No abstract

The ideal magnetohydrodynamic stability of cylindrical equilibria with mass flows is investigated analytically and numerically. The flows modify the local (Suydam) criterion for instability at the resonant surfaces where k⋅B=0. Sheared flows below the propagation speed for the slow wave are found to be destabilizing for the Suydam modes. At a critical velocity, where the shear of the flow exactly balances the propagation of the slow wave along the sheared magnetic field, and the k⋅B=0 surface is at the edge of a slow wave continuum, there is instability regardless of the pressure gradient. Above the critical velocity, the k⋅B=0 surface is stable, but an infinite sequence of unstable modes still exists with frequencies accumulating toward the edge of the slow wave continuum at nonzero Doppler shifted frequency. The stability of the infinite sequences becomes a nonlocal problem whenever the accumulation frequency overlaps with a continuum at some other radial location.

[1] The behavior of the coupled carbon cycle and physical climate system in a global warming scenario is studied using an Earth system model including the atmosphere, land, ocean, and the carbon cycle embedded in these components. A fully coupled carbon-climate simulation and several sensitivity runs were conducted for the period of 1860 -2100 with prescribed IPCC-SRES-A1B emission scenario. Results indicate a positive feedback to global warming from the interactive carbon cycle, with an additional increase of 90 ppmv in the atmospheric CO 2 , and 0.6 degree additional warming, thus confirming recent results from the Hadley Centre and IPSL. However, the changes in various carbon pools are more modest, largely due to the multiple limiting factors constraining terrestrial productivity and carbon loss. The large differences among the three models are manifestations of some of the poorly constrained processes such as the global strength of the CO 2 fertilization effect and the turnover time and rates of soil decomposition.

Axisymmetric toroidal plasma equilibria with mass flows and anisotropic pressure are investigated. The equilibrium system is derived for a general functional form of the pressures, which includes both fluid models, such as the magnetohydrodynamic (MHD) and the double-adiabatic models, and Grad’s guiding center model [Proceedings of the Symposium on Electromagnetics and Fluid Dynamics of Gaseous Plasmas, edited by J. Fox (Polytechnic Inst. of Brooklyn, New York, 1961), p. 37]. This allows for detailed comparisons between the models and clarifies how the ‘‘first hyperbolic region,’’ occurring in fluid theory when the poloidal flow is of the order of the poloidal sound speed, can be eliminated in guiding center theory. In the case of a pure toroidal rotation, macroscopic equations of state are derived from the guiding center model, characterized by a parallel temperature that is constant on each magnetic surface and a perpendicular temperature that varies with the magnetic field. The outward centrifugal shifts of the magnetic axis and of the mass density profile, caused by toroidal rotation, are increased by anisotropy if p∥<p⊥ or decreased (and can even be inverted) if p∥>p⊥. In the guiding center model poloidal flow produces an inward shift of the density profile, in contrast with the MHD result.

[1] Hydrological and current measurements, collected in the Tyrrhenian Sea during May-June 2004, are analyzed with an inverse box model (IBM) to establish the mean circulation patterns of the basin during spring 2004. These patterns are compared with those provided by a high-resolution, primitive equation model (the Princeton ocean model, or POM) implemented over the area to simulate the mean basin circulation during the survey. The good agreement between the two circulation fields, despite the differences and the respective limitations of the employed methods, represents solid evidence for the reliability of the estimated dynamical structures. Moreover, the POM reveals the short spatial variability of the basin not always resolved by IBM because of the low spatial resolution of the in situ measurements. The comparative study indicates the Tyrrhenian basin as an extremely active region of the Mediterranean Sea, characterized by a rich mesoscale dynamics. This work provides, for the first time after more than 25 years, a novel qualitative assessment of the Tyrrhenian Sea spring circulation, together with a robust quantitative estimation of the water mass stream fluxes inside the basin and of the exchanges with the adjacent regions.