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At least 451 records · Page 25

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NASA NTRS1984

Do the Satellites of Uranus Control Its Magnetosphere?

The importance of the satellites of Uranus as sources of magnetospheric plasma were investigated. It is found that neither an Io like plasma source nor a Titan like source is likely at Uranus. The likely presence of a heavy ion plasma torus maintained by charged particle sputtering of the icy satellites is examined. Sputtering of Saturn's icy satellites is considered an important source of heavy ion (oxygen) plasma in Saturn's inner magnetosphere. A major unresolved question is whether this sputtering process does depend on the preexistence of magnetospheric heavy ions derived from another source, Titian.

Cheng, A. F.↗

NASA NTRS1984

Observations of the magnetospheric boundary layers

Results on magnetospheric boundary layers are reviewed, emphasizing their dynamical importance based on hot plasma observations, energetic particle signatures, heavy ion contributions and the effects of wave-particle interactions. Satellite plasma observations show that 1% to 2% of the oncoming solar wind plasma enters the magnetosphere and is initially transported within the magnetospheric boundary layer. Some of this boundary layer plasma is entrained within the Earth's magnetotail where it can be accelerated. Tests are needed to determine the relative contributions of the primary acceleration processes whose effects are especially evident in the plasma sheet boundary layer.

Eastman, T. E.↗

NASA NTRS1984

The application of dimensional analysis to the problem of solar wind-magnetosphere energy coupling

The constraints imposed by dimensional analysis are used to find how the solar wind-magnetosphere energy transfer rate depends upon interplanetary parameters. The analyses assume that only magnetohydrodynamic processes are important in controlling the rate of energy transfer. The study utilizes ISEE-3 solar wind observations, the AE index, and UT from three 10-day intervals during the International Magnetospheric Study. Simple linear regression and histogram techniques are used to find the value of the magnetohydrodynamic coupling exponent, alpha, which is consistent with observations of magnetospheric response. Once alpha is estimated, the form of the solar wind energy transfer rate is obtained by substitution into an equation of the interplanetary variables whose exponents depend upon alpha.

Bargatze, L. F.↗

NASA NTRS1985

The influence of quiet asymmetric magnetosphere on the cutoff rigidities of the main cone

Some earlier studies show that cutoff rigidities of cosmic-ray particles in the model magnetospheric fields of internal and external sources have daily variations caused by asymmetry of the magnetic field due to the currents induced at the magnetopause and tail currents. Cutoff rigidities of the charged particles coming down at the middle latitudes are examined in this paper. The mathematical model of the magnetospheric field is based on the merged IMP-HEOS experimental data set and includes all known at the present time current systems of the magnetosphere: magnetopause, ring, and magnetotail currents.

Tyasto, M. I.↗

NASA NTRS1985

Energy supply processes for magnetospheric substorms and solar flares - Tippy bucket model or pitcher model?

In the past, both magnetospheric substorms and solar flares have almost exclusively been discussed in terms of explosive magnetic reconnection. Such a model may conceptually be illustrated by the so-called 'tippy-bucket model', which causes sudden unloading processes, namely a sudden (catastrophic, stochastic, and unpredictable) conversion of stored magnetic energy. However, recent observations indicate that magnetospheric substorms can be understood as a result of a directly driven process which can conceptually be illustrated by the 'pitcher model' in which the output rate varies in harmony with the input rate. It is also possible that solar flare phenomena are directly driven by a photospheric dynamo. Thus, explosive magnetic reconnection may simply be an unworkable hypothesis and may not be a puzzle to be solved as the primary energy supply process for magnetospheric substorms and solar flares.

Akasofu, S.-I.↗

NASA NTRS1984

Dust-magnetosphere interactions

Many different interactions have been proposed among the magnetospheric particles, fields, and dust grains in the rings of the outer planets. Attention is presently given to the energetic particle absorption signatures obtained by Pioneer 11 and Voyagers 1 and 2, which trace the mass concentrations of particulates in the magnetospheres of Jupiter and Saturn. Particulates immersed in the magnetospheric plasma and exposed to solar UV radiation will charge up to a surface potential that depends on the density and electron energy of the plasma, as well as on the concentration of dust particles. Kinetic effects of charged dust particles arise from the interaction with the planetary magnetic field. Electromagnetic effects are noted which lead to the halo of Jupiter's ring, the dust distribution in Saturn's E ring, and the levitated dust in the Saturn B ring spokes.

Gruen, E.↗

NASA NTRS1985

Plasma-dominated magnetic field configurations for the magnetosphere of Uranus

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.

Ip, A. K.↗

NASA NTRS1985

An MHD model of the earth's magnetosphere

It is pointed out that the earth's magnetosphere arises from the interaction of the solar wind with the earth's geomagnetic field. A global magnetohydrodynamics (MHD) model of the earth's magnetosphere has drawn much attention in recent years. In this model, MHD equations are used to describe the solar wind interaction with the magnetosphere. In the present paper, some numerical aspects of the model are considered. Attention is given to the ideal MHD equations, an equation of state for the plasma, the model as an initial- and boundary-value problem, the shock capturing technique, computational requirements and techniques for global MHD modeling, a three-dimensional mesh system employed in the global MHD model, and some computational results.

Wu, C. C.↗

NASA NTRS1985

A Simulation of High Latitude F-Layer Instabilities in the Presence of Magnetosphere-Ionosphere Coupling

A magnetic-field-line-integrated model of plasma interchange instabilities is developed for the high latitude ionosphere including magnetospheric coupling effects. We show that primary magnetosphere-ionosphere coupling effect is to incorporate the inertia of the magnetospheric plasma in the analysis. As a specific example, we present the first simulation of the E x B instability in the inertial regime, i.e., nu sub i omega where nu sub i is the ion-neutral collision frequency and omega is the wave frequency. We find that the inertial E x B instability develops in a fundamentally different manner than in the collisional case ni sub i omega. Our results show that striations produced in the inertial regime are spread and retarded by ion inertial effects, and result in more isotropic irregularities than those seen in the collisional case.

Mitchell, H. G., Jr.↗

NASA NTRS1985

Supersonic ion outflows in the polar magnetosphere via the geomagnetic spectrometer

Observations of 'supersonic' O(+) outflows by DE-1 in the polar magnetosphere are interpreted as a velocity-filter effect resulting from the transport of these low energy ions from a horizontally narrow source associated with the polar cleft topside ionosphere into the polar cap magnetosphere. Under certain simplifying circumstances, it is shown that the 'Mach number' for the flow is essentially a geometrically determined quantity, approximately the ratio of the horizontal distance of the observation location to the source over the horizontal width of the source. A recently developed two-dimensional kinetic model of ion transport in the polar magnetosphere is used to calculate distribution functions and the associated bulk flow parameters, and these calculated quantities are compared with measured flow parameters from DE-1.

Horwitz, J. L.↗

NASA NTRS1985

Parabolic harmonics in magnetospheric modeling - The main dipole and the ring current

A new representation is developed for that portion of the magnetospheric magnetic field which is produced by currents on the magnetopause or outside it. For a magnetopause which is a paraboloid of rotation and which confines all field lines of magnetospheric sources, the fields due to magnetopause currents are derived for two sources, a dipole and a two-parameter model of the ring current, each arbitrarily inclined to the earth-sun line. Methods are also developed for accomodating changes in the size of the magnetospheric cavity and for using nonparaboloidal boundaries, open at the nightside. Neutral points and polar cusps are derived, and in agreement with observations it is found that a growth of the ring current shifts them equatorward. By extrapolation from these results, it is argued that the observation of such shifts during magnetic storms may help resolve the relative importance of two processes proposed for magnetic storms.

Stern, D. P.↗

NASA NTRS1986

A three-dimensional MHD simulation of the interaction of the solar wind with the earth's magnetosphere - The generation of field-aligned currents

The time-dependent interaction of the solar wind with the earth's magnetosphere is simulated using a three-dimensional MHD model. The bow shock, magnetopause, magnetotail, and plasma sheet of the magnetosphere and Birkeland field-aligned currents that are dependent on the polarity of the z component of the IMF are produced. Twin convection cells and a dawn to dusk electric potential of 30-100 kV are detected at the equator in the magnetosphere. Four types of field-aligned currents are observed: region 1, region 2, dayside magnetopause currents in the dayside cusp region, and the dayside cusp currents for southward IMF. Region 1 and 2 field-aligned currents generated for all IMF conditions are 0.6-1.0 x 10 to the 6th A and 0.15-0.61 x 10 to the 6th A, respectively. The relationship between region 1 currents and field-aligned vorticity, and region 2 currents and pressure gradients are studied. The simulated data are compared with a theoretical analysis of the field-aligned currents and good correlation is observed.

Ogino, T.↗

NASA NTRS1986

Energetic charged particles in the Uranian magnetosphere

The cosmic ray instrument on Voyager 2 obtained detailed charged particle data on the path the spacecraft followed through the newly-discovered Uranian magnetosphere. Notably, the electron counts did not rise above background until the spacecraft was deep in the magnetosphere. The data further indicated that the satellites Miranda, Ubriel and Ariel impose a sweep effect on MeV electrons, which move radially inward from some source in the outer magnetosphere or the magnetotail. The energetic particle data also supported a firm pitch angle dependence for proton energy spectra in the 1-8 MeV range. Finally, the particle density data were commensurate with a dipole magnetic field tilted 60.1 deg in relation to the planet and rotating every 17.4 hr, close to estimates from other instruments.

Stone, E. C.↗

NASA NTRS1986

The magnetosphere of Uranus - Hot plasma and radiation environment

Inferences are drawn on the morphology and composition of the Uranus magnetosphere based on low-energy charged particle data collected by Voyager 2. Proton and electron energies in the magnetosphere attained energies of 4 and 1.2 MeV, respectively, although electron intensities surpassed the proton intensities at most energy levels. Protons dominated in the ion energy regime 0.6-1.0 MeV. The ion and electron spectra were Maxwellian below about 200 keV and had a power law distribution at energies over 590 keV. The power law was reduced by a factor of nearly three inside the orbit of Miranda. The proton population is dense enough to polymerize CO and CH4 ice surfaces within 10,000-100,000 yr. The data indicated that the particles are swept out at least to the orbit of Titania by the satellites. The morphology of the magnetosphere closely resembles that around Jupiter, except that plasma sheet distorsion from particle loading is negligible in regions within 15 Uranus radii.

Krimigis, S. M.↗

NASA NTRS1986

Plasmasphere formation in arbitrarily oriented magnetospheres

The formation of plasmaspheres in planetary magnetospheres with arbitrary orientations of the rotation and magnetic dipole axes is investigated. A traditional plasmasphere with closed orbits inside the plasmapause and open trajectories outside it only occurs for the limiting case of aligned rotation and dipole axes. A time-variable plasmapause exists if the rotation axis is perpendicular to the solar win flow direction. In any other case, no definite plasmapause exists. Solar wind-driven convection transports plasma throughout the magnetosphere with an effectiveness which increases as the orientation goes further from one of the two limiting cases of strict plasmapause formation. The present analysis is applied to earth and Uranus using the actual orientations of the rotation and dipole axes. Particle trajectories at earth deviate only slightly from those obtained with traditional models. Uranus has no plasmasphere, and plasma convects sunwards throughout the inner magnetosphere.

Selesnick, R. S.↗

NASA NTRS1987

Magnetospheric studies using the UKS data

The magnetic field data from the UKS spacecraft were analyzed to learn more about the solar wind interaction with the Earth's magnetosphere and about the magnetosphere itself. The data was reduced from raw experimenter data records to engineering units. The evolution of the waves in the foreshock, the varying structure of the bow shock along the boundary, simultaneous behavior of the magnetopause in the north and south hemisphere and MHD waves in the magnetosphere and magnetosheath were examined.

Russell, C. T.↗

NASA NTRS1986

Magnetospheric particle injection and the upstream ion event of September 5, 1984

Energetic particle data from the AMPTE Charge Composition Explorer (CCE) spacecraft in the outer dayside magnetosphere are examined during the period of an upstream ion event observed by the AMPTE Ion Release Module (IRM) spacecraft on September 5, 1984. The CCE data reveal the following: (1) an ion enhancement was observed at about 0040 UT in near coincidence with a substorm onset at about 0035 UT, approximately 15 minutes prior to the onset of the event upstream of the shock; (b) ions of both solar-wind - H(2+) Fe-group - and ionospheric O(+) - origin over a broad energy range (about 20 keV to greater than 1350 keV) were injected at substorm onset; (3) the time evolution of the H(+), He(2+), and O(+) pitch angle distributions markedly differed, with O(+) exhibiting mostly enhancements at off-90-deg angles for the first hour after injection; (4) an enhancement in the Fe-group ions inside the magnetosphere at L = about 6.4 occurred simultaneously with the appearance of an O(+) burst upstream of the shock. The CCE observations, taken together with the simultaneously observed IRM ion event, suggest that a plausible explanation for the appearance of upstream ions is leakage from the magnetosphere into the upstream region, rather than the alternative explanation which requires in situ acceleration of solar wind ions via the Fermi Mechanims.

Krimigis, S. M.↗

NASA NTRS1986

Magnetospheres of the outer planets

The magnetospheres of the outer planets have been shown by Voyager explorations to strongly interact with the surfaces and atmospheres of their planetary satellites and rings. In the cases of Jupiter, Saturn and Uranus, the processes of charged particle sputtering, neutral gas cloud formation, and rapid plasma injection from the ionization of the neutral clouds, have important implications both for the magnetospheres as a whole and for the surfaces and atmospheres of their satellites. The general methodology employed in these researches has involved comparisons of the planetary magnetospheres in order to identify common physical processes.

Cheng, Andrew F.↗