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  • 1
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    The magnetic field and magnetospheric configuration of Uranus (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: A significant and unique planetary magnetic field discovered by Voyager 2 is presented. A large tilt of 58.6 deg of the magnetic-dipole axis from the rotation axis was found. Combined with a large offset of 0.3 RU of the magnetic dipole from the center of the planet, the moment of 0.23 gauss-RU3 leads to field magnitudes at the surface which vary widely between 0.1 and 1.0 gauss. A simple diagram illustrating the offset tilted dipole of Uranus and some field lines is shown. A more exact and accurate spherical-harmonic model of the planetary field, which includes both dipole and quadrupole moments, is derived. There exists a well-developed bipolar magnetic tail on the night side of the planet which rotates daily about the extended planet-sunline with Uranus because of the large obliquity of the Uranian rotation axis.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 2
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    The magnetosphere of Uranus - Plasma sources, convection, and field configuration (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: It is suggested by qualitative considerations based on analogy with earth, Jupiter, and Saturn that the magnetosphere of Uranus may lack a plasma source able to produce significant internal currents, internal convection, and associated effects. A class of approximately self-consistent quantitative magnetohydrostatic equilibrium configurations for the case of a pole-on magnetosphere with variable plasma parameters is presently constructed in order to test this hypothesis by means of forthcoming Voyager measurements. The configurations that can be computed for the geometries of the magnetic field and of the tail current sheet, for a given distribution of plasma pressure, have a single, funnel-shaped polar cusp pointing into the solar wind and a cylindrical tail plasma sheet whose currents close within the tail, rather than on the tail magnetopause. Interconnection of interplanetary and magnetospheric fields yields a highly asymmetric tail-field configuration.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Astrophysical Journal; vol. 266
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  • 3
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    Plasma-dominated magnetic field configurations for the magnetosphere of Uranus (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: There is significant indirect evidence that the planet Uranus possesses a magnetic field. This evidence is based on the observation of hydrogen Lyman alpha emission from Uranus with the aid of the International Ultraviolet Explorer (IUE) spacecraft. The detection of water ice on the Uranian moons led Cheng (1984) to suggest that charged particle sputtering of the icy satellites could provide a significant internal source of oxygen ions and protons to the Uranian magnetosphere. Cheng concluded that this mechanism would predict aurorae around both magnetic poles of Uranus. Cheng's idea of the presence of a continuous internal plasma supply to the Uranian magnetosphere is further pursued in the present investigation. Questions are considered regarding the evolution of Uranus' magnetosphere from a vacuum configuration toward a plasma pressure dominated equilibrium configuration, taking into account the amount of the thermal plasma pressure as a free parameter.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 90; 6287-629
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  • 4
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    Limits on plasma anisotropy in a tail-like magnetic field (1992)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The condition of magnetohydrostatic equilibrium implies tight constraints on the degree of anisotropy that is supportable in a magnetotail field geometry. If the plasma pressure tensor is assumed to be gyrotropic at the tail midplane (z = 0), then equilibrium requires that it also be nearly isotropic there, with P-perpendicular sub 0/P-parallel sub 0 in the range 1 +/- delta square, where delta of about 0.1 is the ratio of the normal field component at the symmetry plane to the field strength in the tail lobe. The upper and the lower limits are essentially equivalent, respectively, to the marginal mirror and firehose stability conditions evaluated at z = 0, which have been invoked previously to limit the degree of anisotropy in the plasma sheet.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 19; 24; p. 2441-2444.
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  • 5
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    The magnetosphere of Neptune - Its response to daily rotation (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The Neptunian magnetosphere periodically changes every eight hours between a pole-on magnetosphere with only one polar cusp and an earth-type magnetosphere with two polar cusps. In the pole-on configuration, the tail current sheet has an almost circular shape with plasma currents closing entirely within the magnetosphere. Eight hours later the tail current sheet assumes an almost flat shape with plasma currents touching the magnetotail boundary and closing over the tail magnetopause. Magnetic field and tail current sheet configurations have been calculated in a three-dimensional model, but the plasma- and thermodynamic conditions were investigated in a simplified two-dimensional MHD equilibrium magnetosphere. It was found that the free energy in the tail region of the two-dimensional model becomes independent of the dipole tilt angle. It is conjectured that the Neptunian magnetotail might assume quasi-static equilibrium states that make the free energy of the system independent of its daily rotation.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 1705-170
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  • 6
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    The influence of centrifugal forces on the B field structure of an axially symmetric equilibrium magnetosphere (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A model is presented of an axially symmetric pole-on magnetosphere in MHD force balance, in which both plasma thermal pressure gradients and centrifugal force are taken into account. Assuming that planetary rotation leads to differentially rotating magnetotail field lines, the deformation of magnetotail field lines under the influence of both thermal plasma pressure and centrifugal forces was calculated. Analytic solutions to the Grad-Shafranov equation are presented, which include the centrifugal force term. It is shown that the nonrotational magnetosphere with hot thermal plasma leads to a field configuration without a toroidal B(phi) component and without field-aligned Birkeland currents. The other extreme, a rapidly rotating magnetosphere with cold plasma, leads to a configuration in which plasma must be confined within a thin disk in a plane where the radial magnetic field component B(r) vanishes locally.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 94; 8693-870
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  • 7
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    Magnetic field and current structures in the magnetosphere of Uranus (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Voyager 2 magnetic field and plasma data are compared with theoretical model calculations for the magnetosphere of Uranus to derive a global picture from the limited set of measurements. The results suggest that Voyager 2 entered the Uranian magnetosphere during a relaxation phase which followed a compression of the entire magnetosphere. The plasma beta values in the Uranian neutral sheet were found to be smallar by a factor of 3-8 compared to corresponding beta values in the average terrestrial neutral sheet. The excellent agreement found between observed and calculated magnetic tail lobe field strengths indicates that the Uranian magnetosphere reaches the state of quasi-static (i.e., slowly time-dependent MHD equilibrium).
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 92; 15337-15
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  • 8
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    Field line twist and field-aligned currents in an axially symmetric equilibrium magnetosphere (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Field-aligned Birkeland currents and the angle of the magnetic line twist were calculated for an axially symmetric pole-on magnetosphere (assumed to be in MHD equilibrium). The angle of the field line twist was shown to have a strong radial dependence on the axisymmetric magnetotail as well as on the ionospheric conductivity and the amount of thermal plasma contained in closed magnetotail flux tubes. The field line twist results from the planetary rotation, which leads to the development of a toroidal magnetic B-sub-phi component and to differentially rotating magnetic field lines. It was shown that the time development of the toroidal magnetic B-sub-phi component and the rotation frequency are related through an induction equation.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 91; 10995-11
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