Carbon-isotope composition of sediments from the Gulf of Papua

Rivers draining into the Gulf of Papua (GOP) from the Papua New Guinea mainland deliver approximately 340 × 10⁶ t yr^–1 of sediment to the marine environment. The terrestrially derived sediment contains 1.1 ± 0.2% particulate organic carbon with a carbon-isotope composition of –26.5 ± 0.2, and amounts to 3.7 ± 0.7 × 10⁶ t yr^–1. The carbon-isotope composition of sediments in the Gulf of Papua indicates that 40% of the sediment cover contains 75% or more terrestrially derived carbon. Suspended sediments that are transported beyond the delta complex of the Fly River are transported north and northwest, augmented by sediments from other rivers along the coast of the GOP. The carbon-isotope results suggest that a significant quantity of terrestrially derived sediment escapes from the GOP, either along the coastlines to east and west or into the deep ocean via the Moresby and Pandora troughs. Little sediment travels south onto the Great Barrier Reef shelf. Extrapolating the results from this study to the region of Oceania suggests a total flux of particulate organic carbon to the world's oceans from the islands of Oceania of ~ 90 × 10⁶ t yr^–1 or twice the flux of riverine POC from the major rivers of North America, South America, and Africa combined. While such a calculation must be considered illustrative only, the similar tectonic, geomorphologic, and climatic features of the islands of Oceania suggest that the calculation is unlikely to be grossly in error and that the rivers of Oceania therefore represent a major but poorly documented source of sediment and organic carbon to the global ocean.     

Cometary collisions and the dark material on Iapetus

Iapetus (S8) is unique in our solar system in that the albedo of its leading hemisphere is only 0.05 while that of the trailing side is 0.5. Several existing hypotheses are examined and found inadequate. Photometric studies of the dark side are compared to comet nuclei and class D asteroids. It is hypothesized that in the last 10⁶–10⁸ yrs the leading side suffered a high-velocity collision with a cometary body of mass 10¹³–10¹⁵ kg and traveling at a speed of 20 km s^–1. About 5–16% of the excavated material was ejected into space, where the vaporized ices dissipated while the dark carbonaceous/silicate material was reaccreted on the leading side. The collision, although not sufficient to break Iapetus' tidal lock, resulted in a period of oscillation of about 5 yr. Until tidal friction reasserted a lock, the oscillation gave rise to the longitude effect, viz., the observed fact that the dark material covers more than 220 of longitude but only 110 of latitude.     

Interpretation of a special fine structure in type-IV solar radio bursts

A special fine structure (slowly drifting chains of narrowband fiber bursts), firstly observed during the solar type-IV radio burst on April 24, 1985, is interpreted as the radio signature of whistler waves periodically excited by a switch-on/switch-off process of a loss-cone instability in a localized wave packet of the fast magnetoacoustic mode.     

Motions in the field of two rotating magnetic dipoles. III: Zero-velocity curves and surfaces

The zero-velocity curves and surfaces created by two parallel or antiparallel magnetic dipoles moving on concentric circles are computed. It is found that for all combinations of values of the mass parameter µ and the ratio of the magnitudes of the magnetic moments of the two dipoles, there exist closed regions inside of which charged particles are permanently trapped.     

The University of Durham Southern Hemisphere VHE gamma ray telescope

The design of very high energy (VHE) gamma ray telescopes is discussed and it is shown that a triple-mirror telescope operating in threefold fast coincidence provides an effective instrument. The design, construction and operation of a large VHE gamma ray telescope with an energy threshold of 300 GeV (at the zenith) and incorporating several novel features is described. The use of a coincidence guard ring technique is shown to result in the rejection of about 50% of the off-source (proton) events which form a high background and limit the sensitivity of such telescopes.     

Helium resonance lines in the flare of 15 June, 1973

Time sequences of He i and He ii resonance line intensities at several sites within the flare of 15 June, 1973 are derived from observations obtained with the Naval Research Laboratory's Slitless Spectroheliograph on Skylab. The data are compared with predictions in six model flare atmospheres based on two values for the heating rate and three for the flux of photoionizing coronal X-rays and EUV. A peak ionizing flux more than 10³ times that in the quiet Sun is indicated. For most conditions in flare kernels the He ii L and L lines are found to be formed by collisional excitation, thereby contributing to the local cooling of the plasma at temperatures above 6 × 10⁴ K. Emission in the higher Lyman lines is generally the result of a mixture of collisional excitation at these temperatures and photoionization and recombination at temperatures near 2.5 × 10⁴ K. We discuss implications for the common practice of deriving stellar coronal fluxes from He ii 1640 Å fluxes assuming dominance of the recombination mechanism.Chief, Quantum Physics Division, National Institute of Standards and Technology.Operated jointly by the National Institute of Standards and Technology and the University of Colorado.Operated by the National Optical Astronomy Observatories of the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Finite-β effects on the radiative thermal instability in magnetized plasmas

The radiative thermal instability is investigated taking into account finite-, or electromagnetic, effects. The two-fluid model for magnetized plasmas together with the Maxwell equations are used to derive a general dispersion relation valid for compressional perturbations with frequency below the electron-cyclotron frequency. The growth rates of the radiative thermal instabilities involving fast magnetosonic flute-like and low-frequency hydromagnetic perturbations are presented.     

Radiometry of near-earth asteroids

We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids.     

The center-to-limb variations of four Ca i lines in the photospheric spectrum at λ6500

We study the center-limb (CL) variation of the average profiles of four Ca i lines near 6500 and compare these observations with synthetic data obtained from several line formation models having different thermal structures, line parameters, LTE and non-LTE conditions, and micro and macroturbulence values, to assess the formation characteristics of our Ca i lines in the solar photosphere.Comparison of numerical results with observations indicates that non-LTE is indispensable to fit the CL variation of the central residual intensity for the line 6493, and anisotropic microturbulence is indispensable to improve the CL behavior of the equivalent widths for all lines. The Ca i line analysis favors a cool photospheric model, but this cannot be disentangled clearly from the effects of non-LTE and small-scale velocity fields on the grounds of the present line formation models.     

The possibility of transition radiation in planetary nebulae

A new concept —the Paradox of Nebula IC4997 — is the main subject of the present article. The essence of this paradox arises when the variations of the intensities of forbidden lines 4363 [Oiii] andN ₁+N ₂ [Oiii] take place not in unison as is predicted by the classical theory. An attempt is made to solve this paradox, suggesting the possibility both of spontaneous appearance of relativistic electrons in the nebula and the generation of so-calledtransition radiation as a result of electrodynamic interaction of these electrons with dust particles in nebula. The parameters of relativistic electrons and power of transition radiation are obtained. The problems which need further examination are also enumerated.