Three-Dimensional Evolution of the Parker Instability under a Uniform Gravity

Using an isothermal MHD code, we have performed three-dimensional, high-resolution simulations of the Parker instability. The initial equilibrium system is composed of exponentially-decreasing isothermal gas and magnetic field (along the azimuthal direction) under a uniform gravity. The evolution of the instability can be divided into three phases: linear, nonlinear, and relaxed. During the linear phase, the perturbations grow exponentially with a preferred scale along the azimuthal direction but with smallest possible scale along the radial direction, as predicted from linear analyses. During the nonlinear phase, the growth of the instability is saturated and flow motion becomes chaotic. Magnetic reconnection occurs, which allows gas to cross field lines. This, in turn, results in the redistribution of gas and magnetic field. The system approaches a new equilibrium in the relaxed phase, which is different from the one seen in two-dimensional works. The structures formed during the evolution are sheet-like or filamentary, whose shortest dimension is radial. Their maximum density enhancement factor relative to the initial value is less than 2. Since the radial dimension is too small and the density enhancement is too low, it is difficult to regard the Parker instability alone as a viable mechanism for the formation of giant molecular clouds.Comment: 8 pages of text, 4 figures (figure 2 in degraded gif format), to appear in The Astrophysical Journal Letters, original quality figures available via anonymous ftp at ftp://ftp.msi.umn.edu/pub/users/twj/parker3d.uu or ftp://canopus.chungnam.ac.kr/ryu/parker3d.u

Voltage-biased I-V characteristics in the multi-Josephson junction model of high Tc_c superconductor

By use of the multi-Josephson junction model, we investigate voltage-biased I-V characteristics. Differently from the case of the single junction, I-V characteristics show a complicated behavior due to inter-layer couplings among superconducting phase differences mediated by the charging effect. We show that there exist three characteristic regions, which are identified by jumps and cusps in the I-V curve. In the low voltage region, the total current is periodic with trigonometric functional increases and rapid drops. Then a kind of chaotic region is followed. Above certain voltage, the total current behaves with a simple harmonic oscillation and the I-V characteristics form a multi-branch structure as in the current-biased case. The above behavior is the result of the inter-layer coupling, and may be used to confirm the inter-layer coupling mechanism of the formation of hysteresis branches.Comment: 12 pages, Latex, 4 figure

ASCA observations of massive medium-distant clusters of galaxies. II

We have selected seven medium-distant clusters of galaxies (z ~ 0.1 - 0.3) for multi-wavelength observations with the goal of investigating their dynamical state. Following Paper I (Pierre et al. 1999) which reported the ASCA results about two of them, we present here the analysis of the ASCA observations of the other five clusters; RXJ1023.8-2715 (A3444), RXJ1031.6-2607, RXJ1050.5-0236 (A1111), RXJ1203.2-2131(A1451), and RXJ1314.5-2517. Except for RXJ1031.6, whose X-ray emission turned out to be dominated by an AGN, the ASCA spectra are well fitted by a one-temperature thin thermal plasma model. We compare the temperature-luminosity relation of our clusters with that of nearby ones (z<0.1). Two clusters, RXJ1050.5 and RXJ1023.8, show larger luminosities than the bulk of clusters at similar temperatures, which suggests the presence of a cooling flow. The temperature vs. iron-abundance relationship of our sample is consistent with that of nearby clusters.Comment: 9 pages, 20 figures, A&AS in pres

Temporal 1/f^\alpha Fluctuations from Fractal Magnetic Fields in Black Hole Accretion Flow

Rapid fluctuation with a frequency dependence of $1/f^{\alpha}$ (with $\alpha \simeq 1 - 2$) is characteristic of radiation from black-hole objects. Its origin remains poorly understood. We examine the three-dimensional magnetohydrodynamical (MHD) simulation data, finding that a magnetized accretion disk exhibits both $1/f^\alpha$ fluctuation (with $\alpha \simeq 2$) and a fractal magnetic structure (with the fractal dimension of $D \sim 1.9$). The fractal field configuration leads reconnection events with a variety of released energy and of duration, thereby producing $1/f^\alpha$ fluctuations.Comment: 5 pages, 4 figures. Accepted for publication in PASJ Letters, vol. 52 No.1 (Feb 2000