Dynamics of relativistic jets     

K. -I. Nishikawa  J. Frank  D. M. Christodoulou  S. Koide  J. -I. Sakai  Hlne Sol  Robert L. Mutel 《New Astronomy Reviews》1998,42(9-10)

We discuss the structure and relativistic kinematics that develop in three spatial dimensions when a moderately hot, supersonic jet propagates into a denser background medium and encounters resistance from an oblique magnetic field. Our simulations incorporate relativistic MHD in a four-dimensional spacetime and clearly show that (a) relatively weak, oblique fields (at 1/16 of the equipartition value) have only a negligible influence on the propagating jet and they are passively pushed away by the relativistically moving head; (b) oblique fields in equipartition with the ambient plasma provide more resistance and cause bending at the jet head, but the magnitude of this deflection and the associated backflow are small compared to those identified by previous studies. The new results are understood as follows: Relativistic simulations have consistently shown that these jets are effectively heavy and so they do not suffer substantial momentum losses and are not decelerated as efficiently as their nonrelativistic counterparts. In addition, the ambient magnetic field, however strong, can be pushed aside with relative ease by the beam, provided that the degrees of freedom associated with all three spatial dimensions are followed self-consistently during the simulations. The effect is analogous to pushing Japanese “noren” or vertical Venetian blinds out of the way while the slats are allowed to bend and twist in 3-D space. Applied to relativistic extragalactic jets from blazars, the new results are encouraging since superluminal outflows exhibit bending near their sources and their environments are profoundly magnetized – but observations do not provide support for irregular kinematics such as large-scale vortical motions and pronounced reverse flows near the points of origin.  相似文献   

Interfacial energies for quartz and albite in pelitic schist     

Takehiko Hiraga  Osamu Nishikawa  Toshiro Nagase  Mizuhiko Akizuki  David L. Kohlstedt 《Contributions to Mineralogy and Petrology》2002,143(6):664-672

Interfacial energies of quartz/quartz (qz/qz), albite/albite (ab/ab), and quartz/albite (qz/ab) boundaries in low-grade pelitic schist were determined based on measured values of dihedral angles. Three kinds of microstructures were investigated, and the interfacial energies were obtained in two independent ways. (1) Relative values of interfacial energy were calculated from dihedral angles formed at quartz and albite triple junctions. (2) Subgrain boundary energy was calculated using the Read-Shockley theory for a boundary connected to an intergranular pore. Dihedral angles formed at the corners of intergranular pores were measured. From the interfacial tension balance equation, the value of the qz/qz grain boundary energy was then obtained. (3) Dihedral angles formed at intersections of either pericline or albite twin boundaries with either ab/ab or qz/ab boundaries were measured. The twin boundary energy was calculated based on a previously derived equation using Landau potential, twin wall thickness, and critical temperature for a phase transition in albite. With a modified interfacial tension balance equation for a twin boundary fixed to a facet orientation, the interfacial energies of ab/ab and qz/ab boundaries were obtained. Energies obtained by methods of (2) and (3) are in good agreement. The interfacial energies for qz/qz, ab/ab, and qz/ab boundaries obtained in this study are 270뀶, 300끞, and 250끀 mJ/m², respectively.  相似文献   

Facular structures derived from precise two-color contrast observations     

Jun Nishikawa  Tadashi Hirayama 《Solar physics》1990,127(2):211-228

A precise solar surface photometric observing system was developed and intensity observations of the whole Sun were carried out at a fairly quiet period from December 1987 to April 1988, attaining the photometric accuracy of 10^–3. Using 28 days observation of 4 hours duration each and 0.3 s time interval, we obtained the center-to-limb variation of the ratio of the facular intensity contrast (I _f÷I_o) between two colors of 545 nm (G) and 770 nm (R): (I _f÷I₀)_G÷(I_f÷I₀)_R = 2.20 – 1.16, where is the direction cosine between the line of sight and the surface normal. While this relationship was obtained mostly for active regions from the whole Sun data, we also found an almost identical relation for the quiet Sun network by excluding active regions. This suggests the similarity of faculae at both places.Using the above relation and the difference of optical depth, , in two colors, we found that the facular temperature gradient, dT/d _G, is smaller than that of the quiet photosphere if the gradient is measured more or less vertically (i.e., seen at > 0.7) in accord with Foukal and Duvall (1985), while it is larger than that of the quiet photosphere if the temperature gradient is measured more obliquely (i.e., seen at > 0.7). These findings are free from the low spatial resolution of the present observations because the contrast ratio was used, and also independent of a specific model of hot-wall or hillock. In particular, if the true contrast of facular bright points of 0'2 size is taken as 40% in the green from high-resolution observations, the facular point temperature may even be increasing towards higher geometrical levels ar _G 1. We found also that the area filling factor in active regions is on the order of a few percent or so in accordance with the previous studies.  相似文献   

Weakening of the solar extreme-ultraviolet line emission by lyman continuum absorption as derived from line ratios     

Mitsuo Kanno  Yoshinori Suematsu  Takara Nishikawa 《Solar physics》1984,91(1):71-74

The intensity ratios of Niii, Oiii, Oiv, and Ov lines observed by the HCO experiment on Skylab are compared with the results of recent multilevel calculations. It is found that solar transition-region spectra require Lyman continuum absorption. The equivalent optical thickness of the absorbers causing the weakening is found to be 1.6–1.7 for a quiet area, 1.4–1.9 for a coronal hole, and 2.4–2.5 for active regions. These values are consistent with previous estimates from different methods.  相似文献   

Continuum absorption in the solar EUV spectra     

Takara Nishikawa 《Astrophysics and Space Science》1986,119(1):17-19

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Solar differential rotation derived from sunspot observations     

Maspul Aini Kambry  Jun Nishikawa 《Solar physics》1990,126(1):89-100

Sunspot drawings obtained at National Astronomical Observatory of Japan during the years 1954–1986 were used to determine the differential rotation of the Sun. From the limited data set of three solar cycles it was found that three factors (the level of cycle activity, the cycle phase, and sunspot type) affect the solar rotation rate. The differential rotation varies from cycle to cycle in such a way that the rotation velocity in the low activity cycle (cycle 20) is higher than in the high-activity cycle (cycle 19). The equatorial rotation rate shows a systematic variation within each cycle. The rate is higher at the beginning of the cycle and decreases subsequently. Although quite small, the variation of solar differential rotation with respect to Zürich sunspot type was found. The H and J types show the slowest rotation among all the sunspot types.  相似文献