Engineering topics

Thomsen, M. F.

Publications and source records attributed to Thomsen, M. F..

At least 109 records · Page 6

NASA NTRS1982

A nonlocal theory of an electrostatic sinusoidal density drift instability

The stability of space plasmas in which the macroscopic variables vary in the direction x perpendicular to the magnetic field is usually studied by means of the local approximation, in which the fluctuating potential is assumed to be independent of x. To remove this approximation, a nonlocal problem is studied in which the background density, and hence the fluctuating potentials, are periodic in x. From the linear Vlasov/Poisson equations, a set of coupled, linear, homogeneous, algebraic equations relating the Fourier amplitudes of the eigensolution is derived. The equations are solved numerically and a hierarchy of exact eigenmodes characterized by different growth rates and spatial structures is found. At points where the density gradient is locally zero, the mode amplitude is generally several orders of magnitude lower than the peak amplitude. The dependence of these solutions on the parameters which define the background density perturbation is studied and a correspondence between these nonlocal results and those obtained with the local approximation is demonstrated.

Thomsen, M. F.↗

NASA NTRS1982

On the nature of particles in Saturn's spokes

It is noted that an observed deviation of the angular velocity of spoke features from the Keplerian value can yield the charge to mass (q/m) ratio of spoke particles. Observations in the literature of spoke motion are consistent with q/m in the neighborhood of -10 coul/kg. Since a lower limit on q is one electronic charge, this value yields a lower limit to the size of spoke particles of approximately 0.01 micron. The criteria for electrostatically ejecting small particles from larger parent bodies suggest an upper limit of approximately 1,000 m for the parent bodies. The stability of the grains to electrostatic disruption or field emission yields an upper limit of approximately 3 microns for the spoke particle size. It also leads to the conclusion that the spoke particles must consist of material that is stronger than loose dust balls.

Thomsen, M. F.↗

NASA NTRS1982

Dissipation of ionospheric irregularities by wave-particle and collisional interactions

The nonlinear dissipation of plasma irregularities aligned parallel to an ambient magnetic field is studied numerically using a model which employs both wave-particle and collisional diffusion. A wave-particle diffusion coefficient derived from a local theory of the universal drift instability is used. This coefficient is effective in regions of nonzero plasma gradients and produces triangular-shaped irregularities with spectra which vary as f to the -4th, where f is the spatial frequency. Collisional diffusion acts rapidly on the vertices of the irregularities to reduce their amplitude. The simultaneous action of the two dissipative processes is more efficient than collisions acting alone. In this model, wave-particle diffusion mimics the forward cascade process of wave-wave coupling.

Bernhardt, P. A.↗

NASA NTRS1982

An electrostatic parabolic density drift instability

Assuming a uniform magnetic field and using the local approximation, the Vlassov theory for an electrostatic instability driven by a parabolic density gradient is considered. It is found that the instability grows only in the case of a sufficiently large, positive second derivative of the density. Linear growth rate parametric dependences are given, in addition to weakly nonlinear calculations on wave-particle transport due to this instability.

Gary, S. P.↗

NASA NTRS1981

Further observational support for the limited-latitude magnetodisc model of the outer Jovian magnetosphere

A distinction is made between the solar-wind-influenced limited-latitude magnetodisk and magnetic anomaly models of the outer Jovian magnetosphere, and an observational comparison of the two models is presented based on Pioneer and Voyager measurements. Predictions of the two models concerning the location of the current sheet as a function of Jovigraphic latitude, System III longitude and radial distance are contrasted, and it is shown that both models can satisfactorily explain the merging of the current sheep crossings by Voyager 1 and 2. Variations in the energetic particle intensities observed on the outbound pass of Voyager 1 and 2 are observed to correspond to scale heights for energetic particle latitudinal confinement consistent with MHD calculations and Pioneer 10 and Voyager magnetic observations only when the scale heights are calculated on the basis of the limited-latitude magnetodisk model. It is thus suggested that the solar wind must have a greater influence on magnetosphere structure than internal longitudinal plasma asymmetry.

Thomsen, M. F.↗

NASA NTRS1981

Saturn's radio emissions - Rotational modulation

Observational data provided by Pioneer 11 and Voyager 1 have revealed that the Saturnian ring system has important effects on the structures of the planetary ionosphere as well as the magnetospheric environment, such as the unexpected rotational modulation of the Saturn kilometric radiation (SKR) and the Saturn electrostatic discharge (SED) as discovered by the Voyager planetary radio astronomy experiment. The challenge arises to devise a mechanism which can produce electrodynamic coupling between the ring system and the planetary ionosphere and/or magnetosphere in which the associated SKR and SED radio emissions are subject to longitudinal control. One candidate is the establishment of field-aligned current systems, connecting the rings and the ionosphere with longitudinal variation. Local acceleration processes in the vicinity of the rings are also needed. The interplay between the ring-current systems and the geometry of the dipole field and the rings is discussed, and a description is presented of some of the basic ingredients in the longitudinal modulations of SKR and SED.

Goertz, C. K.↗

NASA NTRS1980

Sources and sinks of energetic electrons and protons in Saturn's magnetosphere

The paper deals with some results of continuing analysis and interpretation of energetic particle observations made onboard Pioneer 11 during its August-September 1979 encounter with Saturn. Source strength estimates and the radial dependence of the phase space density of protons of energies above 80 MeV indicate with some confidence that the cosmic-ray neutron albedo from the planets atmosphere and rings is the source of these particles.

Van Allen, J. A.↗

NASA NTRS1980

The energetic charged particle absorption signature of Mimas

In the present paper, the average long-term effect of Saturn's satellite Mimas on the distribution of trapped radiation (macrosignature) is examined, along with a microsignature of satellite absorption, specifically, a brief dip in charged particle intensities observed on the inbound pass of Pioneer 11 as it passed through the orbital range of Mimas. It is hypothesized that it is indeed the shadow of Mimas, in the sense that the observations reveal the effect of this satellite on a distribution of particles which interacted with it in the recent past and then drifted in longitude to the observational location. The hypothesis led to a characterization of the electron energy spectrum in Saturn's inner magnetosphere and to an estimate of the radial diffusion coefficient for such electrons.

Van Allen, J. A.↗

NASA NTRS1980

Theory of scan plane flux anisotropies

When a spacecraft detector measures particle flux as a function of look direction in a plane (the scan plane), anisotropy is often seen. This anisotropy is caused by spatial gradients, by E x B particle drift, and by various spectral and geometric effects. This paper treats all of these effects systematically, starting from the nonrelativistic Vlasov equation. The general analysis is applied to a simple model of an anisotropic distribution to give a relation between the E x B drift, the gradient and the experimentally observed first, second, and third harmonics of the flux as a function of angle in the scan plane. Even with an assumed model, anisotropy observations in one plane alone do not suffice to determine the E x B drift velocity and the spatial gradient independently. If the E x B velocity is assumed (e.g., the corotational velocity in a rotating planetary magnetosphere), the spatial gradient may be deduced, and from it the time rate of change of flux in a nonrotating frame of reference.

Northrop, T. G.↗

NASA NTRS1980

Corotation of Saturn's magnetosphere - Evidence from energetic proton anisotropies

The theory and technique of Northrop and Thomsen (1980) are applied to the observations of energy spectra and directional anisotropies of 0.61- to 3.41 MeV protons in Saturn's magnetosphere. The observations were made by the Goddard Space Flight Center/University of New Hampshire and University of Iowa instruments aboard Pioneer 11 during the Pioneer encounter with Saturn in August-September 1979. Fourier fits to 15-min intervals of data are combined with spectral indices to yield information about the E x B convection velocity and temporal changes in the particle population. There is a fundamental inability to distinguish unambiguously between the two, but if one can be assumed, the other then follows from these calculations. It is found that although these data do not by themselves allow an unambiguous determination of the extent of corotation in Saturn's outer magnetosphere, they are consistent with exact corotation at the nominal rotation period in the presence of significant but not unreasonable temporal variations in the energetic proton population.

Thomsen, M. F.↗

NASA NTRS1980

Motion of trapped electrons and protons in Saturn's inner magnetosphere

A summary is given of basic formulas for the guiding center motion of energetic charged particles trapped in a dipolar magnetic field. These formulas for longitudinal drift rates, latitudinal bounce periods, equatorial gyroradii, and equatorial gyroperiods are then stated in convenient numerical form for electrons and protons as functions of kinetic energy E, magnetic shell parameter L, and equatorial pitch angle alpha 0 for a slightly simplified model of the observed magnetic field of Saturn. To aid in the study of the interaction of charged particles with the rings and inner satellites of Saturn, additional formulas are given for the time interval between successive encounters of charged particles with a satellite in a circular prograde orbit and for the energy of electrons whose longitudinal angular velocity is resonant, or synchronous, with the Keplerian angular velocity of such a satellite.

Thomsen, M. F.↗

NASA NTRS1980

Azimuthal magnetic field at Jupiter

The azimuthal component of the magnetic field at Jupiter is modeled. A current distribution which is the sum of two currents, one flowing along the magnetic field lines and another injected into the current sheet at r(0) (about 10 Jupiter radii). Two cases are examined, one in which current flows along the field lines into the equatorial plane and then radially outward and a second case in which the only current is that injected into the current sheet at r(0). Each of these two cases results in an azimuthal magnetic field which fits the magnetic field data.

Parish, J. L.↗

NASA NTRS1980

Saturn's magnetosphere, rings, and inner satellites

The discovery of the Saturn magnetosphere and its characterization by Pioneer 11 are reported, and findings on the planet's rings and satellites obtained by energetic charged particle measurements within the inner magnetosphere are presented. Bow shock crossings identified by the Pioneer plasma analyzer and magnetometer at distances of 24.1, 23.1 and 20.0 Saturn radii indicate the presence of a magnetosphere with physical dimensions and charged particle populations intermediate between those of the earth and Jupiter, with a scale more similar to that of the earth. Particle angular distributions on the inbound leg of the trajectory are consistent with a dipole magnetic field approximately perpendicular to the planet's equator, while on the outbound leg the distributions indicate the presence of an equatorial current sheet. Charged particle absorption features are detected at the orbits of Dione and Mimas, encompassing the orbits of Tethys and Enceladus, and at 2.534 and 2.343 Saturn radii indicating the presence of satellites of diameters greater than 170 km. Charged particle measurements also confirm the Pioneer division in the rings between 2.292 and 2.336 Saturn radii, a suspected satellite at 2.82 Saturn radii, the presence of the F ring between 2.336 and 2.371 Saturn radii and the outer radius of the A ring at 2.292 Saturn radii.

Van Allen, J. A.↗

NASA NTRS1979

On the inference of properties of Saturn's Ring E from energetic charged particle observations

The paper demonstrates that information about Saturn's Ring E particle size is potentially obtainable from observations of Saturnian trapped radiation. It is shown that observations of the radial dependence of the intensities, energy spectra, electron-to-proton intensity ratio, and pitch angle distributions of energetic charged particles trapped outside of Ring A can potentially provide information (1) on the existence of Ring E, (2) on the effective size of the particulate matter therein, and (3) on the magnitude of the radial diffusion coefficient for energetic particles. A parametric study of these possibilities is specialized to the characteristics of the University of Iowa detectors on Pioneer 11 which was scheduled to make a close encounter with Saturn in 1979.

Thomsen, M. F.↗

NASA NTRS1979

The dynamics of the Jovian magnetosphere

The current status of the understanding of the dynamics of Jupiter's magnetosphere is reviewed. A brief summary is presented of the concepts and processes which were identified as being of probable importance by pre-Pioneer 10 and 11 work (both theoretical and observational). The insights provided by the in situ Pioneer flights are then discussed. The Jovian magnetosphere consists of several relatively distinct regions: the inner magnetosphere, the intermediate magnetosphere, the outer magnetosphere, a transition region just inside the magnetopause, and the magnetosheath. The basic particle and magnetic field characteristics of these regions are summarized, and the dynamical processes which are currently thought to be significant in each of them are reviewed. Finally, some outstanding questions and problems are identified for future treatment based on Pioneer data or on data from the upcoming Voyager and Galileo missions.

Goertz, C. K.↗

NASA NTRS1979

Precipitation fluxes of energetic electrons at Jupiter - An estimated upper limit

Divine's (1976) model for the observed energetic electron fluxes in the inner Jovian magnetosphere is used to calculate space densities, and the combined source and loss term is evaluated for steady-state radial diffusion conserving the first and second adiabatic invariants. Upper-limit estimates for the ionospheric precipitation fluxes at various values of L and over various electron energy ranges are derived by assuming that contributions to the source term are negligible and that particle precipitation due to pitch-angle diffusion in the loss cone is the main contributor to the loss term. The energy deposition rates corresponding to these precipitated particles are estimated, along with the bremsstrahlung X-ray emissions that should result. It is found that the X-ray emissions should be 5 to 6 orders of magnitude below the reported upper observational limit.

Thomsen, M. F.↗

NASA NTRS1979

Radial diffusion of Io-injected plasma

The paper reexamines the problem of particle injection by Io and subsequent radial diffusion by flux tube interchange using a proper phase space density formulation. The mathematical formalism is developed, and the theoretical results are compared with the observations, taking into account the pitch angle and energy coverage of the detector on Pioneer 10. Two objectives are pursued: (1) to test the hypothesis of Siscoe and Chen (1977) that Io is the source for all of the plasma observed by Frank et al. (1976) inside 10 R(J); and (2) to describe a simple but flexible method intended to analyze the observations of any other plasma instrument flown through the inner Jovian magnetosphere, e.g., those on Voyager 1 and 2 and Galileo.

Goertz, C. K.↗

NASA NTRS1979

Further observational support for the lossy radial diffusion model of the inner Jovian magnetosphere

A mathematical model describing radial diffusion, with violation of the third adiabatic invariant and with local losses, is applied to the Pioneer 10 and 11 observations of omnidirectional integral intensities of electrons with energy exceeding 21 MeV. Local losses outside L = 3 are much stronger than synchrotron losses but weaker than the losses one would expect for strong pitch angle diffusion. If radial diffusion is driven by ionospheric winds, as was suggested by Brice and McDonough (1973), it is found that the theoretical solution which best fits the data requires that the radial diffusion coefficient equals 3 x 10 to the -9th L-cubed/s and the lifetime against local losses is approximately a few million seconds.

Goertz, C. K.↗