63,479 research outputs found

    Mechanism of Magnetic Flux Loss in Molecular Clouds

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    We investigate the detailed processes working in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities nH<104cmβˆ’3n_{\rm H} < 10^4 {\rm cm}^{-3}, and charged grains contribute more than 90% at nH>106cmβˆ’3n_{\rm H} > 10^6 {\rm cm}^{-3}. Thus grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time tBt_B by a power law tB∝Bβˆ’Ξ³t_B \propto B^{-\gamma}, where BB is the mean field strength in the cloud, we find Ξ³β‰ˆ2\gamma \approx 2, characteristic to ambipolar diffusion, only at nH<107cmβˆ’3n_{\rm H} < 10^7 {\rm cm}^{-3}. At higher densities, Ξ³\gamma decreases steeply with nHn_{\rm H}, and finally at nHβ‰ˆndecβ‰ˆafewΓ—1011cmβˆ’3n_{\rm H} \approx n_{\rm dec} \approx {\rm a few} \times 10^{11} {\rm cm}^{-3}, where magnetic fields effectively decouple from the gas, Ξ³<<1\gamma << 1 is attained, reminiscent of Ohmic dissipation, though flux loss occurs about 10 times faster than by Ohmic dissipation. Ohmic dissipation is dominant only at nH>1Γ—1012cmβˆ’3n_{\rm H} > 1 \times 10^{12} {\rm cm}^{-3}. While ions and electrons drift in the direction of magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, where grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities as nHβ‰ˆndecn_{\rm H} \approx n_{\rm dec}, the process going on at high densities is quite different from ambipolar diffusion in which particles of opposite charges are supposed to drift as one unit.Comment: 34 pages including 9 postscript figures, LaTex, accepted by Astrophysical Journal (vol.573, No.1, July 1, 2002

    Quasi-Monte Carlo methods for Choquet integrals

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    We propose numerical integration methods for Choquet integrals where the capacities are given by distortion functions of an underlying probability measure. It relies on the explicit representation of the integrals for step functions and can be seen as quasi-Monte Carlo methods in this framework. We give bounds on the approximation errors in terms of the modulus of continuity of the integrand and the star discrepancy.Comment: 6 page

    Inverse stochastic optimal controls

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    We study an inverse problem of the stochastic optimal control of general diffusions with performance index having the quadratic penalty term of the control process. Under mild conditions on the drift, the volatility, the cost functions of the state, and under the assumption that the optimal control belongs to the interior of the control set, we show that our inverse problem is well-posed using a stochastic maximum principle. Then, with the well-posedness, we reduce the inverse problem to some root finding problem of the expectation of a random variable involved with the value function, which has a unique solution. Based on this result, we propose a numerical method for our inverse problem by replacing the expectation above with arithmetic mean of observed optimal control processes and the corresponding state processes. The recent progress of numerical analyses of Hamilton-Jacobi-Bellman equations enables the proposed method to be implementable for multi-dimensional cases. In particular, with the help of the kernel-based collocation method for Hamilton-Jacobi-Bellman equations, our method for the inverse problems still works well even when an explicit form of the value function is unavailable. Several numerical experiments show that the numerical method recover the unknown weight parameter with high accuracy
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