Scott Jay Bolton

Outflow of hydrogen ions from Ganymede

On 6 September 1996 plasma measurements were obtained in the vicinity of Ganymede as the Galileo spacecraft passed by this moon with a closest approach distance of 261 km. Near Ganymede a dense, cold plasma region was found to be embedded in Jupiters hot plasma sheet. The cold plasmas are hydrogen ions flowing outwards from Ganymede at supersonic speeds. Temperatures and maximum number densities of these ions were about 4 × 104 K and 100 /cm³, respectively. Over Ganymedes polar caps there is strong plasma convection with speeds in the range of 50 km/s, i.e., the flow is not primarily directed parallel to the local magnetic fields. The corresponding potentials across a Ganymede diameter are in excess of 100 kV. The primary source of the hydrogen ions is believed to be the water ices on the surface of Ganymede. However, the anticipated oxygen ion outflow is not present, which implies that the oxygen is left on the moons surface. The loss of hydrogen from Ganymedes surface is about 3 × 109 grams/year.

A revised model of Jupiter's inner electron belts: Updating the Divine radiation model

[1]xa0In 1983, Divine presented a comprehensive model of the Jovian charged particle environment that has long served as a reference for missions to Jupiter. However, in situ observations by Galileo and synchrotron observations from Earth indicate the need to update the model in the inner radiation zone. Specifically, a review of the model for 1 MeV < E < 100 MeV trapped electrons suggests that, based on the new synchrotron observations, the pitch angle distributions within L < 4 need to be updated by introducing two additional components: one near the Jovian magnetic equator and one at high magnetic latitudes. We report modifications to the model that reproduce these observations. The new model improves the fit to synchrotron emission observations and remains consistent with the original fit to the in situ Pioneer and Voyager data. Further modifications incorporating observations from the Galileo and Cassini spacecraft will be reported in the future.

Low‐energy electron measurements at Ganymede with the Galileo spacecraft: Probes of the magnetic topology

During the close flyby of Ganymede by the Galileo spacecraft on 6 September 1996 a plasma analyzer was used to obtain comprehensive measurements of the thermal electron plasmas in the vicinity of this moon. Our initial analyses are directed toward the character of energy influxes into Ganymedes polar caps and the pitch angle distributions for warm electrons in the energy range of 70 eV to 4.5 keV. The source of these electrons is Jupiters plasma sheet within which Ganymede was embedded during the flyby. These electrons were present along the entire trajectory and provide the opportunity to use their pitch angle distributions with respect to the magnetic field in order to investigate the magnetic field topology at Ganymede. Observations of the loss cones for these pitch angle distributions are a simple means of determining whether or not a given magnetic field line intersects the moons surface. It is found that the observed pitch angle distributions are inconsistent with the current magnetostatic model for the magnetic fields in the vicinity of Ganymede. Thus a major revision of this model is required that accounts for the contributions of the plasma dynamical interaction in order to achieve a more accurate assessment of an intrinsic magnetic moment in the interior of this moon. In addition, the electron energy fluxes into the polar cap at the time of flyby were measured to be about 1 erg/cm²-s, or a total of approximately 5 × 1016 ergs/s over each polar cap available for excitation and ionization of any atmosphere near the moons surface.

Changes in Jupiter's 13-cm Synchrotron Radio Emission Following the Impact of Comet Shoemaker-Levy-9

Results of an observing program to monitor the synchrotron radio emission from Jupiters inner radiation belts before, during and after the impact of Comet SL-9 are reported. The observations were made at 2295 Mhz as part of the NASA-JPL Jupiter Patrol, a long-term radio astronomy monitoring program begun in 1971. The data indicate that the intensity of the synchrotron emission at 13 cm wavelength increased by 27 percent within a few days after the comet impacts; the longitudinal beaming curve was distorted during the week of impacts; the magnetic latitude beaming curves flattened after the week of impacts suggesting an increase in the emission at higher magnetic latitudes; and the decay of the enhanced emission is consistent with an exponential with a time constant of ∼125 days. The reported changes following the SL-9 impact are unprecedented in the 23-year history of the Jupiter Patrol.

Atmospheric loss of energetic electrons in the Jovian synchrotron zone

Abstract For decades, ground-based radio observations of Jovian synchrotron radiation have shown emission originating predominantly from the equatorial region and from high-latitude regions (lobes) near L∼2.5. The observations show a longitudinally asymmetric gap between the emission peaks of the lobes and the atmosphere of Jupiter. One possible explanation for these gaps is the loss of electrons through collisions with atmospheric neutrals as the electrons bounce along magnetic field lines and drift longitudinally in the presence of asymmetric magnetic fields. To assess this hypothesis, we applied the recently developed O6 and VIP4 magnetic field models to calculate the trajectories of electrons as they drift longitudinally in Jupiters magnetic field, and derive the sizes of their equatorial drift loss cones. We then identified the shells on which electrons would be lost due to collisions with the atmosphere. The calculated drift loss cone sizes could be applied in future to the modeling of electron distribution functions in this region and could also be applied to the study of Jovian auroral zone. This method also allowed us to compute the shell-splitting effects for these drifting electrons and we find the shell-splitting to be small (⩽0.05RJ). This justifies a recent modeling assumption that particles drift on the same shells in a three-dimensional distribution model of electrons. We also compared the computed gaps with the observed gaps, and found that the atmospheric loss mechanism alone is not able to sufficiently explain the observed gap asymmetry.

Cassini/Huygens flyby of the Jovian system

[1]xa0This is an introduction to the series of 15 papers that follow. These report scientific results from the fields-and-particles instruments on Cassini/Huygens. For these works, their data acquisition started as early as 16 months before the passage by Jupiter and, for some, continued up to the approach to Saturn. The flyby of Jupiter provided a gravitational assist, which was necessary for the spacecraft to reach its destination, the Saturnian system. In addition to the scientific results, the Cassini/Huygens team gained much because Jupiter provided a dress rehearsal for skills needed at Saturn. The flyby also fostered international cooperation between the Cassini/Huygens teams, the Galileo teams, and the scientific teams using instruments in orbit about the Earth and on the ground. A number of important discoveries and insights came from these cooperative efforts. The boundary physics and compression of the Jovian magnetosphere was observed by both fields and particle instruments. The propagation of the solar wind between Earth and Jupiter was studied. The source of Jovian radio emission was further characterized. The first images of energetic neutral atoms at Jupiter were obtained.

The slow growth of humility

Galileos stunning discovery of the four largest satellites of Jupiter forced the over throw of the Earth-centered cosmology that had dominated astronomy for centuries. Such a fundamental transformation of the Western Worlds view of its importance in the cosmos could be expected to produce some humility in society. However, the deep desire for our uniqueness continues to struggle with the astronomical evidence.