We use IMP 8 plasma, magnetic field, and energetic ion observations within the Earth's foreshock from January through August 1995 to determine the effects of energetic ion bursts on the ambient solar wind for comparison with model predictions. Owing to the spiral interplanetary magnetic field orientation, the events are far more common upstream from the prenoon than postnoon bow shock. Pressures associated with the energetic ions depress foreshock magnetic field strengths and plasma densities. The magnitude of the depression is proportional to the intensity of energetic ions. The excavated plasma and magnetic field sometimes pile up in narrow regions of enhanced plasma densities and magnetic field strengths, but depressed flow velocities, just outside the foreshock cavities. Typical amplitudes of the depressions and enhancements at IMP 8 far upstream from the bow shock are far less (20% and 10%, respectively) than those seen in past case studies of events observed just outside the bow shock. The cavities occur preferentially during high‐speed solar wind streams but show no clear dependence upon other solar wind parameters. The distribution of burst durations resembles those for interarrival times for interplanetary magnetic field discontinuities, magnetopause motion, and flux transfer events, suggesting causal relationships between these phenomena.
Voyager 2 observations have shown that Uranus possesses a well‐developed bipolar magnetotail similar in certain characteristics to that of Earth, in spite of an anomalously large tilt of the planetary magnetic dipole to the rotation axis at Uranus. The intensity of the magnetic field in the tail lobes decreases with increasing distance down the tail from the planet as | x SM | −0.59±0.03 . This gradient is similar to that found in the Earth's tail but significantly less steep than that observed in the tails of Jupiter and Saturn. The thickness of the plasma sheet is a minimum (∼10 R U ) near the tail center, increasing toward the flanks as at Earth. Pressure balance within the plasma sheet is maintained predominantly by protons and electrons with energies 10 eV to 6 keV. Except in transient events, the contribution of ≥28‐keV protons to pressure balance in the sheet is <5%. This is in contrast to the dominant role played by more energetic plasma ions in the Jovian magnetotail. An average value of β ∼ 7 was found in the plasma sheet at Uranus and ∼0.1 in the lobe plasma. The Uranian magnetic tail was observed to rotate 360° about its longitudinal axis, a result of the approximately sunward pointing planetary rotation axis at the time of encounter. This, together with the large tilt (60°) of the magnetic dipole, results in a small but measurable twist in the tail's magnetic lines of force, with a derived helical pitch of 5.5° ± 3.0°. The B yz (SM) component of the tail field can be modeled as the sum of the twist component, a radial (diverging or converging from the x SM axis) component, and a component parallel to the z SM axis that closes through the neutral sheet and is strongest there. The cross‐tail current density at the neutral sheet is estimated to be ≃3 × 10 −11 A m −2 . Large temporal variations observed in magnetic fields and plasmas during the Voyager 2 traverse of the magnetotail may have been produced by substorm activity.
A correlative survey of Magnetometer (MAG) and Planetary Radio Astronomy (PRA) 1.2 kHz continuum radiation measurements from Voyager 2 provide evidence for at least eight distant Jovian magnetotail sightings occurring about once a month over the first 2/3 of 1981 at distances of ∼ 5,000 to ∼ 9,000 R J . The occurrences of these events are in good agreement with prior Plasma Wave Science and Plasma Science identifications. Observations of these distant magnetotail, or tail filament, encounters appear most prevalent in both MAG and PRA data sets when the spacecraft was closest to the Jupiter‐Sun axis at ≃ 6,500 R J from the planet; the PRA events are also most intense during those times. A specific tail encounter occurring in mid‐February 1981 is analyzed and shown to possess a remarkably symmetric structure in its central region. Tail bipolarity is characteristic of most of the eight events.
An updated analysis and interpretation are presented of the magnetic field observations obtained during the Mariner 10 encounter with the planet Mercury on March 29, 1974. The combination of data relating to position of the detached bow shock wave and magnetopause and the geometry and magnitude of the magnetic field within the magnetospherelike region surrounding Mercury lead to the conclusion that an internal planetary field exists with dipole moment approximately 5.1 × 1022 G cm³. The limited data set precludes quantitative determination of an intrinsic field more complex than a centered dipole. The dipole axis has a polarity sense similar to that of earth and is tilted 7° from the normal to Mercury's orbital plane. The magnetic field observations reveal a significant distortion of the modest Hermean field (350 γ at the equator) by the solar wind flow and the formation of a magnetic tail and neutral sheet which begins close to the planet on the night side. Presently, an active dynamo mechanism in the planetary interior appears to be favored in the interpretation of the field origin, although fossil remanent magnetization cannot be excluded. The composite data set is not consistent with a complex induction process driven by the solar wind flow.
Data from the Goddard Space Flight Center magnetometers on Voyager 2 have yielded on inbound trajectory observations of multiple crossings of the bow shock and magnetosphere near the Jupiter-sun line at radial distances of 99 to 66 Jupiter radii ( R J ) and 72 to 62 R J , respectively. While outbound at a local hour angle of 0300, these distances increase appreciably so that at the time of writing only the magnetopause has been observed between 160 and 185 R J . These results and the magnetic field geometry confirm the earlier conclusion from Voyager I studies that Jupiter has an enormous magnetic tail, approximately 300 to 400 R J 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. The spacecraft passed within 62,000 kilometers of Ganymede (radius = 2,635 kilometers) and observed characteristic fluctuations interpreted tentatively as being due to disturbances arising from the interaction of the Jovian magnetosphere with Ganymede.
Magnetic field measurements made over a 21‐hour interval during the Mariner 10 encounter with Venus were used to study the downstream region of the solar wind‐Venus interaction over a distance of ≈ 100 R v (Venus radii). Mariner 10 encountered Venus on February 5, 1974, with closest approach at 1702 UT. For most of the day before closest approach the spacecraft was located in a sheathlike region which was apparently bounded by the planet's bow shock on the outer side and either a planetary ‘wake boundary’ or a transient boundarylike feature on the inner side. The spacecraft made multiple encounters with the wakelike boundary during the 21‐hour interval with an increasing frequency as it approached the planet. Each pass into the wake boundary from the sheath region was consistently characterized by a slight decrease in magnetic field magnitude, a marked increase in the frequency and amplitude of field fluctuations, and a systematic clockwise rotation of the field direction when viewed from above the plane of Venus' orbit. These boundary crossings were not accompanied strictly by hydromagnetic directional discontinuities, however, but occasionally (∼⅓ of the crossings) such a discontinuity was sufficiently close to the crossing zone to be considered part of the boundary transition. There were a significantly larger number of discontinuities in the overall 21‐hour period than were observed on average during other comparable periods both before and after encounter. A simple large‐scale draped field model in the sense of a magnetic ‘comet tail’ was found not to hold for the downstream region. The sporadic observation of the wake during the near‐encounter period may have been controlled by changes in the direction of the interplanetary field.
We present simultaneous observations of magnetic fields and flow anisotropies of solar energetic (several MeV) protons and electrons made by the ISEE 3 and IMP 8 spacecraft during the passage of an interplanetary magnetic cloud on February 16 ‐ 17, 1980. ISEE 3 was situated ∼250 R E upstream of Earth at the L l libration point, and IMP 8 was crossing the Earth's northern tail lobe and magnetosheath in an dusk‐dawn direction at a mean downtail distance of ∼27 R E . Several solar particle onsets were seen at ISEE 3 during cloud passage, with corresponding energetic particle signatures being observed in the geomagnetic tail by IMP 8. Various differences in the particle signatures at the two spacecraft occur, however, and these may be interpreted in terms of the magnetic field line topology of the cloud, the connectivity of the cloud field lines to the solar surface, and the interconnection between the magnetic fields of the magnetic cloud and of the Earth. The observations are consistent with a model of magnetic clouds according to which these mesoscale configurations are curved magnetic flux ropes attached at both "ends" to the Sun's surface and extending to 1 AU. The energetic particles stream out of the Sun along the cloud's helical field lines, arrive with appropriate time delays at an observation site inside the cloud, and become counterstreaming after mirroring within the cloud. When the field in the magnetic cloud had a persistent southward component, we can directly confirm the interconnection between the terrestrial and the cloud magnetic fields at the location of IMP 8 in the magnetotail. While at ISEE 3 inside the magnetic cloud the energetic particles have a strong east‐west bidirectional flow, IMP 8 observes a tail‐aligned bidirectional flow (i.e., roughly orthogonal to the flow direction at ISEE 3) as the energetic particles travel earthward along lobe field lines closer to Earth. Thus we have a case of solar particles gaining access into the magnetosphere at low altitudes first through magnetic connection of the magnetic cloud to the Sun and, subsequently, via the interconnection of the cloud's field lines to those of the Earth.
A prolonged period of B y > 0 dominated IMF conditions was monitored by WIND on Dec., 18–19, 1994. Observations on convection over the southern polar cap in the middle of this period (IMF B z ∼ 0) were available from AKEBONO. Detection of the well‐known dawn‐to‐dusk flow observing simultaneously energy‐dispersed ions in the cusp strongly indicates that the field lines reconnected at the dayside were azimuthally accelerated. GEOTAIL happened to be in the duskside flank at (X GSM , Z GSM ) ∼ (−15 ∼ −23, −1 ∼ −7)R E . The GEOTAIL data not only prove that this near‐tail flank region in the southern hemisphere is filled with azimuthally accelerated flux tubes that are loaded with solar wind plasma, but they also show that O + ions are transported tailward along these field lines. The latter is a new finding which suggests a route for transporting O + ions from the ionosphere to the magnetotail.
