Steven Peter Joy

Europa and Callisto: Induced or intrinsic fields in a periodically varying plasma environment

Magnetometer data from four Galileo passes by the Jovian moon Europa and three passes by Callisto are used to interpret the properties of the plasma surrounding these moons and to identify internal sources of magnetic perturbations. Near Europa the measurements are consistent with a plasma rich in pickup ions whose source is freshly ionized neutrals sputtered off of the moons surface or atmosphere. The plasma effects vary with Europas height above the center of Jupiters extended plasma disk. Europa is comet-like when near the center of the current sheet. It is therefore likely that the strength of the currents coupling Europa to Jupiters ionosphere and the brightness of a Europa footprint will depend on System III longitude. Magnetic perturbations on the scale of Europas radius can arise from a permanent dipole moment or from an induced dipole moment driven by the time-varying part of Jupiters magnetospheric field at Europas orbit. Both models provide satisfactory fits. An induced dipole moment is favored because it requires no adjustable parameters. The inductive response of a conductive sphere also fits perturbations on two passes near Callisto. The implied dipole moment flips direction as is predicted for greatly differing orientations of Jupiters magnetospheric field near Callisto in the two cases. For both moons the current carrying shells implied by induction must be located near the surface. An ionosphere cannot provide the current path, as its conductivity is too small, but a near surface ocean of ∼10 km or more in thickness would explain the observations.

Ganymede's magnetosphere: Magnetometer overview

Ganymede presents a unique example of an internally magnetized moon whose intrinsic magnetic field excludes the plasma present at its orbit, thereby forming a magnetospheric cavity. We describe some of the properties of this mini-magnetosphere, embedded in a sub-Alfvenic flow and formed within a planetary magnetosphere. A vacuum superposition model (obtained by adding the internal field of Ganymede to the field imposed by Jupiter) organizes the data acquired by the Galileo magnetometer on four close passes in a useful, intuitive fashion. The last field line that links to Ganymede at both ends extends to ∼2 Ganymede radii, and the transverse scale of the magnetosphere is ∼5.5 Ganymede radii. Departures from this simple model arise from currents flowing in the Alfven wings and elsewhere on the magnetopause. The four passes give different cuts through the magnetosphere from which we develop a geometric model for the magnetopause surface as a function of the System III location of Ganymede. On one of the passes, Ganymede was located near the center of Jupiters plasma disk. For this pass we identify probable Kelvin-Helmholtz surface waves on the magnetopause. After entering the relatively low-latitude upstream magnetosphere, Galileo apparently penetrated the region of closed field lines (ones that link to Ganymede at both ends), where we identify predominantly transverse fluctuations at frequencies reasonable for field line resonances. We argue that magnetic field measurements, when combined with flow measurements, show that reconnection is extremely efficient. Downstream reconnection, consequently, may account for heated plasma observed in a distant crossing of Ganymedes wake. We note some of the ways in which Ganymedes unusual magnetosphere corresponds to familiar planetary magnetospheres (viz., the magnetospheric topology and an electron ring current). We also comment on some of the ways in which it differs from familiar planetary magnetospheres (viz., relative stability and predictability of upstream plasma and field conditions, absence of a magnetotail plasma sheet and of a plasmasphere, and probable instability of the ring current).

The magnetic field and magnetosphere of Ganymede

Within Jupiters magnetosphere, Ganymedes magnetic field creates a mini-magnetosphere. We show that the magnetic field measured during Galileo‧s second pass by Ganymede, with closest approach at low altitude almost directly over the moons polar cap, can be understood to a large measure in terms of the structure of a vacuum superposition model of a uniform field and a Ganymede-centered dipole field. Departures from the simple model can be attributed principally to magnetopause currents. We show that the orientation of the observed magnetopause normal is qualitatively consistent with expectations from the vacuum superposition model. The magnetopause currents inferred from the inbound boundary crossing are closely related to expected values, and the magnetic structure of the boundary is similar to that observed at the magnetopause of Earth. We use the vacuum magnetic field model to infer the magnetic field near Ganymedes surface, and thereby predict the particle loss cones that should be present along the spacecraft trajectory. By mapping a fraction of the corotation electric field into the polar cap, we determine expected flow velocities near closest approach to Ganymede as a function of reconnection efficiency. We conclude by discussing prospects for measurements on Galileos remaining passes by Ganymede.

Dawn Arrives at Ceres: Exploration of a Small Volatile-Rich World

On 6 March 2015, Dawn arrived at Ceres to find a dark, desiccated surface punctuated by small, bright areas. Parts of Ceres’ surface are heavily cratered, but the largest expected craters are absent. Ceres appears gravitationally relaxed at only the longest wavelengths, implying a mechanically strong lithosphere with a weaker deep interior. Ceres’ dry exterior displays hydroxylated silicates, including ammoniated clays of endogenous origin. The possibility of abundant volatiles at depth is supported by geomorphologic features such as flat crater floors with pits, lobate flows of materials, and a singular mountain that appears to be an extrusive cryovolcanic dome. On one occasion, Ceres temporarily interacted with the solar wind, producing a bow shock accelerating electrons to energies of tens of kilovolts.

Mirror mode structures in the Jovian magnetosheath

[1] Mirror mode waves are commonly observed in planetary magnetosheaths. Their magnetic signatures are often periodic but occasionally appear as intermittent increases of field magnitude (peaks) or as intermittent decreases (dips). We define quantitative mirror structure identification criteria and statistically analyze the distributions of the various forms. A survey of all the relevant magnetometer data in the Jovian magnetosheath reveals that mirror mode structures are present 61.5% of the time. Two-thirds of the events include waves that are either quasi-periodic or aperiodic, while 19% contain dips and 14% contain peaks. The amplitude and period of quasi-periodic and periodic structures appear to increase as the residence time of the flowing plasma within the sheath increases. Peaks are primarily observed on the dayside in the high β plasmas of the middle magnetosheath. Dips are observed mostly in low β plasma near the magnetopause and on the flanks. A phenomenological model for the evolution of mirror structures that accounts for these observations has been developed. We propose that the mirror structures form near the bow shock and undergo an initial growth phase during which their amplitude increases linearly. Structures that dwell in anisotropic, high β plasma may saturate nonlinearly as described by Kivelson and Southwood [1996]. We interpret field magnitude peaks as the signatures of such nonlinear saturation. Finally, we ascribe the dip signatures to the process of stochastic decay of mirror structures as flow away from the subsolar point carries the structures into lower β plasma.

Distribution of phyllosilicates on the surface of Ceres

INTRODUCTION The surface of the dwarf planet Ceres is known to host phyllosilicate minerals, but their distribution and origin have not previously been determined. Phyllosilicates are hydrated silicates, and their presence on the surface of Ceres is intriguing given that their structure evolves through an aqueous alteration process. In addition, some phyllosilicates are known to bear NH4, which places a constraint on the pH and redox conditions during the evolution of Ceres. We studied the distribution of phyllosilicates across the planet’s surface to better understand the evolutionary pathway of Ceres. RATIONALE Using the data acquired by the mapping spectrometer (VIR) onboard the Dawn spacecraft, we mapped the spatial distribution of different minerals on Ceres on the basis of their diagnostic absorption features in visible and infrared spectra. We studied the phyllosilicates through their OH-stretch fundamental absorption at about 2.7 µm and through the NH4 absorption at about 3.1 µm. From our composition maps, we infer the origin of the materials identified. RESULTS We found that Mg- and NH4-bearing phyllosilicates are ubiquitous across the surface of Ceres and that their chemical composition is fairly uniform. The widespread presence of these two types of minerals is a strong indication of a global and extensive aqueous alteration—i.e., the presence of water at some point in Ceres’ geological history. Although the detected phyllosilicates are compositionally homogeneous, we found variations in the intensity of their absorption features in the 3-µm region of the reflectance spectrum. Such variations are likely due to spatial variability in relative mineral abundance (see the figure). CONCLUSION The large-scale regional variations evident in the figure suggest lateral heterogeneity in surficial phyllosilicate abundance on scales of several hundreds of kilometers. Terrains associated with the Kerwan crater (higher concentration of phyllosilicates) appear smooth, whereas the Yalode crater (lower concentration of phyllosilicates) is characterized by both smooth and rugged terrains. These distinct morphologies and phyllosilicate concentrations observed in two craters that are similar in size may reflect different compositions and/or rheological properties. On top of this large-scale lateral heterogeneity, small-scale variations associated with individual craters could result from different proportions of mixed materials in a stratified upper crustal layer that has been exposed by impacts. Variations associated with fresh craters, such as the 34-km-diameter Haulani, indicate the presence of crustal variations over a vertical scale of a few kilometers, whereas much larger craters, such as the 126-km-diameter Dantu, suggest that such stratification may extend for at least several tens of kilometers. Abundance maps. Qualitative maps of the abundances of (top) phyllosilicates and (bottom) NH4, based on the depth of their absorption features. The two maps have a similar global pattern, although they differ in some localized regions such as Urvara. The scale bar is valid at the equator. The dwarf planet Ceres is known to host phyllosilicate minerals at its surface, but their distribution and origin have not previously been determined. We used the spectrometer onboard the Dawn spacecraft to map their spatial distribution on the basis of diagnostic absorption features in the visible and near-infrared spectral range (0.25 to 5.0 micrometers). We found that magnesium- and ammonium-bearing minerals are ubiquitous across the surface. Variations in the strength of the absorption features are spatially correlated and indicate considerable variability in the relative abundance of the phyllosilicates, although their composition is fairly uniform. These data, along with the distinctive spectral properties of Ceres relative to other asteroids and carbonaceous meteorites, indicate that the phyllosilicates were formed endogenously by a globally widespread and extensive alteration process.

The dusk flank of Jupiter's magnetosphere

Limited single-spacecraft observations of Jupiters 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 Jupiters magnetopause is very similar to that of the Earth, confirming that the understanding built on studies of the Earths magnetosphere is valid. The data also reveal evidence for a well-developed boundary layer just inside the magnetopause.

Magnetized or unmagnetized: Ambiguity persists following Galileo's encounters with Io in 1999 and 2000

Magnetometer data from Galileos close encounters with Io on October 11, 1999, and February 22, 2000, do not establish clearly either the existence or absence of an internal magnetic moment because they were acquired in regions where plasma currents contribute large magnetic perturbations. Data from an additional encounter on November 26, 1999, with closest approach beneath Ios south polar regions, were lost. The recent passes add to our understanding of the interaction of the torus with Io and its flux tube and tighten the limits on possible internal sources of magnetic fields. Simple field-draping arguments account for some aspects of the observed rotations. Interpretations in terms of both a magnetized and an unmagnetized Io are considered. Data from the February 2000 pass (closest approach altitude 201 km, latitude 18°) rule out a strongly magnetized Io (surface equatorial field larger than the background field) but do not rule out a weakly magnetized Io (surface equatorial field of the order of Ganymedes but smaller than the background field at Io). Models suggest that if Io is magnetized, its magnetic moment is not strictly antialigned with the rotation axis. The inferred tilt is consistent with contributions from an inductive field analogous to that observed at Europa and Callisto. If an induced field is present, the currents would flow in the outer mantle or aesthenosphere. Wave perturbations differ on flux tubes that do or do not link directly to Io and its ionosphere suggesting that the latter flux tubes are virtually stagnant in Ios frame and that a unipolar inductor appropriately models the currents linking Io to Jupiters ionosphere.

Solar wind interaction with small bodies. 2: What can Galileo's detection of magnetic rotations tell us about Gaspra and Ida

Abstract As the Galileo spacecraft passed the asteroids Gaspra in 1990 and Ida in 1993, the magnetometer recorded changes in the solar wind magnetic field that we associate with the presence of the nearby body. This paper focuses on the types of interactions that can produce perturbations in the solar wind. We have suggested that the interaction at Gaspra is consistent with expectations of flow diversion by a magnetic dipole moment and an associated “magnetosphere” whose scale size is much larger than the diameter of the solid body. The conditions for the Ida flyby leave more room for ambiguity. The observations could plausibly be related to either interaction with a magnetized body or with a conducting body. We will report on details of the observations that may enable us to distinguish between the different types of interaction and to provide quantitative estimates of the physical properties of the asteroids themselves.

GALILEO AT JUPITER: CHANGING STATES OF THE MAGNETOSPHERE AND FIRST LOOKS AT IO AND GANYMEDE*

Several investigations based on data from the magnetometer on the Galileo Orbiter are described. From Galileo’s initial inbound pass, we learn that the magnetosphere can experience large changes in its magnetic configuration. Between 50 and 30 R,, the radial magnetic forces on the plasma were larger in 1995 than in 1973 when Pioneer 10 passed through the same region of the magnetosphere, implying that either external or internal plasma forces were also larger. Although there are several ways to interpret the change of the magnetic configuration, we suggest that the variations are governed principally by the solar wind dynamic pressure and that the dayside magnetosphere as far in as -30 RJ may be more strongly affected by the solar wind than has previously been recognized. Minor effects of higher mass loading may also be present. Data from the flyby of 10 show a large magnetic perturbation; we argue that it is plausible that 10 has an intrinsic magnetic field and that also contributions from plasma perturbations are significant. We find unexpected evidence that molecular ions are being picked up over a large spatial region in the vicinity of the moon. During the pass by Ganymede we observed a large magnetic perturbation consistent with an intrinsic dipole field. The multiple flybys of Ganymede scheduled for later portions of Galileo’s mission will allow us to test our understanding of the magnetic signature.