Galileo Energetic Particle Detector Research Articles

Energetic charged particles near Europa

Galileo energetic particles detector (EPD) data are presented from several close encounters of the spacecraft with Jupiter's moon Europa. These data reveal significant decreases in electron and ion count rates very close to the moon and a few Europa radii downstream. The relation between count rate decreases and satellite absorption is analyzed because these particles sputter and chemically modify Europa's surface. Decreases on the plasma wake side of the satellite have radial extents comparable to satellite dimensions. Furthermore, some energetic particle wakes are shifted toward Jupiter from predicted locations. At some wake locations, electron fluxes do not get above instrument background levels. Corresponding electrons are lost preferentially on Europa's trailing hemisphere. Upstream of Europa during the E12 encounter, deep decreases in energetic ion count rates are observed for some instrument look directions. These decreases are associated with satellite absorption because the E12 closest approach altitude is smaller than relevant ion gyroradii. Other E12 decreases, such as a “bite‐out” corresponding to a peak in the measured magnetic field intensity, are associated with “secondary” drifts in the radial direction caused by the field gradient. Pitch angle distributions measured during the E15 encounter (when Galileo was in Europa's wake) show much higher fluxes of locally mirroring particles than would be expected in a simple absorption signature. We suggest that a fraction of the upstream population escapes absorption because of the pileup of magnetic flux on Europa's trailing hemisphere.

Storm‐like dynamics of Jupiter's inner and middle magnetosphere

The discovery of energy‐time dispersed, charged particle signatures of dynamic, longitudinally confined charged particle injections within Jupiter's inner magnetosphere has been reported previously as measured in >20‐keV particle intensities by the Galileo energetic particles detector (EPD). While these events have similarities to so‐called substorm injections observed within the Earth's magnetosphere, it is unknown whether the driving mechanisms are similar. Over 100 Jovian injection events have now been documented between radial distances of ∼9 RJ (the inner boundary of most of the observations) and 27 RJ Injections occur at all System III longitudes and local time positions. Similar to Earth magnetospheric injections, the Jovian injections occur throughout the broad radial region of transition between the quasi‐dipolar and neutral sheet magnetic field configurations, and where the charged particle energy densities are competitive with the magnetic energy densities. The Jovian injections can be clustered in time, analogous to what often happens during well‐known magnetic storms that occur in the Earth's magnetosphere. During one particular periapsis of Galileo's orbital trajectory, the magnetosphere was observed by EPD to become suddenly very disturbed with multiple injections following a prolonged period (>24 Earth hours) of relative quiescence. Because of the prestorm coincidence of a signature of an apparent Earth‐like global magnetospheric disturbance, we hypothesize that this Jovian storm occurred when the inner and middle magnetosphere were triggered out of marginal stability by the passage of a magnetohydrodynamic fast mode wave launched at the magnetopause by a pressure variation in the interplanetary (solar wind) environment.

On Europa's magnetospheric interaction: A MHD simulation of the E4 flyby

The global three‐dimensional interaction of Europa with the Jovian magnetosphere is modeled by using a complete set of ideal magnetohydrodynamic (MHD) equations. The model accounts for exospheric mass loading, ion‐neutral charge exchange, recombination, and a possible intrinsic dipole magnetic field of Europa. The single‐fluid MHD equations are solved by using a modern, finite volume, higher‐order, Godunov‐type method on an adaptively refined unstructured grid, which allows detailed modeling of the region near Europa while still resolving both the upstream region and the satellite's wake. The magnetic field and plasma density measured during Galileo's E4 flyby of December 19, 1996, are reproduced reasonably well in the simulation. We find the agreement between the data and our model particularly convincing if we assume that the plasma velocity during the E4 flyby deviated from the nominal corotation direction by approximately 20P. Evidence from the Galileo energetic particle detector also supports this assumption. In this case, we can fit the data using a dipole with orientation close to that of an induced dipole arising from the interaction of a hypothetical conducting subsurface layer on Europa with the periodically changing magnetic field of Jupiter. However, the magnitude of the dipole in our model is somewhat smaller (70%) than that suggested by Khurana et al. [1998]. The total mass loading and ion‐neutral charge exchange rates are consistent with the estimates of Europa's atmosphere and ionosphere.

Inference of Europa's conductance from the Galileo Energetic Particles Detector

On Galileo's first approach to Europa on December 19, 1996, the Energetic Particles Detector (EPD) observed decreases in the count rates of low‐energy ions in Europa's plasma wake. We understand these decreases to be “microsignatures” of Europa, that is, the signatures of recent satellite absorptions of energetic ions. Using a previously developed simulation of count rates measured by a detector, we compare simulated data with measurements from Europa's immediate and distant wakes. We find that if Europa is considered to be an inert, solid body absorber of charged particles, we cannot account for the depth of observed microsignatures using our model. Instead, if Europa, its ionosphere, or its magnetosphere is sufficiently conducting that Jovian field lines become “hung up” on the moon with contact times of the order of the bounce time of energetic ions, then we can explain the decreases in the data using our model. For a conducting Europa, our simulation gives the best order‐of‐magnitude agreement with a global radial diffusion of the order of DLL ≈ 10−10L4. Simulation results put a lower limit of ΣE ≈ 8 mho on the global conductance of Europa and/or its environs, ∼50% the value for Io. Finally, higher‐energy microsignatures in the EPD data can be understood when we include the effects of a bend in magnetic field lines near Europa that causes a local curvature drift perpendicular to the direction of plasma motion.

Plasma flow in the magnetosphere of Ganymede

Galileo Energetic Particles Detector (EPD) measurements indicate that convective flow speeds near Ganymede decrease to less than one‐third of rigid corotation well before any significant increase in the magnetic field strength along the trajectory is observed. In an Alfvén wing model, such a deceleration implies that the Ganymede magnetosphere is a conducting object. We assess the magnitude of various conductivities and find that closure of the Alfvén wing current is attainable only through the body of Ganymede itself. We discuss briefly the planetological implications of this magnetospheric result.

Galileo‐measured depletion of near‐Io hot ring current plasmas since the Voyager epoch

The first mass‐discriminated, hot ion distribution moments (pressure, energy intensity) are determined for hot >50‐keV ions in Jupiter's inner magnetosphere at the outer edge of Io's plasma torus by using the Galileo energetic particle detector (EPD) data. These hot plasmas were significantly depleted during the Galileo encounter in 1995 as compared with the Voyager epoch of 1979. The depletion of the hot ions is apparently caused by enhanced charge exchange losses of hot ions, perhaps associated with enhanced emissions of neutral gases from the volcanoes of Io. Such neutral gas enhancements could simultaneously explain increases, reported elsewhere, in the densities of the cooler Io torus plasmas. The hot plasma changes may explain why radial transport interchange turbulence has been observed by Galileo in the Io torus regions, whereas such turbulence was not apparent during the Voyager encounters in 1979. The hot ion depletion could also play a role in explaining the apparent differences between the Jovian auroral configuration observed in recent years by the Hubble space telescope and ground observers and the configuration observed by Voyager. This possibility is much less certain, however.

Pitch angle diffusion at Jupiter's moon Ganymede

The magnetic field of Jupiter's moon Ganymede, provides a unique, large B value mirror point near the equator of Jovian field lines that intersect the moon. Ganymede creates a large loss cone in the distributions of all charged particles populating those field lines, emptying them of particles that mirror at radial distances < ∼ 8 RJ. Such a loss cone is seen when the Galileo energetic particles detector (EPD) views along the field line toward Ganymede. When the EPD views in the antimoon direction, the loss cone is nearly filled in for low energy electrons and remains empty at high energies. Because these electrons travel between their Ganymede mirror points and their near‐Jupiter mirror points much faster than they drift across the Ganymede magnetosphere, they are effectively trapped between the moon and Jupiter for several bounce periods. Thus the amount of filling observed in the antimoon viewing loss cone gives a direct measure of the amount of pitch angle scattering occurring during the electrons half‐bounce period. Using general wave‐particle scattering theory, we have fit the pitch angle distributions within the antimoon viewing loss cone and have extracted values of the diffusion coefficient and its variation with energy. We find weak diffusion to dominate, yielding lifetimes of ∼103 s at low energies to ∼104 s at high energies. The diffusion coefficient, assumed independent of pitch angle, varies as ∼E−1 or equivalently ∼γ−3.5 (E is the electron kinetic energy and γ is the relativistic correction factor) at energies above ∼50 keV. Finally, we note that the existence of loss cones in the antimoon direction is evidence that Jovian field lines intersecting Ganymede effectively move with the moon.

Electron beams and ion composition measured at Io and in its torus.

Intense, magnetic field-aligned, bidirectional, energetic (>15 kiloelectron volts) electron beams were discovered by the Galileo energetic particles detector during the flyby of Io. These beams can carry sufficient energy flux into Jupiter's atmosphere to produce a visible aurora at the footprint of the magnetic flux tube connecting Io to Jupiter. Composition measurements through the torus showed that the spatial distributions of protons, oxygen, and sulfur are different, with sulfur being the dominant energetic (> approximately 10 kiloelectron volts per nucleon) ion at closest approach.