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  • Lunar and Planetary Science and Exploration  (8)
  • 1
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    Structure of the Jovian Magnetodisk Current Sheet: Initial Galileo Observations (2001)
    In:  Other Sources
    Details
    Publication Date: 2011-08-23
    Description: The ten-degree tilt of the Jovian magnetic dipole causes the magnetic equator to move back and forth across Jupiter's rotational equator and tile Galileo orbit that lies therein. Beyond about 24 Jovian radii, the equatorial current sheet thins and tile magnetic structure changes from quasi-dipolar into magnetodisk-like with two regions of nearly radial but antiparallel magnetic field separated by a strong current layer. The magnetic field at the center of the current sheet is very weak in this region. Herein we examine tile current sheet at radial distances from 24 55 Jovian radii. We find that the magnetic structure very much resembles tile structure seen at planetary magnetopause and tail current sheet crossings. Tile magnetic field variation is mainly linear with little rotation of the field direction, At times there is almost no small-scale structure present and the normal component of the magnetic field is almost constant through the current sheet. At other times there are strong small-scale structures present in both the southward and northward directions. This small-scale structure appears to grow with radial distance and may provide the seeds for tile explosive reconnection observed at even greater radial distances oil tile nightside. Beyond about 40 Jovian radii, the thin current sheet also appears to be almost constantly in oscillatory motion with periods of about 10 min. The amplitude of these oscillations also appears to grow with radial distance. The source of these fluctuations may be dynamical events in tile more distant magnetodisk.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Planetary and Space Science (ISSN 0032-0633); Volume 47; 1101-1109
    Format: text
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  • 2
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    The Dusk Flank of Jupiter's Magnetosphere (2002)
    In:  Other Sources
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    Publication Date: 2011-08-23
    Description: 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.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Nature: Letters to Nature; Volume 415; 991-994
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  • 3
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    Oceans in Europa and Callisto: Independent Evidence from Magnetic Perturbations (1998)
    In:  Other Sources
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    Publication Date: 2018-06-08
    Description: Galileo, the first artificial satellite of an outer planet, has been orbiting Jupiter since Dec 7, 1995. The spacecraft encounters one of the four Galilean satellites on each orbit.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Nature
    Format: text
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  • 4
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    Sounding of Icy Galilean Satellites by Surface Observatories (2001)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: Several independent geological and geophysical investigations suggest that Europa and Ganymede contain subsurface oceans. Using Jupiter's rotating magnetic field as a primary signal, the magnetometer experiment onboard Galileo has measured secondary induction signals emanating from Europa, Ganymede, and surprisingly Callisto. The strong electromagnetic induction from these moons suggests that large global electrical conductors are located just below their icy crusts. A detailed analysis reveals that global salty oceans with salinity similar to the Earth's ocean and thicknesses in the range of approx. 6-100 kms can explain the induction observed by the Galileo magnetometer. Additional information is contained in the original extended abstract.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Forum on Innovative Approaches to Outer Planetary Exploration 2001-2020; 48; LPI-Contrib-1084
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  • 5
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    Probing Ganymede's Magnetosphere with Field Line Resonances (1999)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We report on the spectrum of field line resonances identified in data acquired by the Galileo spacecraft within Ganymede's magnetosphere on a relatively low latitude pass. We infer properties of the plasma distribution and its transport from the observed spectrum. The harmonic structure in the spectrum of the magnetometer data agrees very well with the frequencies predicted for resonances of a dipole field. The spectrum implies a density of 2 amu per cubic centimeter near the equator on closed field lines of Ganymede's magnetic field inside of 2 RG (Ganymede radii). This density is significantly reduced relative to the local density of the Jovian plasma sheet near Ganymede (less than or equal to 8 electrons per cubic centimeter, or approximately equal to 100 amu per cubic centimeter for an average ion mass per charge of 20 amu and average charge of 1.5 electron charges). A shadowing effect of Ganymede for the flow of particles injected at a reconnection layer on the side of the moon downstream relative to the direction of torus plasma flow accounts for the marked density depletion. Implications for conducting paths near Ganymede's surface are considered.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Paper-1999JA900161 , UCLA-IGPP-Publ-5105 , Journal of Geophysical Research (ISSN 0148-0227); 104; A7; 14,729-14,738
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  • 6
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    Wave Activity in Europa's Wake: Implications for Ion Pickup (2001)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Intense wave power at frequencies near and below the cyclotron frequencies of heavy ions was detected in Europa's wake during the E11 and E15 flybys. The fluctuations are mainly transverse to the background magnetic field. Wave characteristics indicate that they are ion cyclotron waves driven by positively charged pickup ions. In both flybys there is evidence, derived from the wave polarization, for pickup of negatively charged chlorine ions. When the moon is near the center of the Jovian current sheet, the pickup rate inferred for the E15 flyby is larger than that for the E11 flyby, when the moon is outside the Jovian current sheet. The wave power does not provide exact pickup density values because the waves are observed in regions where their growth has not yet fully developed. At the edges of the wake region, low-frequency (〈 K+ gyrofrequency) magnetohydrodynamic waves are also present. We identify magnetic field signatures that are reminiscent of interchange/ballooning of mass-loaded flux tubes from the wake/pickup region expanding into ambient medium that is less dense.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Paper-2000JA000347 , Journal of Geophysical Research (ISSN 0148-0227); 106; A11; 26,033-26,048
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  • 7
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    The Permanent and Inductive Magnetic Moments of Ganymede (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: Data acquired by the Galileo magnetometer on five passes by Ganymede have been used to characterize Ganymede's internal magnetic moments. Three of the five passes were useful for determination of the internal moments through quadrupole order. Models representing the internal field as the sum of dipole and quadrupole terms or as the sum of a permanent dipole field upon which is superimposed an induced magnetic dipole driven by the time varying component of the externally imposed magnetic field of Jupiter's magnetosphere give equally satisfactory fits to the data. The permanent dipole moment has an equatorial field magnitude 719 nT and is tilted by 176 degrees from the spin axis with the pole in the southern hemisphere rotated by 24 degrees from the Jupiter-facing meridian plane towards the trailing hemisphere. The data are consistent with an inductive response of a good electrical conductor of radius approximately 1 Ganymede radius. Although the data do not enable us to establish the presence of an inductive response beyond doubt, we favor the inductive response model because it gives a good fit to the data using only 4 parameters to describe the internal sources of fields, whereas the equally good dipole plus quadrupole fit requires 8 parameters. An inductive response is consistent with a buried conducting shell, probably liquid water with dissolved electrolytes, somewhere in the first few hundred km below Ganymede's surface. The depth at which the ocean is buried beneath the surface is somewhat uncertain, but our favored model suggests a depth of order 150 kilometers. As both temperature and pressure increase with depth and the melting temperature of pure ice decreases to a minimum at approximately 170 kilometer depth, it seems possible that near this location, a layer of water would be sandwiched between layers of ice.
    Keywords: Lunar and Planetary Science and Exploration
    Type: UCLA-IGPP-Publ-5562
    Format: application/pdf
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  • 8
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    Implications of Depleted flux Tubes in the Jovian Magnetosphere (2000)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A rare but persistent phenomenon in the jovian magnetosphere is the occurrence of apparently depleted flux tubes, whose magnetic pressures are significantly above ambient levels. These flux tubes occur about 0.25% of the observing time in the region of the Io torus in the Galileo high resolution data. The importance of these tubes is that they can return to the inner magnetosphere the magnetic flux that has been convected radially outward with the iogenic plasma to the tail. The paucity of these tubes is consistent with the expected flux return rates if the tubes are moving inward at an average rate of about 5-10 km/s in the torus. Depleted flux tubes have yet to be observed inside of the lo orbit where the plasma beta is lower than in the hot torus. Estimates of the plasma density outside the tube from plasma wave measurements enable the average perpendicular temperature to be obtained from the magnetic field change. Extrapolating this temperature back to lo, we obtain an average ion temperature of approximately 60 eV. These values are generally consistent with earlier Voyager observations but on the low side of their range of uncertainty, and agree quite well with contemporaneous Galileo measurements where these are available.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Paper 2000GL003815 , Geophysical Research Letters (ISSN 0094-8276); 27; 19; 3133-3136
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