In an earlier work we identified a global, nonaxisymmetric instability associated with the presence of an extreme in the radial profile of the key function ...L(r){identical_to}({sigma}{omega}/{kappa}{sup 2})S{sup 2/{gamma}} in a thin, inviscid, nonmagnetized accretion disk. Here {sigma}(r) is the surface mass density of the disk, {omega}(r) is the angular rotation rate, S(r) is the specific entropy, {gamma} is the adiabatic index, and {kappa}(r) is the radial epicyclic frequency. The dispersion relation of the instability was shown to be similar to that of Rossby waves in planetary atmospheres. In this paper, we present the detailed linear theory of this Rossby wave instability and show that it exists for a wider range of conditions, specifically, for the case where there is a ''jump'' over some range of r in {sigma}(r) or in the pressure P(r). We elucidate the physical mechanism of this instability and its dependence on various parameters, including the magnitude of the ''bump'' or ''jump,'' the azimuthal mode number, and the sound speed in the disk. We find a large parameter range where the disk is stable to axisymmetric perturbations but unstable to the nonaxisymmetric Rossby waves. We find that growth rates of the Rossby wave instability can be high, {approx}0.2{omega}{sub K} for relative small jumps or bumps. We discuss possible conditions which can lead to this instability and the consequences of the instability. (c) 2000 The American Astronomical Society.
► We study a model of line-tied MHD modes, with no nulls or closed field lines. ► We analyze quasi-separatrix layers, the squashing factor, and two scalar potentials. ► Comparing the potentials ...distinguishes cases with and without magnetic reconnection. ► These cases are distinguished in terms of the tearing width and the geometric width. ► The squashing degree detects magnetic configurations with potential for reconnection.
In three-dimensional magnetic configurations for a plasma in which no closed field line or magnetic null exists, no magnetic reconnection can occur, by the strictest definition of reconnection. A finitely long pinch with line-tied boundary conditions, in which all the magnetic field lines start at one end of the system and proceed to the opposite end, is an example of such a system. Nevertheless, for a long system of this type, the physical behavior in resistive magnetohydrodynamics (MHD) essentially involves reconnection. This has been explained in terms comparing the geometric and tearing widths
1,2. The concept of a quasi-separatrix layer
3,4 was developed for such systems. In this paper we study a model for a line-tied system in which the corresponding periodic system has an unstable tearing mode. We analyze this system in terms of two magnetic field line diagnostics, the
squashing factor
5–7 and the electrostatic potential difference
8,9 which has been used in kinematic reconnection studies. We discuss the physical and geometric significance of these two diagnostics and compare them in the context of discerning tearing-like (reconnection-like) behavior in line-tied modes.
Implicit time differencing of the resistive magnetohydrodynamic (MHD) equations can step over the limiting time scales—such as Alfvén time scales—to resolve the dynamic time scales of interest. ...However, nonlinearities present in these equations make an implicit implementation cumbersome. Here, viable paths for an implicit, nonlinear time integration of the MHD equations are explored using a 2D reduced viscoresistive MHD model. The implicit time integration is performed using the Newton–Raphson iterative algorithm, employing Krylov iterative techniques for the required algebraic matrix inversions, implemented Jacobian-free (i.e., without ever forming and storing the Jacobian matrix). Convergence in Krylov techniques is accelerated by preconditioning the initial problem. A “physics-based” preconditioner, based on a semi-implicit approximation to the original set of partial differential equations, is employed. The preconditioner employs low-complexity multigrid techniques to invert approximately the resulting elliptic algebraic systems. The resulting 2D reduced resistive MHD implicit algorithm is shown to be successful in dealing with large time steps (on the order of the dynamical time scale of the problem) and fine grids. The algorithm is second-order accurate in time and scalable under grid refinement. Comparison of the implicit CPU time with an explicit integration method demonstrates CPU savings even for moderate (64×64) grids, and close to an order of magnitude in fine grids (256×256).
Magnetic Reconnection Null point Finn, John M
Nature physics,
200607, 20060701, 2006-07-01, Letnik:
2, Številka:
7
Journal Article
Recenzirano
The reordering of field lines during magnetic reconnection plays an important part in many astrophysical and terrestrial plasma phenomena. Satellite measurements of a so-called null point during ...magnetic reconnection should help refine theoretical models of this process.
We have measured the proton recoil polarization in the He-4((e) over right arrow ,e(')(p) over right arrow)H-4 reaction at Q(2)=0.5, 1.0, 1.6, and 2.6 (GeV/c)(2). The measured ratio of polarization ...transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model. In addition, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.
The Kelvin–Helmholtz (KHI)/tearing (TMI) instability is studied with a 2D incompressible Hall MHD model. In the equilibrium configuration of interest, the magnetic and ion velocity fields are ...parallel and identically sheared. While in resistive MHD simultaneous growth of a TMI and a KHI is precluded, Hall physics, by decoupling electrons and ions, destabilizes both modes, leading to a more complex interaction. Nonlinearly, saturation occurs with the formation of a magnetic island and an ion flow vortex in both sub- and super-Alfvénic regimes. For moderately large
c/
ω
pi
, the electron flow shows good alignment with the magnetic field, while demagnetized ions still show KH activity.
We present the first measurement of the Q{sup 2} dependence of the neutron spin structure function g{sub 2}{sup n} at five kinematic points covering 0.57 (GeV/c){sup 2}{<=}Q{sup 2}{<=}1.34 ...(GeV/c){sup 2} at x{approx_equal}0.2. Though the naive quark-parton model predicts g{sub 2}=0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g{sub 2}{sup n} can be predicted using next-to-leading order fits to world data for g{sub 1}{sup n}. Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q{sup 2}, indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g{sub 1}{sup n} are consistent with next-to-leading order fits to world data.
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