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Levy, E. H.

Publications and source records attributed to Levy, E. H..

35 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

A Space Station-based search for other planetary systems

The physical forces shaping and maintaining the form of the solar system and disk galaxies are reviewed to define the basis for an observational campaign from the Space Station, to find other planetary systems. The evolution of the distribution of types of matter in the solar system is regarded as typical of the formation of planetary systems around other stars. The observation campaign would cover 100 stars out to 10 pc and last 15-30 yr. Technological challenges which must be met to realize the telescope on the Station are described.

Levy, E. H.↗
NASA NTRS1986

Astrometric telescope of ten microarcsecond accuracy on the Space Station

The Astrometric Telescope Facility (ATF) will be operated in the NASA Space Station in the 1990s, furnishing long term, highly accurate relative astrometry of nearby stars in order to detect gravitational perturbations by companion stars with masses as small as that of Neptune. An accuracy of 10 microarcsec is required; this is 100 times better than ground observatory performance. In the Gatewood et al. (1980) astrometric technique used, the relative positions of star images in the telescope focal plane are indicated by the relative phases of the modulations of star brightnesses introduced by translating a Ronchi ruling across the focal plane at uniform speed. Space Station vibration damping, fine guiding accuracy, optical configuration, Ronchi ruling metric accuracy, and the choice of detectors, are discussed.

Levy, E. H.↗
NASA NTRS1985

Protostars and planets - Overview from a planetary perspective

Today the simultaneous formation of a star and planetary system is understood to be natural and coupled consequences of the collapse of an interstellar cloud of molecular gas and dust. Ideas put forward to account for the existence of the solar system seem generally applicable to star formation throughout the universe. If this is indeed the case, then planetary systems should be present in large numbers throughout the universe. Even the gross structural features of the planetary system in the solar system seem largely to be the result of nearly deterministic processes, rather than the result of chance alone; many planetary systems throughout the universe may resemble this one. This chapter summarizes and examines some of the implications of ideas about the formation of stars and planetary systems.

Levy, E. H.↗
NASA NTRS1984

Oscillating dynamo magnetic field in the presence of an external nondynamo field - The influence of a solar primordial field

Dynamo magnetic fields are self-excited and, once started, can perpetrate themselves with no outside source of magnetic flux, as long as the necessary fluid motions persist. Such dynamo fields behave completely independently of the field's overall polarity. In the presence of an external field of separate origin this polarity symmetry of the dynamo states is broken; the dynamo states become asymmetric with respect to polarity. In this paper a calculation is performed of the characteristics of a spherical shell dynamo in the presence of a fossil magnetic field penetrating into the dynamo from below. The asymmetric periodic states are found as a function of the strength of underlying fossil field. Applying these results to the sun, there appears to be no evidence of any intense large-scale primordial magnetic flux, having either dipole-like or quadrupole-like symmetry about the sun's equator, penetrating into the convection zone from the sun's radiative core. Indeed, the calculations indicate, even on the basis of the presently crude observations, that any such primordial field must have an intensity smaller than a few gauss.

Boyer, D. W.↗
NASA NTRS1982

Oscillating dynamo in the presence of a fossil magnetic field - The solar cycle

Hydromagnetic dynamo generation of oscillating magnetic fields in the presence of an external, ambient magnetic field introduces a marked polarity asymmetry between the two halves of the magnetic cycle. The principle of oscillating dynamo interaction with external fields is developed, and a tentative application to the sun is described. In the sun a dipole moment associated with the stable fluid beneath the convection zone would produce an asymmetrical solar cycle.

Levy, E. H.↗
NASA NTRS1980

Electric field effects on galactic cosmic rays at the heliosphere boundary

The boundary condition for galactic cosmic rays at the outer edge of the modulation region was examined. With few exceptions authors have in the past uncritically used a time-independent, spherically-symmetric condition. It was found that, depending on the mechanism by which the heliospheric magnetic field connects to the general galactic magnetic field, the induced electric field may have important effects on the boundary condition. Sample calculations are presented which illustrate the potentially large, nonspherically-symmetric effects which may be expected.

Jokipii, J. R.↗
NASA NTRS1978

Magnetic field in the primitive solar nebula

Carbonaceous chondrites have apparently been magnetized in their early history in magnetic fields with intensities of 0.1 to 10 G, but the origin of the magnetizing field has remained obscured. It is suggested that the magnetic field recorded in the remanence of carbonaceous chondrites may have been produced by a self-excited hydromagnetic dynamo in the gaseous preplanetary nebula from which the solar system is thought to have formed. Recently computed models for the evolution of the preplanetary nebula, consisting of turbulent and differentially rotating gaseous disks with characteristic radial scales of several AU, are used to demonstrate the feasibility of this hypothesis. The maximum field intensity that might be realized by the dynamo production process is estimated to be as high as 1 to 10 G, taking into account two dynamical mechanisms that limit the strength of the field (the Coriolis force and ambipolar diffusion).

Levy, E. H.↗
NASA NTRS1978

Magnetic field generation at high magnetic Reynolds number

The lowest-order contribution of finite electrical resistivity to the process of magnetic-field regeneration at high magnetic Reynolds number is calculated. It is found that finite resistivity changes the calculated regeneration rate by less than a factor of 2.

Levy, E. H.↗
NASA NTRS1977

Effects of particle drift on cosmic-ray transport. I - General properties, application to solar modulation

Although gradient and curvature drifts are explicitly contained in the general equations of cosmic-ray transport, they have been almost universally neglected in applications of these equations. The drifts are evaluated explicitly for the Parker (1965) spiral magnetic field, and it is shown that for particles with rigidities greater than about 0.3 GV in the solar wind, they are larger than the solar-wind velocity over much of the heliosphere. Hence most current models of solar modulation and solar-flare particle events neglect terms which in many cases are as important as those retained. Calculations are presented which demonstrate the importance of the effects for simple modulation models. It is concluded that comparisons of presently available model calculations with observations do not provide a fair test of transport theory since they neglect drifts, which may substantially reduce the net modulation.

Jokipii, J. R.↗
NASA NTRS1977

Effects of particle drifts on the solar modulation of galactic cosmic rays

Gradient and curvature drifts in an Archimedean-spiral magnetic field are shown to produce a significant effect on the modulation of galactic cosmic rays by the solar wind. The net modulation, heliocentric radial gradient, and average energy change of particles which reach the inner solar system are significantly reduced. The effects of drifts are due to the fact that cosmic rays for which the drift velocity is comparable to the wind velocity or larger, have more rapid access to the inner solar system than in the absence of drifts.

Jokipii, J. R.↗
NASA NTRS1976

The interplanetary magnetic field structure

The seasonal bias in the sector structure of the interplanetary magnetic field has led to the suggestion that the field in each hemisphere of the solar cavity has the same polarity as the average magnetic field at the corresponding solar pole, and that the surface which separates the two polarity regions is only slightly warped. In this scheme the observed sector structure results from corotation of the warped surface past the earth with the angular velocity of the sun. This picture of the interplanetary field structure provides a simple explanation for the solar magnetic cycle periodicity of the diurnal variation of energetic cosmic rays. A discrepancy between the large latitudinal extent of the photospheric sector structure and the apparently small extent of the interplanetary sector structure places constraints on models of the origin of the solar wind. The discrepancy can be resolved if the solar wind originates high in the solar atmosphere where the geometry of the interplanetary field is simplified by magnetic stresses, or if coronal holes produce a large portion of the solar wind.

Levy, E. H.↗
NASA NTRS1974

Possible acceleration of charged particles through the reconnection of magnetic field lines in interplanetary space

Prominent intensity spikes in the flux of protons and alphas with less than 0.5 MeV per charge have been observed in the region several hours behind an interplanetary shock front. The small spatial scale of these events and the high anisotropy of the particle flux suggest local acceleration. The spectra of the particles, which are cut off at equal energy per charge, suggest acceleration through an electric field. The possibility that these events have their origin in active magnetic neutral sheets in the shocked solar wind is examined.

Levy, E. H.↗
NASA NTRS1974

Possible acceleration of charged particles through the reconnection of magnetic field lines in interplanetary space

Prominent intensity spikes in the flux of protons and alphas with less than 0.5 MeV per charge were observed in the region several hours behind an interplanetary shock front. The small spatial scale of these events and the high anisotropy of the particle flux suggest local acceleration. The spectra of the particles, which are cut off at equal energy per charge, suggest acceleration through an electric field. The possibility is examined that these events have their origin in active magnetic neutral sheets in the shocked solar wind.

Levy, E. H.↗
NASA NTRS1972

Kinematic reversal schemes for the geomagnetic dipole.

Fluctuations in the distribution of cyclonic convective cells, in the earth's core, can reverse the sign of the geomagnetic field. Two kinematic reversal schemes are discussed. In the first scheme, a field maintained by cyclones concentrated at low latitude is reversed by a burst of cyclones at high latitude. Conversely, in the second scheme, a field maintained predominantly by cyclones in high latitudes is reversed by a fluctuation consisting of a burst of cyclonic convection at low latitude. The precise fluid motions which produce the geomagnetic field are not known. However, it appears that, whatever the details are, a fluctuation in the distribution of cyclonic cells over latitude can cause a geomagnetic reversal.

Levy, E. H.↗