Potential field extrapolation using three components of a solar vector magnetogram with a finite field of view

The potential magnetic field from a finite planar boundary is extrapolated into the upper hemisphere using information from all three magnetic field components. The method determines, first, the transverse field associated with the observed normal magnetic intensity. Then by subtraction, the method determines the associated transverse magnetic field observed in the interior (i.e., in the field of view) of the magnetogram which is due to the normal flux exterior to the field of view of the magnetogram. Inverting this information gives an approximation to the exterior normal flux. The combination of the observed normal flux of the interior and the approximation of the exterior normal flux is employed to calculate the potential field. The formulation of the problem results in an ill-posed integral inversion problem in which a regularized solution is obtained using the singular value decomposition (SVD) technique in conjunction with an appropriate Tikhonov-Phillips filter. The technique can be applied to correcting potential field calculations which are influenced by out-of-view fluxes, e.g., for a high spatial resolution vector magnetogram with a small field of view in which there is no supporting exterior data. The problem studied is also important in providing a regularized solution of the Cauchy potential problem. The method provides a much larger range of convergence than the method of Gary and Musielak (1992), and, in fact, is stable in the total upper hemisphere.The U.S. Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.     

Zircon Ce4+/Ce3+ ratios and ages for Yulong ore-bearing porphyries in eastern Tibet

Yulong ore-bearing porphyries, along the northwestern extension of the Red River–Ailao Shan fault system in eastern Tibet, consist of five porphyry deposits, containing a total of more than 8 million tons of copper resources. U–Th–Pb laser inductively coupled plasma mass spectrometry dating of zircon shows that the porphyries were emplaced in Early Tertiary (41.2–36.9 Ma), covering a period of ∼4.3 Ma, with formation ages decreasing systematically from northwest to southeast. The start of porphyry magmatism coincided with the onset of transpressional movement along the Red River–Ailao Shan fault system, implying a close link between these two events. Age sequence in intrusions can be plausibly explained by assuming that a region of melting in the lower northwestern plate moved southeasternward along the Tuoba–Mangkang fault relative to the upper plate. Zircon grains from the Yulong ore-bearing porphyries have higher Ce⁴⁺/Ce³⁺ than those from barren porphyries in the region. This suggests that the ore-bearing porphyries crystallized from a relatively oxidized magma, which has important implications for future ore exploration in the region and other Cu deposits in convergent margin environments in general.     

The Danebury Iron Age meteorite—An H5 ordinary chondrite “find” from Hampshire,England

What remains of a 30 g sample, first recognized as a meteorite in 1989 during characterization of metalworking debris from Danebury, an Iron Age hillfort, in Hampshire, England, has been classified as an H5 ordinary chondrite. Its arrival on Earth has been dated as 2350 ± 120 yr BP, making it contemporary with the period of maximum human activity at the recovery site. Despite its considerable terrestrial residence age, the interior of the specimen is remarkably fresh with a weathering index of W1/2. There is, however, no evidence of human intervention in its preservation. Its near‐pristine state is explained in terms of its serendipitous burial during the back‐fill of a pit dug into chalk by prehistoric people for the storage of grain. This chance discovery has interesting ramifications for the survival of meteorites in areas having a high pH because of a natural lime content arising as a result of the local geology.     

Removing the Pressure Limitations on Flux Pile-Up Reconnection

In a recent Letter, Litvinenko, Forbes, and Priest (1996) claimed that the rate of the very fast flux-pile up reconnection is severely limited by the effects of plasma pressure. They considered however only two-dimensional, zero-vorticity flows. Here we show that this limitation is a feature of these restrictive assumptions and can be removed by relaxing either of them.     

Evidence for Flash Mixing in He-rich sdB Stars

We present FUSE spectra of three He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines, suggesting that they have mixed triple-α carbon from the deep interior out to their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of C and N but still retains a significant amount of hydrogen. In contrast, the third star, LB1766, is devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions arise from a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the stellar envelope, thereby mixing the envelope with the He- and C-rich core. Such “flash-mixed” stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical EHB stars. Two types of flash mixing are possible:“deep” and “shallow”, depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG1544+488 and JL87 are examples of “deep” and “shallow” flash mixing, respectively.     

Molecular Cloud Collapse

Theoretical models of star formation from the collapse of molecular cloud cores have been evolving in complexity for many years. This work describes the evolution of models from the nonrotating, nonmagnetized singular isothermal sphere to rotating, magnetized singular isothermal toroids. Four members of the same family are studied–the four combinations of nonrotation/rotation and nonmagnetization/magnetization. It is found that although rotation alone can hinder collapse to a pointmass, addition of braking from a magnetic field can transport away much of the angular momentum upward in a low-velocity outflow during the collapse phase. For all magnetized cases, accretion proceeds at a constant rate similar to that of the isothermal sphere.     

Echelle observations of C iii λ1909 and Si iii λ1892

Profiles of C iii 1909 and Si iii 1892 obtained on and near the limb during the 1976 flight of the University of Hawaii echelle rocket spectrograph were reduced and analyzed to determine electron densities and mass motions. The electron pressure derived (N _eT_e 4 × 10¹⁵ cm^–3) agrees well with that determined by Cook and Nicolas (1979) from ATM data. Nonthermal velocities in the region of formation of Si iii 1892 on the disk were found to be 10–12 km s^–1, somewhat lower than the values obtained by Doschek et al. (1976), also from ATM spectra. However, velocities derived at and above the limb were in closer agreement, about 17 km s^–1.Geosciences Systems Department, Computer Sciences Corporation, 8728 Colesville Road, Silver Spring, Md. 20910, U.S.A.     

A quantitative technique for assessing the roundness of pottery sherds in water currents

The shape property of transposed pottery sherds that changes most rapidly due to wear during water transport in natural sedimentary environments is roundness, the degree of curvature of edges and corners. of the many methods for the measurement of particle roundness, which are briefly reviewed, that proposed by Wadell (1932, 1933) is considered to specify roundness best. A simple and rapid procedure for the measurement of the Wadell projection roundness is described, which yields roundness values having a precision of the order of a few percent, the precision improving with the degree of roundness. the technique is therefore suitable for the analysis of large samples of transposed and transported sherds.