope that the reader can envision the direction of future research necessary to achieve an understanding of the dynamics of the magnetotail from 60 to 240 Re and perhaps beyond.

The Jovian magnetospheric field measured by Pioneer 10 and 11 can be well modeled by a combination of current systems composing an azimuthally symmetric current disc, a dusk-dawn current sheet in both the dayside and the nightside magnetosphere, and an image dipole to represent the effects of currents on the magnetopause. The inclusion of a dusk-dawn current sheet in the dayside magnetosphere allows observations obtained both inbound and outbound to be simultaneously fit by an azimuthally symmetric current disc (i.e., without the need for local time dependent current densities). Similar disc current intensities are found to describe both Pioneer 10 and Pioneer 11 encounters. During the Pioneer 10 inbound passage the magnetopause was rapidly pushed inside the spacecraft position by a solar wind compression event. The changes that occurred in the magnetospheric field at this time can be described by relatively simple changes in our model parameters. When the field lines of our model are extended beyond the region of fitting, both the Pioneer 10 and the Pioneer 11 encounters give similar profiles with magnetopause distances that are consistent with the actual encounters. The most striking feature of the models is that they suggest that the Jovian cusp is at much lower latitudes than is the case with the earth’s magnetosphere.

The properties of waves with frequencies below 3 Hz observed upstream of low Mach number (2–3) interplanetary shocks are discussed. High-frequency emissions (0.2–2 Hz in the spacecraft frame) are commonly detected immediately upstream (<2 RE) of the shocks, whereas lower frequency emissions (∼0.05 Hz) are found to extend upstream to much greater distances (typically 10 RE). Both emissions are right-hand circularly or elliptically polarized and generally propagate within a 15° cone angle relative to the ambient magnetic field. The lack of a significant compressional component for either of these waves is in agreement with propagation parallel to the ambient magnetic field. Upstream waves are detected principally in association with quasi-parallel shocks (θBn < 65°). Assuming the waves are propagating outward from the shock, an expression is derived for the wave frequency in the solar wind rest frame. The waves are found to have rest frame frequencies of 0.05–1 Hz and 10−3 to 10−2 Hz. Arguments are presented which exclude the possibility that the high-frequency waves are standing whistler mode waves. The most likely source of these emissions is generation at the shock by 100 eV to 1 keV electrons and propagation of the whistler mode waves into the upstream region. The lower frequency 10−3 to 10−2 Hz waves propagate at speeds near the Alfvén velocity, and hence cannot outrun the super-Alfvénic shocks. These waves must be locally generated by plasma instabilities in the upstream region. Generation by Landau electrons or ions can be ruled out due to the observation of parallel propagation of the waves. The most likely source is 1–10 keV cyclotron-resonant ions propagating away from the shock. The upstream waves bear many similarities to those observed in the earth's foreshock. The frequencies, polarization, and typical upstream extent are nearly identical. It is also deduced that the lower frequency waves of both regions are generated by keV ions streaming away from the shock. There are some differences, however. Waves upstream of interplanetary shocks are found to propagate parallel to the magnetic field (<15°), are noncompressive (ΔB/B ≤ 0.25) and are generally lower in amplitude. Additionally, there are extraordinary interplanetary events, for which the scale of the upstream wave region is greatly extended (∼1300 RE or 0.04 AU) and the field takes on a more turbulent character. The latter events should be of interest in modeling Fermi acceleration of ions at collisionless shocks.

Observations by Pioneer 10 and 11 show that the strongest azimuthal fields are observed near the dawn meridian (Pioneer 10) while the weakest occur near the noon meridian (Pioneer 11), suggesting a strong local time dependence for the corresponding radial current system. Modeling studies of the radial component of the field observed by both spacecraft suggest that the corresponding azimuthal current system must also be a strong function of local time. Both the azimuthal and the radial field component signatures exhibit sharp dips and reversals, requiring thin radial and azimuthal current systems. There is also a suggestion that these two current systems either are interacting or are due, at least in part, to the same current. We suggest that a plausible current model consists of the superposition of a thin, local time independent, azimuthal current system plus the equatorial portion of a taillike current system that extends into the dayside magnetosphere.

The magnetic field of the Jovian current disc has been modeled by using Euler functions and the Biot-Savart law applied to a series of concentric, but not necessarily coplanar, current rings. We find that a best fit to the Pioneer 10 outbound perturbation magnetic field data (Btotal - Bdipole) is obtained if the current disc is twisted (outer edges increasingly lag behind inner edges with radial distance) and also bent so as to tend toward parallelism with the Jovigraphic equator. The inner and outer radii of the disc appear to be about 7 RJ and 150 RJ, respectively, although some indication of a changing magnetopause location is apparent in the data. Because of the observed current disc penetrations, the bent disc also requires a deformation in the form of a bump or wrinkle whose axis tends also to exhibit spiraling. The radial dependence of the azimuthal current in the disc is not described by a simple power law, the outer region showing a smaller power law dependence. Modeling of the azimuthal field shows it to be due to a thin radial current sheet, but there is some evidence that this may, in fact, be due in large part to penetration of a tail current sheet as suggested by the Voyager observations.