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NASA NTRS1979

On the origin of chondrules

A new mechanism for primordial melting of chondrules based upon heating by relativistic electrons accelerated by reconnecting magnetic field lines is discussed. This mechanism is free of the creation-annihilation problem of collisions used for heating. The basic requirements for reconnection are an interplanetary magnetic field with regions of reversals. An early magnetic field is inferred from the paleomagnetism of meteorites and the requirements of solar spin-down, while the field reversals upon which the reconnection is based are a common property of the present epoch solar wind.

Sonett, C. P.↗

NASA NTRS2009

The Invariant Twist of Magnetic Fields in the Relativistic Jets of Active Galactic Nuclei

The origin of cosmic magnetic (B) fields remains an open question. It is generally believed that very weak primordial B fields are amplified by dynamo processes, but it appears unlikely that the amplification proceeds fast enough to account for the fields presently observed in galaxies and galaxy clusters. In an alternative scenario, cosmic B fields are generated near the inner edges of accretion disks in Active Galactic Nuclei (AGNs) by azimuthal electric currents due to the difference between the plasma electron and ion velocities that arises when the electrons are retarded by interactions with photons. While dynamo processes show no preference for the polarity of the (presumably random) seed field that they amplify, this alternative mechanism uniquely relates the polarity of the poloidal B field to the angular velocity of the accretion disk, resulting in a unique direction for the toroidal B field induced by disk rotation. Observations of the toroidal fields of 29 AGN jets revealed by parsec-scale Faraday rotation measurements show a clear asymmetry that is consistent with this model, with the probability that this asymmetry came about by chance being less than 1 %. This lends support to the hypothesis that the Universe is seeded by B fields that are generated in AGN via this mechanism

Contopoulos, Ioannis↗

NASA NTRS1978

Spin-related magnetism of interstellar grains

The magnetic dipole moments and internal magnetic fields due to the spin of electrically charged elongated nonmagnetic interstellar grains in kinetic equilibrium with their surroundings are computed for the grain-size range from 0.01 to 1.0 micron. It is shown that the induced magnetic moments and internal magnetic fields of charged spinning nonmagnetic grains of arbitrary composition and prolate spheroidal shape can be appreciable, possibly even exceeding 0.01 emu/cu cm for 0.01-micron grains. The results indicate that virtually all grains smaller than 0.1 micron in mean diameter, and all elongated grains smaller than about 1 micron in length, are immersed in local magnetic fields due to spin that are much larger than the ambient galactic field. Some implications of this effect are discussed in relation to the polarization of starlight by aligned dust grains and the primordial remanent magnetization found in primitive carbonaceous chondrites.

Srnka, L. J.↗

NASA NTRS1973

Electromagnetic evidence concerning the lunar interior and its evolution

Reviews of the evidence for magnetization of the moon found from discovery of remanence in lunar samples, direct measurements of fields on the surface of the moon, and direct and indirect determination of fields from lunar orbit. It is shown that the evidence implies that the fields are not only local but that regional properties are found though there is still no direct evidence for a global dipole moment. Limits on the detectibility of a global dipole are given and it is shown that the strength of magnetization for reasonable thermal gradients places possible dipole moments just below the threshold of detectibility of current experiments. The hypothesis of plate magnetics is reviewed. Current ideas regarding the source of the background magnetic field presumed responsible for the magnetization are critically considered. These are the dynamo hypothesis and primordial magnetization. Consequences of both are discussed and finally the constraints placed upon the thermal evolution of the moon are considered.

Sonett, C. P.↗

NASA NTRS1977

The interior structure of Jupiter (Consequences of Pioneer 10 data)

Models of the Jovian interiors based on theoretical equations of state of hydrogen and helium supported by a few experimental points and on observed parameters such as oblateness, gravitational coefficients, heat emission, magnetic fields, are discussed. The models fall into three categories: (1) those that assume a uniform and rather low H2/He ratio throughout the planet; (2) those in which this ratio is solar and thus higher; and (3) those that take into account the lack of complete miscibility of the two elements in the condensed state. It appears now also that within the limits of error the planet is in a hydrostatic equilibrium. The large heat emission and the need for an efficient source of internal heat are confirmed, but the results do not indicate which one of the various possible mechanisms is favored, although new evolutionary models suggest that the primordial heat may be insufficient. A new red spot has been discovered. Finally, the presence of a highly eccentric and inclined magnetic field poses new problems related to the pattern of internal convection and to the possibility of a north-south asymmetry of the interior. Further analysis of the available data may throw additional light on these questions.

Smoluchowski, R.↗

NASA NTRS1973

On the primordial condensation and accretion environment and the remanent magnetization of meteorites

In the context of various models for the early evolution of a solar nebula, the possible roles assigned to ambient magnetic fields and the paleointensities required to establish the stable natural remanent magnetization observed in meteorites, are discussed. It is suggested that the record of paleofields present during condensation, growth, and accumulation of grains is likely to have been preserved as chemical or thermochemical remanence in unaltered meteoritic material. Fine particle theories appear adequate for treating meteoritic remanence, if models based on corresponding types of permanent magnet materials, e.g., powder ferrites for chondrites and diffusion hardened alloys for iron meteorites, are adopted.

Brecher, A.↗

NASA NTRS1988

Properties of chondrules

Chondrules contain a record of many essential characteristics of the primordial solar nebula, whose high-temperature chondrule-generating events have spanned nearly 10 million years. As chondrules cooled, they became magnetized by nebular fields; chondrule rims show that dust was present in the vicinity of chondrule formation, and that multiple melting events were possible. The chondrule precursors included many large grains, showed variable degrees of Fe-oxidation, and encompassed many isotopic reservoirs. The properties of these nebular materials systematically changed from the enstatite, to ordinary, and finally to carbonaceous chondrite formation regions.

Grossman, Jeffrey N.↗

NASA NTRS1985

Ringed planets - Still mysterious. II

The morphology of the rings of Saturn and the mechanisms proposed to explain their fluctuations and origin are discussed and illustrated with photographs and diagrams in a general review. The kinks and braids of the F ring are attributed to the gravitational effects of the known shepherding satellites and/or a number of as yet unseen moonlets; the irregular gaps in the B and inner A rings, to particle interactions (diffusion-instability processes); and the flickering spokes of the central B ring, to the rotation of the Saturn magnetic field. It is pointed out that the composition of the single Jovian ring and of the Jovian satellites can be explained if both are remnants of the planet's primordial disk, but that the rock-ice compositions of the rings and inner satellites of Saturn require a more sophisticated model, perhaps involving meteoroid erosion. The rings of Uranus and Neptune are briefly considered.

Cuzzi, J. N.↗

NASA NTRS1987

The theory of the Galactic magnetic field

The paper discusses the role of the magnetic field in determining the large scale structure and dynamics of the interstellar medium. It then discusses the origin and maintenance of the Galactic field. The two major competing theories are that the field is primordial and connected to an intergalactic field or that the field is removed from and regenerated within the Galaxy. Finally, cosmic ray acceleration and confinement in the interstellar medium are discussed.

Zweibel, Ellen G.↗

NASA NTRS1993

Magnetically controlled solar nebula

It is widely believed that a primordial solar nebula, the precursor of the Sun and its planetary system, could be best described in terms of an accretion disk. Such an accretion disk is though to be turbulent, and it is usually imagined that turbulent viscosity alone provides the torque responsible for the structure and the evolution of the nebula. However, it was found that an MHD dynamo operating in a turbulent nebula can contemporaneously produce magnetic fields capable of significantly altering or even dominating the total torque. Thus, it seems that no model of a viscous solar nebula is complete without taking magnetic fields into consideration. It was demonstrated that there are usually two distinct regions of nebular disk where a dynamo can operate: the inner region, where the magnetic field coupled to gas due to relatively high thermal ionization; and the outer region, where this coupling is achieved due to nonthermal ionization. Most models also show the existence of an intermediate region, 'the magnetic gap,' where neither thermal nor nonthermal sources can produce enough ionization to provide the necessary coupling between the magnetic field and the gas. The location and width of the gap change substantially from one model to another. At present, we can only estimate the strength of a generated magnetic field. It seems that a large-scale magnetic field is likely to be in the equipartition with the turbulent kinetic energy; however, the intense magnetic fluctuations may greatly exceed this equipartition strength on short time and length scales. To show how a dynamo-generated magnetic field changes the structure of a viscous nebula, we consider four nebula models extensively.

Stepinski, T. F.↗

NASA NTRS2005

Unveiling the High Energy Obscured Universe: Hunting Collapsed Objects Physics

A large part of energy from space is coming from collapsing stars (SN, Hypernovae) and collapsed stars (black holes, neutron stars and white dwarfs). The peak of their energy release is in the hard-X and gamma-ray wavelengths where photons are insensitive to absorption and can travel from the edge the Universe or the central core of the Galaxy without loosing the primordial information of energy, time signature and polarization. The most efficient process to produce energetic photons is gravitational accretion of matter from a "normal" star onto a collapsed companion (L~GxMcollxdMacc/dtx( 1Rdisc)-dMacc/dt x c2), exceeding by far the nuclear reaction capability to generate high energy quanta. Thus our natural laboratory for "in situ" investigations are collapsed objects in which matter and radiation co-exist in extreme conditions of temperature and density due to gravitationally bent geometry and magnetic fields. This is a unique opportunity to study the physics of accretion flows in stellar mass and super-massive Black Holes (SMBHs), plasmoids generated in relativistic jets in galactic microQSOs and AGNs, ionised plasma interacting at the touching point of weakly magnetized NS surface, GRB/Supernovae connection, and the mysterious origins of "dark" GRB and X-ray flash.

Ubertini, P.↗

NASA NTRS2005

Planetary Geophysics and Tectonics

The broad objective of this work is to improve understanding of the internal structures and thermal and stress histories of the solid planets by combining results from analytical and computational modeling, and geophysical data analysis of gravity, topography and tectonic surface structures. During the past year we performed two quite independent studies in the attempt to explain the Mariner 10 magnetic observations of Mercury. In the first we revisited the possibility of crustal remanence by studying the conditions under which one could break symmetry inherent in Runcorn's model of a uniformly magnetized shell to produce a remanent signal with a dipolar form. In the second we applied a thin shell dynamo model to evaluate the range of intensity/structure for which such a planetary configuration can produce a dipole field consistent with Mariner 10 results. In the next full proposal cycle we will: (1) develop numerical and analytical and models of thin shell dynamos to address the possible nature of Mercury s present-day magnetic field and the demise of Mars magnetic field; (2) study the effect of degree-1 mantle convection on a core dynamo as relevant to the early magnetic field of Mars; (3) develop models of how the deep mantles of terrestrial planets are perturbed by large impacts and address the consequences for mantle evolution; (4) study the structure, compensation, state of stress, and viscous relaxation of lunar basins, and address implications for the Moon s state of stress and thermal history by modeling and gravity/topography analysis; and (5) use a three-dimensional viscous relaxation model for a planet with generalized vertical viscosity distribution to study the degree-two components of the Moon's topography and gravity fields to constrain the primordial stress state and spatial heterogeneity of the crust and mantle.

Zuber, Maria↗

NASA NTRS1992

The spectrum of random magnetic fields in the mean field dynamo theory of the Galactic magnetic field

The fluctuation spectrum that must arise in a mean field dynamo generation of galactic fields if the initial field is weak is considered. A kinetic equation for its evolution is derived and solved. The spectrum evolves by transfer of energy from one magnetic mode to another by interaction with turbulent velocity modes. This kinetic equation is valid in the limit that the rate of evolution of the magnetic modes is slower than the reciprocal decorrelation time of the turbulent modes. This turns out to be the case by a factor greater than 3. Most of the fluctuation energy concentrates on small scales, shorter than the hydrodynamic turbulent scales. The fluctuation energy builds up to equipartition with the turbulent energy in times that are short compared to the e-folding time of the mean field. The turbulence becomes strongly modified before the dynamo amplification starts. Thus, the kinematic assumption of the mean dynamo theory is invalid. Thus, the galactic field must have a primordial origin, although it may subsequently be modified by dynamo action.

Kulsrud, Russell M.↗

NASA NTRS1975

Sheath-limited unipolar induction in the solar wind

A model of the steady-state interaction between the solar wind and an electrically conducting body having neither an atmosphere nor an intrinsic magnetic field sufficient enough to deflect the plasma flow is presented which considers some effects of a plasma surface sheath on unipolar induction. The Sonett-Colburn (1967, 1968) unipolar dynamo model is reviewed, and it is noted that the unipolar dynamo response of an electrically conducting body in the solar wind's motional field can be controlled by sheath effects in certain cases where the body radius is less than a certain critical value. It is shown that sheath effects do not limit the unipolar response of the moon or Mercury since their body radii are much larger than their critical radii. Sheath effects are also considered for asteroids, the Martian satellites, the irregular Jovian satellites, the outer satellites of Saturn, and meteorite parent bodies in a primordial enhanced solar wind.

Srnka, L. J.↗

NASA NTRS1980

The ancient sun: Fossil record in the earth, moon and meteorites; Proceedings of the Conference, Boulder, CO, October 16-19, 1979

Papers are presented concerning theories of solar variability and their consequences for luminosity, particle emission and magnetic field changes within the past 4.5 billion years, and on the records of such solar behavior in lunar, meteoritic and terrestrial materials. Specific topics include the neutrino luminosity of the sun, the relation of sunspots to the terrestrial climate of the past 100 years, solar modulation of galactic cosmic rays, the historical record of solar activity, C-14 variations in terrestrial and marine reservoirs, and solar particle fluxes as indicated by track, thermoluminescence and solar wind measurements in lunar rocks. Attention is also given to the spin-down of the solar interior through circulation currents and fluid instabilities, grain surface exposure models in planetary regoliths, rare gases in the solar wind, nitrogen isotopic variations in the lunar regolith, the influence of solar UV radiation on climate, and the pre-main sequence evolution of the sun and evidence of the primordial solar wind in the electromagnetic induction heating of the asteroids and moon.

Pepin, R. O.↗