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NASA NTRS1979

Magnetic field reconnection

A reasonably detailed description is obtained of the current status of our understanding of magnetic field reconnection. The picture that emerges is of a process, simple in concept but extremely complicated and multifaceted in detail. Nonlinear MHD processes in the external flow region, governed by distant boundary conditions, are coupled to nonlinear microscopic plasma processes in the diffusion region, in a manner not clearly understood. It appears that reconnection may operate in entirely different ways for different plasma parameters and different external boundary conditions. Steady reconnection may be allowed in some cases, forbidden in others, with intermediate situations involving impulsive or pulsative events.

Sonnerup, U. O.↗

NASA NTRS1986

Cascade model of coronal heating

It is suggested that the quasi-static evolution of coronal magnetic structures is characterized by a cascade of magnetic energy to smaller length scales. This cascade process takes place on a time scale t sub b determined entirely by the photospheric motions. The Ohmic heating rate E sub H in the statistically stationary state was estimated using observational data on the diffusivity of photospheric motions; E sub H turned out to be too small by a factor of 40 when compared with observed coronal energy losses. However, given the fact that the theoretical estimate is based on a rather uncertain extrapolation to the diffusive regime, current heating cannot be ruled out as a viable mechanism of coronal heating.

Vanballegooijen, A. A.↗

NASA NTRS1988

Cosmic-ray transport in accelerating flows

The quasi-linear transport equation of energetic charged particles is derived, including scattering by Alfven waves propagating parallel and antiparallel to a uniform magnetic field, losses by synchrotron radiation, and acceleration of the cold background fluid supporting the waves. As in the comoving frame equations of radiative transfer, scattering, and loss terms are evaluated in the frame in which the fluid is locally at rest. The diffusion approximation is applied to the resulting equation, yielding a transport equation for the isotropic part of the distribution function and a first-order approximation to the anisotropy. The conditions under which this approximation may be applied are discussed. In the nonrelativistic regime, the standard equation of the diffusion approximation is recovered. As contributions previously investigated, new effects appear from the spatial and temporal variations of the fluid velocity as well as the synchrotron radiation terms. These are discussed in detail.

Kirk, John G.↗

NASA NTRS1994

Cosmic bullets as particle accelerators and radio sources

We have simulated in two dimensions the dynamical evolution of dense gas clouds(`cosmic bullets') moving supersonically through a uniform low-density medium. The diffusive shock acceleration of relativistic protons (cosmic rays) and their dynamical feedback on the background flow are included by the two-fluid model for this process. The acceleration of relativistic electrons is approximated by a test-particle model, and a passive magnetic field is followed by a simple advection scheme. Strong bow shocks, with Mach numbers similar to that of a bullet's motion, are the most important particle accelerators in the flow, while tail shocks and shocks inside the bullets do not play generally significant roles in this regard. For our simulation parameters, approximately greater than 10% of the initial bullet kinetic energy is converted to a combination of internal energy of gas and cosmic-ray protons by the time the bullets begin to be disrupted. Characteristically, the cosmic rays gain several percent of the available kinetic energy. Bullet destruction on timescales only a little larger than the ram pressure bullet crushing time begins in response to Kelvin-Helmholtz and especially to Rayleigh-Taylor instabilities along the forward bullet surface. For dense bullets this happens before the bullet is stopped by ram pressure. According to our simple model for synchrotron emission from relativistic electrons accelerated and transported within the flows, that emission increases rapidly as the bullet begins to fragment, when it is strongly dominated by field enhancement in sheared flows. Synchrotron emission from the acceleration region within the bow shock is, by contrast, much weaker.

Jones, T. W.↗

NASA NTRS1993

Absence of magnetic trapping on closed field lines at Neptune

The large offset of Neptune's magnetic field in an offset tilted dipole (OTD) approximation allows some fraction of magnetospheric field lines to have monotonically increasing field magnitude over their entire length in the magnetosphere. The usual magnetic trapping is impossible on such field lines, meaning that all charged particles on these field lines will precipitate after mirroring at most once. These field lines can be thought of as the part of the global magnetic anomaly where no particles on the corresponding drift shells remain trapped for an entire rotation of the planet. In this paper we examine both the morphology and size of the affected surface area in the OTD2 and O8 magnetic field models. For regions where there are indications that part of the aurora could be due to monotonic field line precipitation we analyze whether these field lines could be sufficiently populated to produce auroral signatures by considering photoelectron production rates and radial diffusion. We conclude that monotonic field line precipitation may contribute to the observed Neptune aurora but does not explain the global phenomenon.

Paranicas, C.↗

NASA NTRS1974

The differential rotation of the solar surface

The large-scale flow in the solar convection zone is discussed. The objective is to deduce from observation the principal physical balances in the governing equations. The simplest set of equations that seem potentially realistic are utilized. It is assumed that magnetic fields are negligible, that mixing-length theory gives an accurate representation of the mean structure, and that rigid-body rotation exists at the base. It is deduced that the zonal glow is geostrophic, the meridional flow is controlled by friction, and diffusive heating balances advective cooling due to vertical motion. Next, a detailed calculation of the latitude profile of surface angular velocity is performed, based on approximate equations containing only the principal balances, and agrees well with observation. Finally, it is demonstrated that the amplitude of the flow may be determined by the constraint that the net equatorward angular momentum flux vanish. The conclusions are consistent with the hypothesis that the flow is primarily a single large axisymmetric convection cell in each hemisphere.

Gierasch, P. J.↗

NASA NTRS1975

The quiet time structure of energetic /35-560 keV/ radiation belt electrons

Detailed Explorer 45 observations of quiet time pitch angle distributions and equatorial radial profiles of energetic radiation belt electrons are presented. The observed pitch angle distributions in the slot region and the outer regions of the plasmasphere are found to agree with the values expected from resonant interactions with the plasmaspheric whistler mode wave band, and Coulomb collisions are found to be the dominant electron loss mechanism in the inner zone. The overall structure of quiet time radial profiles is shown to agree with the equilibrium structure expected to result from a balance between pitch angle scattering losses and radial diffusion from an outer zone source, and this agreement suggests that the dominant quiet time source and loss mechanisms have been correctly identified. Electron fluxes in the inner plasmasphere are found to remain nearly constant during the quiet periods due to their decoupling from magnetic activity variations in the outer plasmasphere.

Lyons, L. R.↗

NASA NTRS1982

The magnetosphere of Saturn

Information about the magnetosphere of Saturn is provided: the magnetic dipole moment is axisymmetric, the bow shock stand-off distance is about 22 R sub S. The satellites Titan, Dione, and Tethys are probably the primary sources of magnetospheric plasma. Outside of approx. 4 R sub S, energetic particles are energized by diffusing inward while conserving their first and second adiabatic invariants. Particles are lost by satellite sweep-out, absorption byt the E ring and probably also by plasma interactions. The inner magnetosphere is characterized.

Schardt, A. W.↗

NASA NTRS1990

The diffuse interstellar medium

The last 20 years of the efforts to understand the diffuse ISM are reviewed, with recent changes of fundamental aspects being highlighted. Attention is given to the interstellar pressure and its components, the weight of the ISM, the midplane pressure contributions, and pressure contributions at 1 kpc. What velocity dispersions, cosmic ray pressure, and magnetic field pressure that can be expected for a gas in a high magnetic field environment is addressed. The intercloud medium is described, with reference to the work of Cox and Slavin (1989). Various caveats are discussed and a number of areas for future investigation are identified. Steps that could be taken toward a successful phase segregation model are discussed.

Cox, Donald P.↗

NASA NTRS1992

Static and dynamic loop models and their observational signatures

The magnetically closed regions of the outer solar atmosphere can be studied in terms of one dimensional hydrodynamic loops models. These regions include the bright plasma loops that are readily visible in EUV and X-ray images as well as the fainter, more diffuse-appearing plasma that surrounds them. The basic theoretical properties of static, steady state, and time dependent loop models are discussed, and these properties are related to observations of emission measure distributions and Doppler shifts. Both existing observations and future observations from the Solar and Heliospheric Observatory (SOHO) are considered. In particular, the dependence of the models on the form of the energy input (coronal heating) is emphasized.

Klimchuk, James A.↗

NASA NTRS1992

Distribution of particles and fields in turbulent media

It is shown that fluctuations of kinetic coefficients can lead to the development of an instability that tends to increase the gradients of the distribution of particles and fields in turbulent media. It is found that high rank correlation functions of the cosmic medium turbulent velocity must be taken into account in most real cases. Turbulence in compressible media can lead to a decrease of the CR diffusion coefficient. In some cases the diffusion coefficient may become negative, which implies the development of an instability with respect to the formation of clusters of particles. A similar instability may exist for fields in turbulent media and in media with fluctuation parameters. It may lead to the bunching of magnetic field lines and to the fractal structure of waves propagating in the media.

Dolginov, A. Z.↗

NASA NTRS2003

Effect of Crucible Diameter Reduction on the Convection, Macrosegregation, and Dendritic Morphology during Directional Solidification of Pb-2.2 Wt Pct Sb Alloy

The Pb-2.2 wt pct Sb alloy has been directionally solidified in 1-, 2-, 3-, and 7-mm-diameter crucibles with planar and dendritic liquid-solid interface orphology. For plane front solidification, the experimentally observed macrosegregation along the solidified length follows the relationship proposed by Favier. Application of a 0.4 T transverse magnetic field has no effect on the extent of convection. Reducing the ampoule diameter appears to decrease the extent of convection. However, extensive convection is still present even in the 1-mm-diameter crucible. An extrapolation of the observed behavior indicated that nearly diffusive transport conditions require ampoules that are about 40 microns in diameter. Reduction of the crucible diameter does not appear to have any significant effect on the primary dendrite spacing. However, it results in considerable distortion of the dendrite morphology and ordering. This is especially true for the 1-mm diameter samples.

Chen, Jun↗

NASA NTRS1972

Instabilities and turbulence in highly ionized plasmas in a magnetic field

Physical mechanisms were considered which are responsible for plasma turbulence and the establishment of necessary conditions for energy exchange and transfer in the frequency spectrum. In addition, work was performed to better understand the drift instability in the highly inhomogeneous Rensselaer arc, and methods to suppress this instability using feedback stabilization techniques. Correlation techniques were refined to study plasma turbulence, the diffusion wave technique for monitoring cross-field diffusion was extended to include regimes of high turbulence levels, and a technique for coupling stabilizing RF power to the Rensselaer arc was developed.

Jennings, W. C.↗

NASA NTRS1980

Composition of the nighttime ionospheric F 1 region near the magnetic equator

The effects of vertical E x B transport on NO(+), O2(+) and O(+) densities in the nighttime equatorial ionospheric F 1 region are investigated. Ion densities are calculated as functions of altitude, latitude and local time by the numerical solution of coupled, time-dependent ion continuity equations, taking into account production, loss by charge exchange and dissociative recombination and transport by diffusion and E x B drift. The results of the calculations are compared with measurements of NO(+), O2(+) and O(+) ion densities obtained at low altitudes by a mass spectrometer on board the Atmospheric Explorer C satellite, and are found to be consistent with the observations, suggesting that in the equatorial region, vertical transport by E x B drift is primarily responsible for producing the observed NO(+), O2(+) and O(+) density profiles. In addition, the reaction of O2(+) with N(4S) is found to be an important sink for O2(+) and a source of NO(+) ions. Implications of the observed and calculated near constancy of electron and ion densities with altitude when NO(+) is the dominant ion on the growth of large-scale irregularities are also considered

Anderson, D. N.↗

NASA NTRS1978

Industrial ion source technology

An analytical model was developed to describe the development of a coned surface texture with ion bombardment and simultaneous deposition of an impurity. A mathematical model of sputter deposition rate from a beveled target was developed in conjuction with the texturing models to provide an important input to that model. The establishment of a general procedure that will allow the treatment of manay different sputtering configurations is outlined. Calculation of cross sections for energetic binary collisions was extened to Ar, Kr.. and Xe with total cross sections for viscosity and diffusion calculated for the interaction energy range from leV to 1000eV. Physical sputtering and reactive ion etching experiments provided experimental data on the operating limits of a broad beam ion source using CF4 as a working gas to produce reactive species in a sputtering beam. Magnetic clustering effects are observed when Al is seeded with Fe and sputtered with Ar(?) ions. Silicon was textured at a micron scale by using a substrate temperature of 600 C.

Kaufman, H. R.↗

NASA NTRS1981

The UV resonance lines of three hot Ap stars: HD 133029, HD 175362 and HD 219749

High resolution spectra of the Ap stars HD 133929, HD 175329 and HD 219749 were obtained during two 16 hour shifts with the IUE satellite at Goddard Space Flight Center. Stellar wind, extended atmosphere in rigid corotation with the stellar surface, and the influence of the strong magnetic field on the upper part of the atmosphere would explain the shape and the strength of the resonance lines. The resonance lines of HD 175362 in particular show very peculiar behavior. The necessary driving forces for the expanding envelope are not compatible with the diffusion theory of the Ap atmospheres.

Rakos, K. D.↗

NASA NTRS1986

Coupled hydromagnetic wave excitation and ion acceleration upstream of the Jovian bow shock

We extend the Lee (1982) self-consistent theory of upstream wave excitation and particle energization to address observations by Voyager 2 of sunward propagating MHD waves and diffuse suprathermal particle populations upstream of the Jovian bow shock. Two new ideas are incorporated into the theory. First, the interplanetary seed wave population is taken to be an equal admixture of waves propagating both toward and away from the shock parallel to the interplanetary magnetic field. Second, finite connection times are incorporated approximately into the theory in an effort to understand whether the particle spectra at high energy are limited by particle escape or finite connection time. It is found that finite connection times dominate the particle distribution at energies above 40 keV. In this manner the suprathermal proton distributions can be accounted for by a multiple reflection, shock acceleration theory. We find that the theory can also account for the low-frequency waves observed upstream of the shock in conjunction with the suprathermal ions.

Smith, C. W.↗

NASA NTRS1987

Simulations of the sun's polar magnetic fields during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, the evolution of the radial component of the solar photospheric magnetic field is simulated during 1976-1984, and the corresponding evolution of the line-of-sight polar fields as seen from earth is derived. The observed timing and strength of the polar-field reversal during cycle 21 can be accounted for by supergranular diffusion alone, for a diffusion coefficient of 800 sq km/sec. For an assumed 300 sq km/sec rate of diffusion, on the other hand, a poleward meridional flow with a moderately broad profile and a peak speed of 10 m/s reached at about 5 deg latitude is required to obtain agreement between the simulated and observed fields. Such a flow accelerates the transport of following-polarity flux to the polar caps, but also inhibits the diffusion of leading-polarity flux across the equator. For flows faster than about 10 m/s, the latter effect dominates, and the simulated polar fields reverse increasingly later and more weakly than the observed fields.

Devore, C. Richard↗