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NASA NTRS1988

Experiments on the formation, properties and processing of cosmic dust

The results from various experiments on the formation, properties, and processing of potential silicate components of cosmic dust are reviewed. Consideration is given to the role of evaporation and condensation in grain formation, nucleation experiments using volatile compounds and refractory species, and efforts to produce particles that might represent cosmic grains. Other studies include particle scattering experiments, studies on the growth of clusters, the sputtering process, the impact of temperature on sputtering yield and optical properties, and experimental and theoretical studies of the structure of particulate aggregates.

Donn, B. D.↗

NASA NTRS1990

Cosmic-ray streaming in the Born approximation

The present work invokes the Born approximation to derive a more accurate expression for the streaming of cosmic rays parallel to the mean magnetic field. While all prior results pertaining to the helicity dependence of the diffusion coefficient and convection speed can be recovered as special cases from this streaming equation, it is concluded that a new set of transport parameters presented here is more appropriate for the solar modulation of galactic cosmic rays. In addition, a new parameter related to time variability, which may be a dominant cause of charge sign-dependent transport of solar particles, is introduced.

Bieber, J. W.↗

NASA NTRS1991

Cosmic Ray Nuclei (CRN) detector investigation

The Cosmic Ray Nuclei (CRN) detector was designed to measure elemental composition and energy spectra of cosmic radiation nuclei ranging from lithium to iron. CRN was flown as part of Spacelab 2 in 1985, and consisted of three basic components: a gas Cerenkov counter, a transition radiation detector, and plastic scintillators. The results of the experiment indicate that the relative abundance of elements in this range, traveling at near relativistic velocities, is similar to those reported at lower energy.

Meyer, Peter↗

NASA NTRS1991

A measurement of the energy spectra of cosmic rays from 20 to 1000 GeV per amu

A group collaboration was made in the development of the Bristol University Gas Spectrometer number 4 (BUGS 4). The BUGS 4 detector is designed to measure the charge spectrum for species between oxygen and the iron peak as a function of energy per nucleon, between 20 and 1000 GeV/amu. It is particularly concerned with energies above 50 GeV/amu. The high energy component is considerably less affected by propagation through the interstellar medium than the lower energy component and is expected to approach the original charge spectrum of the source more closely. This information allows one to unravel the effects of cosmic ray production, acceleration, and propagation. The detector is described in total detail. The method of estimating the charge and energy of a cosmic ray depends on the energy of the particle. Calculations and experiments lead to the expectation of a nearly constant charge resolution of about 0.2 charge units over the whole energy range except 4.5 less than gamma less than 20. In this band, the experiment is insensitive to energy. A balloon flight is planned in 1993.

Gregory, John C.↗

NASA NTRS1990

The heliospheric current sheet and modulation of Galactic cosmic rays

The posssible effect of the heliospheric current sheet on the modulation of cosmic rays is examined by examining the number and the nature of coronal mass ejections on the basis of data collected on an abrupt onset of cosmic ray modulation observed in May 1987 on earth and in September 1987 by Pioneer 10 and 11 and in a previous study of modulation for the years 1976-1986. The measure used to examine the gradient-drift theory (according to which modulation is associated with solar cycle changes in the current sheet) is the value of the difference in the maximum latitudinal extents for the current sheet in the northern and the southern solar hemispheres. These were obtained from contours of the current sheet produced by extrapolating photospheric magnetic field measurements to a solar wind source surface. The observations are found to be consistent with the predictions of the gradient drift model.

Smith, Edward J.↗

NASA NTRS1990

Cosmic-ray gradient measurements and modulation beyond the inner solar wind termination shock

Data provided by the IMP, Voyager, and Pioneer spacecraft for the cosmic-ray particles with E greater than 60 MeV show the existence of integral radial cosmic-ray-density gradients which were nearly constant from 1977 to 1982. In this paper, these measurements are explained using a model in which significant modulation occurs in the turbulent shocked plasma flow between the inner solar wind termination and the outer contact discontinuity separating the interstellar medium flow from the heliospheric plasma.

Quenby, J. J.↗

NASA NTRS1990

Galactic hydrostatic equilibrium with magnetic tension and cosmic-ray diffusion

Three gravitational potentials differing in the content of dark matter in the Galactic plane are used to study the structure of the z-distribution of mass and pressure in the solar neighborhood. A P(0) of roughly (3.9 + or - 0.6) x 10 to the -12th dyn/sq cm is obtained, with roughly equal contributions from magnetic field, cosmic ray, and kinetic terms. This boundary condition restricts both the magnitude of gravity and the high z-pressure. It favors lower gravity and higher values for the cosmic ray, magnetic field, and probably the kinetic pressures than have been popular in the past. Inclusion of the warm H(+) distribution carries a significant mass component into the z about 1 kpc regime.

Boulares, Ahmed↗

NASA NTRS1991

Cosmic X-ray background from hot gas

This paper considers constraints on models of the cosmic X-ray background (XRB) in which the XRB is produced by optically thin thermal bremsstrahlung from hot gas. It is shown that models in which the gas is gravitationally confined in a spherical configuration and is heated only once are contradicted by the observed number of gravitationally lensed quasars together with the lower limit on the number of XRB sources required by limits on fluctuations in the XRB and the cosmic microwave background. In addition, it is shown that, for models in which the gas is not gravitationally confined, the expansion time of the gas is much shorter than the radiative cooling time, so that such models cannot explain the XRB. It is concluded that thermal bremsstrahlung models cannot account for the XRB if the emitting gas is heated only once.

Rogers, R. D.↗

NASA NTRS1991

Thermal design of the Cosmic Background Explorer cryogenic optical assembly

The Cosmic Background Explorer (COBE) is a satellite which is designed to study the 3-K cosmic background radiation. Two cryogenic instruments and a common support structure comprise the Cryogenic Optical Assembly (COA). In order to make definitive measurements of the low-energy FIR radiation, the temperature of the COA must be maintained below 3 K. Temperature control of the COA elements is achieved by the use of a superfluid helium dewar. The thermal design also minimizes power dissipation, transient operational effects, and thermal gradients within the COA. This is accomplished by isolating selected components and heat-sinking others to various locations on the cryogen tank. The thermal design was developed primarily through analysis. Analytical models were created to perform parametric design studies. Ground testing was performed to correlate the thermal models and validate the thermal design. Both test and flight data confirmed that all design requirements were either met or exceeded.

Mosier, Carol L.↗

NASA NTRS1991

A medium-scale measurement of the cosmic microwave background at 3.3 millimeters

A system has been developed for making measurements of spatial fluctuations in the cosmic microwave background radiation, on an angular scale of 5 arcmin to a few degrees. The system consists of an off-axis Gregorian telescope with a nearly Gaussian response with FWHM adjustable from 20 to 50 arcmin, an SIS coherent receiver operating at 3.3 mm, and a pointing system capable of better than 1 arcmin rms stabilization. This paper reports on results from the system's first balloon flight in August 1988, and ground-based measurements made from the South Pole in December 1988. A portion of the South Pole data is used to place a 95-percent confidence level upper limit of Delta T/T less than 0.000035 for Gaussian sky fluctuations in the background radiation at 20-arcmin angular scale and a limit of Delta T/T less than 0.000033 on overall excess intrinsic sky noise. In addition, dust contamination in cosmic background radiation data is estimated using measurements of the Galaxy from this flight and a previous one, along with the IRAS 100-micron map. These anisotropy results give the most stringent limits on cold dark matter theories to date.

Meinhold, Peter↗

NASA NTRS1991

Closed timelike curves produced by pairs of moving cosmic strings - Exact solutions

Exact solutions of Einstein's field equations are presented for the general case of two moving straight cosmic strings that do not intersect. The solutions for parallel cosmic strings moving in opposite directions show closed timelike curves (CTCs) that circle the two strings as they pass, allowing observers to visit their own past. Similar results occur for nonparallel strings, and for masses in (2+1)-dimensional spacetime. For finite string loops the possibility that black-hole formation may prevent the formation of CTCs is discussed.

Gott, J. Richard, III↗

NASA NTRS1991

Cosmic rays from primordial black holes

The quark and gluon emission from primordial black holes (PBHs) which may have formed from initial density perturbations or phase transitions in the early universe are investigated. If the PBHs formed from scale-invariant initial density perturbations in the radiation dominated era, it is found that the emission can explain or contribute significantly to the extragalactic photon and interstellar cosmic-ray electron, positron, and antiproton spectra around 0.1-1 GeV. In particular, the PBH emission strongly resembles the cosmic-ray gamma-ray spectrum between 50 and 170 MeV. The upper limits on the PBH density today from the gamma-ray, e(+), e(-), and antiproton data are comparable, provided that the PBHs cluster to the same degree as the other matter in the Galactic halo.

Macgibbon, Jane H.↗

NASA NTRS1991

The charge state of the anomalous component of cosmic rays

The ionic charge state of anomalous cosmic-ray oxygen has been determined by comparing measurements obtained inside the magnetosphere on a series of Cosmos satellite flights with simultaneous observations outside the magnetosphere from IMP 8 and ICE. A mean charge state of 0.9 + 0.3, - 0.2 is found for abount 10 MeV/nucleon anomalous oxygen, consistent with the model of Fisk et al. (1974), in which the anomalous cosmic rays originate from the neutral component of the local interstellar medium. This same approach gives results consistent with a mean charge state of +7 for solar energetic oxygen ions.

Adams, J. H., Jr.↗

NASA NTRS1991

Cosmic ray produced Be-10 and Al-26 in Antarctic rocks - Exposure and erosion history

Cosmic-ray produced Be-10 and Al-26 were measured in purified quartz fractions of selected rock samples from Antarctic mountains. From these data, mean erosion rates were calculated for the limiting case of steady-state surface exposure to cosmic rays, and minimum exposure ages, for the limiting case of no erosion. Calculated mean erosion rates are very low, on the order of a few times 0.00001 cm/yr; the sampling is believed to be sufficient to generalize this result to exposed bedrock in Antarctica. In favorable cases it is possible to distinguish between the limiting cases; steady-state erosion seems a better description in such cases. Most samaples, including some taken a few meters above the present ice level, seem to have been exposed for millions of years, without major episodes of burial or abrasion by ice.

Nishiizumi, K.↗

NASA NTRS1991

Studies of low-energy Galactic cosmic-ray composition at 22 AU. I - Secondary/primary ratios

Data from the High Energy Telescope of the CRS experiment on Voyager 2 have been used to measure the intensity, spectra, and elemental abundances of Galactic cosmic rays from Be to Ni at about 100 MeV/n. The charge resolution of this telescope varies from sigma = 0.034 for oxygen to sigma = 0.11 for iron. The solar modulation deceleration parameter Phi relevant for these data is estimated to be around 300 MV (Phi = 150 MeV/n for particles with A/Z = 2), an unprecedently low level for such measurements. This low modulation parameter is a result of the measurements being made in the outer heliosphere at a heliocentric distance of 22 AU, and centered on the solar minimum period of cycle 21. The results on secondary/primary ratios are used to test the Leaky-Box model of cosmic ray propagation, using the most recent cross sections data in hydrogen and helium, and taking into account the effects of the ionized fraction of the interstellar medium. It is found that all the secondary/primary ratios up to P/S are completely consistent with an exponential pathlength distribution (PLD). This PLD shape also accounts for the Sc-V/Fe ratio.

Ferrando, P.↗

NASA NTRS1991

A Compton reflection model for the cosmic X-ray and gamma-ray backgrounds

The gamma-ray spectrum of an AGN in the Compton reflection model is calculated and the contribution of such sources to the cosmic gamma-ray background is considered. The spectrum is composed of three parts: an X-ray power law, a broad hump from roughly 10 to 120 keV due to reprocessing of the power-law X-ray spectrum by cold gas, and gamma rays which are due to the inverse-Compton scattering of X-rays. The contribution of such sources to the cosmic gamma-ray background is computed and it is found that they provide an excellent fit to the entire background spectrum from 3 keV to 10 MeV. Constraints on the physical conditions in these sources are derived based on these results and the possibility of observing such sources with GRO is considered.

Rogers, R. D.↗

NASA NTRS1991

A measurement of the cosmic-ray H-2 and He-3 spectra and H-2/He-4 and He-3/He-4 ratios in 1989

The energy spectra of H-2 and H-3 nuclei up to energies of 450 and 800 MeV nucleon, respectively, were measured during a balloon flight of the NASA/NMSU Balloon Borne Magnet Facility on September 5, 1989. These isotopes were cleanly resolved from the more abundant H-1 and He-4 using a measurement of rigidity versus velocity as measured in a high-resolution scintillation counter. After correction for atmospheric secondaries the measured H-2/He-4 and He-3/He-4 ratios are both consistent with the production expected if H-1 and He-4 have the same path length in the galaxy as heavier cosmic-ray nuclei such as C and Fe, to within about 20 percent. Solar modulation effects play an important role in this uncertainty. This result favors a generally similar propagation history, along with similar types of sources for H-1 and He-4 and the heavier cosmic-ray nuclei.

Webber, W. R.↗

NASA NTRS1990

Low energy cosmic ray studies from a lunar base

Studies of cosmic ray nuclei with energies less than about 7 GeV/nucleon in low earth orbit are hampered by the geomagnetic field. Even in high inclination orbits these effects can be significant. The lunar surface (or lunar orbit) provides an attractive site for carrying out low energy cosmic ray studies which require large detectors. The rationale and requirements for this type of experiment are described.

Wiedenbeck, Mark E.↗