Magnetic Field Studies at Jupiter by Voyager 2: Preliminary Results

Magnetosheath jets at Jupiter and across the solar system

The study of jets in the Earth's magnetosheath has been a subject of extensive investigation for over a decade due to their profound impact on the geomagnetic environment and their close connection with shock dynamics. While the variability of the solar wind and its interaction with Earth's magnetosphere provide valuable insights into jets across a range of parameters, a broader parameter space can be explored by examining the magnetosheath of other planets. Here we report the existence of anti-sunward and sunward jets in the Jovian magnetosheath and show their close association with magnetic discontinuities. The anti-sunward jets are possibly generated by a shock-discontinuity interaction. Finally, through a comparative analysis of jets observed at Earth, Mars, and Jupiter, we show that the size of jets scales with the size of bow shock.

Formation, habitability, and detection of extrasolar moons

The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoon's stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1-0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.

Magnetic fields at uranus

The magnetic field experiment on the Voyager 2 spacecraft revealed a strong planetary magnetic field of Uranus and an associated magnetosphere and fully developed bipolar masnetic tail. The detached bow shock wave in the solar wind supersonic flow was observed upstream at 23.7 Uranus radii (1 R(U) = 25,600 km) and the magnetopause boundary at 18.0 R(U), near the planet-sun line. A miaximum magnetic field of 413 nanotesla was observed at 4.19 R(U ), just before closest approach. Initial analyses reveal that the planetary magnetic field is well represented by that of a dipole offset from the center of the planet by 0.3 R(U). The angle between Uranus' angular momentum vector and the dipole moment vector has the surprisingly large value of 60 degrees. Thus, in an astrophysical context, the field of Uranus may be described as that of an oblique rotator. The dipole moment of 0.23 gauss R(3)(U), combined with the large spatial offset, leads to minimum and maximum magnetic fields on the surface of the planet of approximately 0.1 and 1.1 gauss, respectively. The rotation period of the magnetic field and hence that of the interior of the planet is estimated to be 17.29+/- 0.10 hours; the magnetotail rotates about the planet-sun line with the same period. Thelarge offset and tilt lead to auroral zones far from the planetary rotation axis poles. The rings and the moons are embedded deep within the magnetosphere, and, because of the large dipole tilt, they will have a profound and diurnally varying influence as absorbers of the trapped radiation belt particles.

Magnetic fields at neptune

The National Aeronautics and Space Administration Goddard Space Flight Center-University of Delaware Bartol Research Institute magnetic field experiment on the Voyager 2 spacecraft discovered a strong and complex intrinsic magnetic field of Neptune and an associated magnetosphere and magnetic tail. The detached bow shock wave in the supersonic solar wind flow was detected upstream at 34.9 Neptune radii (R(N)), and the magnetopause boundary was tentatively identified at 26.5 R(N) near the planet-sun line (1 R(N) = 24,765 kilometers). A maximum magnetic field of nearly 10,000 nanoteslas (1 nanotesla = 10(-5) gauss) was observed near closest approach, at a distance of 1.18 R(N). The planetary magnetic field between 4 and 15 R(N) can be well represented by an offset tilted magnetic dipole (OTD), displaced from the center of Neptune by the surprisingly large amount of 0.55 R(N) and inclined by 47 degrees with respect to the rotation axis. The OTD dipole moment is 0.133 gauss-R(N)(3). Within 4 R(N), the magnetic field representation must include localized sources or higher order magnetic multipoles, or both, which are not yet well determined. The obliquity of Neptune and the phase of its rotation at encounter combined serendipitously so that the spacecraft entered the magnetosphere at a time when the polar cusp region was directed almost precisely sunward. As the spacecraft exited the magnetosphere, the magnetic tail appeared to be monopolar, and no crossings of an imbedded magnetic field reversal or plasma neutral sheet were observed. The auroral zones are most likely located far from the rotation poles and may have a complicated geometry. The rings and all the known moons of Neptune are imbedded deep inside the magnetosphere, except for Nereid, which is outside when sunward of the planet. The radiation belts will have a complex structure owing to the absorption of energetic particles by the moons and rings of Neptune and losses associated with the significant changes in the diurnally varying magnetosphere configuration. In an astrophysical context, the magnetic field of Neptune, like that of Uranus, may be described as that of an "oblique" rotator.

The dusk flank of Jupiter's magnetosphere

Limited single-spacecraft observations of Jupiter's magnetopause have been used to infer that the boundary moves inward or outward in response to variations in the dynamic pressure of the solar wind. At Earth, multiple-spacecraft observations have been implemented to understand the physics of how this motion occurs, because they can provide a snapshot of a transient event in progress. Here we present a set of nearly simultaneous two-point measurements of the jovian magnetopause at a time when the jovian magnetopause was in a state of transition from a relatively larger to a relatively smaller size in response to an increase in solar-wind pressure. The response of Jupiter's magnetopause is very similar to that of the Earth, confirming that the understanding built on studies of the Earth's magnetosphere is valid. The data also reveal evidence for a well-developed boundary layer just inside the magnetopause.

Plasma Observations Near Saturn: Initial Results from Voyager 1

Extensive measurements of low-energy plasma electrons and positive ions were made during the Voyager 1 encounter with Saturn and its satellites. The magnetospheric plasma contains light and heavy ions, probably hydrogen and nitrogen or oxygen; at radial distances between 15 and 7 Saturn-radii (Rs) on the inbound trajectory, the plasma appears to corotate with a velocity within 20 percent of that expected for rigid corotation. The general morphology of Saturn's magnetosphere is well represented by a plasma sheet that extends from at least 5 to 17 Rs, is symmetrical with respect to Saturn's equatorial plane and rotation axis, and appears to be well ordered by the magnetic shell parameter L (which represents the equatorial distance of a magnetic field line measured in units of Rs). Within this general configuration, two distinct structures can be identified: a central plasma sheet observed from L = 5 to L = 8 in which the density decreases rapidly away from the equatorial plane, and a more extended structure from L = 7 to beyond 18 Rs in which the density profile is nearly flat for a distance +/- 1.8 Rs off the plane and falls rapidly thereafter. The encounter with Titan took place inside the magnetosphere. The data show a clear signature characteristic of the interaction between a subsonic corotating magnetospheric plasma and the atmospheric or ionospheric exosphere of Titan. Titan appears to be a significant source of ions for the outer magnetosphere. The locations of bow shock crossings observed inbound and outbound indicate that the shape of the Saturnian magnetosphere is similar to that of Earth and that the position of the stagnation point scales approximately as the inverse one-sixth power of the ram pressure.

Plasma Observations Near Jupiter: Initial Results from Voyager 2

The first of at least nine bow shock crossings observed on the inbound pass of Voyager 2 occurred at 98.8 Jupiter radii (R(J)) with final entry into the magnetosphere at 62 R(J). On both the inbound and outbound passes the plasma showed a tendency to move in the direction of corotation, as was observed on the inbound pass of Voyager 1. Positive ion densities and electron intensities observed by Voyager 2 are comparable within a factor of 2 to those seen by Voyager 1 at the same radial distance from Jupiter; the composition of the magnetospheric plasma is again dominated by heavy ions with a ratio of mass density relative to hydrogen of about 100/1. A series of dropouts of plasma intensity near Ganymede may be related to a complex interaction between Ganymede and the magnetospheric plasma. From the planetary spin modulation of the intensity of plasma electrons it is inferred that the plasma sheet is centered at the dipole magnetic equator out to a distance of 40 to 50 R(J) and deviates from it toward the rotational equator at larger distances. The longitudinal excursion of the plasma sheet lags behind the rotating dipole by a phase angle that increases with increasing radial distance.

Plasma Wave Observations Near Jupiter: Initial Results from Voyager 2

This report provides an initial survey of results from the plasma wave instrument on the Voyager 2 spacecraft, which flew by Jupiter on 9 July 1979. Measurements made during the approach to the planet show that low-frequency radio emissions from Jupiter have a strong latitudinal dependence, with a sharply defined shadow zone near the equatorial plane. At the magnetopause a new type of broadband electric field turbulence was detected, and strong electrostatic emissions near the upper hybrid resonance frequency were discovered near the low-frequency cutoff of the continuum radiation. Strong whistler-mode turbulence was again detected in the inner magnetosphere, although in this case extending out to substantially larger radial distances than for Voyager 1. In the predawn tail region, continuum radiation was observed extending down to extremely low frequencies, approximately 30 hertz, an indication that the spacecraft was entering a region of very low density, approximately 1.0 x 10(-5) per cubic centimeter, possibly similar to the lobes of Earth's magnetotail.

Hot Plasma Environment at Jupiter: Voyager 2 Results

Measurements of the hot (electron and ion energies >/=20 and >/= 28 kiloelectron volts, respectively) plasma environment at Jupiter by the low-energy charged particle (LECP) instrument on Voyager 2 have revealed several new and unusual aspects of the Jovian magnetosphere. The magnetosphere is populated from its outer edge into a distance of at least approximately 30 Jupiter radii (R(J)) by a hot (3 x 10(8) to 5 x 10(8) K) multicomponent plasma consisting primarily of hydrogen, oxygen, and sulfur ions. Outside approximately 30 R(J) the hot plasma exhibits ion densities from approximately 10(-1) to approximately 10(-6) per cubic centimeter and energy densities from approximately 10(-8) to 10(-13) erg per cubic centimeter, suggesting a high beta plasma throughout the region. The plasma is flowing in the corotation direction to the edge of the magnetosphere on the dayside, where it is confined by solar wind pressure, and to a distance of approximately 140 to 160 R(J) on the nightside at approximately 0300 local time. Beyond approximately 150 R(J) the hot plasma flow changes into a "magnetospheric wind" blowing away from Jupiter at an angle of approximately 20 degrees west of the sun-Jupiter line, characterized by a temperature of approximately 3 x 10(8) K (26 kiloelectron volts), velocities ranging from approximately 300 to > 1000 kilometers per second, and composition similar to that observed in the inner magnetosphere. The radial profiles of the ratios of oxygen to helium and sulfur to helium (</= 1 million electron volts per nucleon) monotonically increase toward periapsis, while the carbon to helium ratio stays relatively constant; a significant amount of sodium (Na/O approximately 0.05) has also been identified. The hydrogen to helium ratio ranges from approximately 20 just outside the magnetosphere to values up to approximately 300 inside; the modulation of this ratio suggests a discontinuity in the particle population at approximately 50 to 60 R(J). Large fluctuations in energetic particle intensities were observed on the inbound trajectory as the spacecraft approached Ganymede, some of which suggest the presence of a "wake." Five-and 10-hour periodicities were observed in the magnetosphere. Calculations of plasma flow velocities with the use of Compton-Getting formalism imply that plasma is mostly corotating to large radial distances from the planet. Thus the Jovian magnetosphere is confined by a plasma boundary (as was implied by the model of Brice and Ioannidis) rather than a conventional magnetopause. Inside the plasma boundary there exists a discontinuity at approximately 50 to 60 R(J) we have named the region inside this discontinuity the "inner plasmasphere."

Planetary Radio Astronomy Observations from Voyager 2 Near Jupiter

The Voyager 2 Planetary Radio Astronomy experiment to Jupiter has confirmed and extended to higher zenomagnetic latitudes results from the identical experiment carried by Voyager 1. The kilometric emissions discovered by Voyager 1 often extended to 1 megahertz or higher on Voyager 2 and often consisted of negatively or, less frequently, positively drifting narrowband bursts. On the basis of tentative identification of plasma wave emissions similar to those detected by Voyager 1, the plasma torus associated with Io appeared somewhat denser to Voyager 2 than it did to Voyager 1. We report here on quasiperiodic sinusoidal or impulsive bursts in the broadcast band range of wavelengths (800 to 1800 kilohertz). A Faraday effect appears at decametric frequencies, which probably results from propagation of the radiation near its sources on Jupiter. Finally, we discuss the occurrence of decametric emission in homologous arc families.

Voyager 2: Energetic Ions and Electrons in the Jovian Magnetosphere

The Voyager 2 encounter has enhanced our understanding of earlier results and provided measurements beyond 160 Jupiter radii (R(J)) in the magnetotail. Significant fluxes of energetic sulfur and oxygen nuclei (4 to 15 million electron volts per nucleon) of Jovian origin were observed inside 25 R(J), and the gradient in phase space density at 12 R(J) indicates that the ions are diffusing inward. A substantially longer time delay versus distance was found for proton flux maxima in the active hemisphere in the magnetotail at Jovicentric longitudes lambda(III), = 260 degrees to 320 degrees than in the inactive hemisphere at lambda(III), = 85 degrees to l10 degrees . These delays can be related to the radial motion of plasma expanding into the magnetotail, and differences in the expansion speeds between the active and inactive hemispheres can produce rarefaction regions in trapped particles. It is suggested that the 10-hour modulation of interplanetary Jovian electrons may be associated with the arrival at the dawn magnetopause of a rarefaction region each planetary rotation.