The three-dimensional geometry of the heliospheric current sheet

The three-dimensional geometry of the heliospheric current sheet seen from fixed points in interplanetary space is constructed for idealized (sinusoidal) magnetic neutral lines (equators) and for an observed magnetic equator on the basis of the “kinematic method” developed by Hakamada and Akasofu (1982). The cross-sections of the wavy current sheet at distances 1, 2 and 5 a.u. are also constructed for the idealized magnetic neutral lines.     

Water transport from the North-east Irish Sea to western Scottish coastal waters: Further observations from time-trend matching of Sellafield radiocaesium

Radiocaesium isotopes, discharged into the North-east Irish Sea from the Sellafield (formerly Windscale) nuclear fuel reprocessing plant in Cumbria, have been employed as flow monitors to update and extend the record of coastal water movement from the Irish Sea to the Clyde Sea area and, further north, to Loch Etive. The temporal trends in radiocaesium levels have been used to determine the extent of water mixing en route and to define mean advection rates. Flow conditions from the Irish Sea have changed considerably since the mid-1970s, the residence time of northern Irish Sea waters being ～12 months during 1978–1980 inclusive. Average transport times of four and six months are estimated for the Sellafield to Clyde and Sellafield to Etive transects respectively. Sellafield ¹³⁷Cs levels in seawater were diluted by factors of 27 and 50 respectively during current movement to the Clyde and Etive areas. The decrease in salinity-corrected ¹³⁷Cs concentrations between the Clyde and Etive suggests that dilution by Atlantic water occurs, the latter mainly entering the Firth of Lorne from the west. The majority (～94%) of the radiocaesium supply to Loch Etive enters the Firth of Lorne via the portion of the coastal current circulating west of Islay, only ～6% arriving via the Sound of Jura.     

Anisotropy of turbulent transport in a convective layer

An estimate for the anisotropy of the turbulent viscositys is given in a convective layer heated from below and rotating around a vertical axis. In the case of two-dimensional convection, there is a stationary regime withs⊇2 regardless of the rotation. In the case of three-dimensional convection in a slowly rotating layer (with the Taylor number equal to 1600), nonstationary turbulent regimes take place withs⊇1.6 forR=2.5×10⁴ (R is the Rayleigh number) ands⊇1.2 forR=10⁴. The parameters plays an, important role in the theory of differential rotation of the convective solar or stellar envelopes. So far, it has been evaluated empirically or semi-empirically. Some prospects in the development of the theory of differential rotation are discussed here in terms of the moment theory of hydrodynamic fields. The relation between this strict approach and an anisotropic viscosity approximation is considered.     

Opposition Effects in Brightness,Color, and Polarization of Comet 1P/Halley: Comparison with Atmosphereless Solar System Bodies

Systematic and uniform sets of photometric and polarimetric observations of comet 1P/Halley have been analyzed. The phase dependence of brightness for comet Halley was obtained at phase angles α ranging from 1.4° ≤ α ≤ 65°. The following parameters were determined: the amplitude of the opposition effect Δm = 0.75^m ± 0.06^m; the half-width at a half-maximum of intensity HWHM = 6.4° ± 1.6°; the linear phase coefficient β = 0.0045 ± 0.0001 mag/deg for α from 30° ≤ α ≤ 65°; and the phase angle at which a nonlinear increase in brightness starts, α _opp ≈ 31°. For the first time, the phase-angle dependence was obtained for the color of the dust of comet Halley: the color index BC-RC systematically decreases with increasing phase angle. Such a phase behavior of the dust color can be caused by the decrease in the mean size of dust particles that occurs when the comet approaches the Sun. For comet Halley, the negative polarization branch is almost symmetric; the minimal value of polarization is P _min = −1.54% at a phase angle α_min = 10.5°, and the inversion angle is α_inv = 21.7°. A comparative analysis of the phase functions of brightness and polarization has been performed for the cometary dust and atmosphereless bodies. Among the latter are low-albedo asteroids of the P and C types (102 Miriam and 47 Aglaja, respectively), as well as Deimos; high-albedo objects, such as the E-type asteroid 64 Angelina and the icy satellite of Jupiter Europa; and the Moon with its intermediate albedo. The possibility of a weak depression in the negative polarization branch of comets Halley and 47P/Ashbrook-Jackson at phase angles smaller than 2° is discussed.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 353–363.Original Russian Text Copyright © 2005 by Rosenbush.     

A simulation study of distortion of the heliospheric sheet caused by solar activity

During major geomagnetic storms, the interplanetary magnetic field angle φ (phi) changes often abruptly, either from 135° to 315° or 315° to 135°, suggesting that the heliospheric current sheet is pushed upward or downward by disturbed solar wind. The distortion of the heliospheric current sheet by three successive solar flares is simulated to show that such a flapping motion can occur.     

Pb, Sr and Nd isotope study of two co-existing magmas in the Nízke Tatry Mountains, Western Carpathians (Slovakia)

Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley – Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite and the Raztocna diorite. The ⁸⁷Sr/⁸⁶Sr₍₀₎ ratios vary between 0.7075 and 0.7118, the ε Nd₍₀₎ values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and a lower crustal source. Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material.     

Design of a high update-rate star sensor for arcsec-level attitude determination from balloon-borne X/γ astronomy platforms

This paper describes the design of a star sensor based upon a high dynamic range CCD in order to reach an arcsec-level attitude determination in balloon-borne missions. A custom star identification software was developed and laboratory-tested on a prototype assembled using commercial components. A set of numerical simulations have been carried out to study the dependence on the pointing precision of the centroid position accuracy, the number of detected stars and the effect of the image focusing. Moreover, the role of the electronic noise and the discrete pixel structure on the light signals is identified by the analysis of numerical simulations. Laboratory tests confirm that the arcsec pointing accuracy with a 1 Hz update rate can be achieved with our combination of custom-developed software and selected hardware components.