5,491 research outputs found
Magnetic Energy Dissipation in the Solar Corona
No full textIn this paper we discuss the problem of small scale generation in a nonuniform magnetized plasma. In a MHD framework, we show that the free-energy stored in the inhomogenous, large scale coronal magnetic fields can be transferred on smaller and smaller scales up to the dissipative ones. This energy transfer is induced by the presence of MHD waves of “modest” amplitude (i.e. for which the non-linear time is much longer than the Alfvén time) which are likely to be generated in the solar athmosphere
Editorial: Special issue: Present achievements and new frontiers in space plasmas
No full textSpace plasma physics, pushed by the impressive recent technological developments, is undergoing a period of intense progress. This progress is achieved first at the level of observations, including both remote and in situ measurements, but also on the theoretical side, mainly by means of large scale numerical simulations made possible by the dramatic increase of computational resources. In particular, three-dimensional mainly hybrid but also fully kinetic simulations are today feasible, and large intervals in spatial and time scales can at last be accessed by fluid simulations. Addressing fundamental problems such as, e.g. magnetic reconnection, nonlinear dynamics or turbulence development in the kinetic range, are no longer just a heart’s desire today
Small scale processes and macroscopic effects in collisionless plasmas
No full textIn high temperature space and laboratory plasmas, energy, initially injected at large scales, is transferred towards small scales. In a number of cases these plasmas can be considered as almost collisionless and dissipative effects become important at length scales much smaller than those where kinetic effects come into play. Here we discuss the role of microscopic small scale kinetic effects and their possible feedback on the large scale dynamics by considering two problems that exhibit interesting analogies: the nonlinear Landau damping of an electrostatic wave and the nonlinear evolution of collisionless magnetic reconnection
A numerical algorithm for geophysical and astrophysical inhomogeneous fluid flows
No full textWe present a 3-dimensional explicit high accuracy numerical code for the solution of the Navier-Stokes equations in the Boussinesq approximation. The code is conceived to investigate inhomogeneous fluid flow characterized by the presence of nonlinear interactions and of very strong gradients of the physical fields delocalized in the inhomogeneous direction. In the linear regime the code has been tested by solving the well-known convective instability problem for which analytical solutions are available. To check its correctness and the stability in the nonlinear regime, we have solved the temporal mixing layer problem and reproduced results well established in the current literature. The code is optimized for massively parallel computers
Nonlinear energy cascade in nonuniform media
No full textThe nonlinear energy cascade in a nonuniform magnetized plasma is studied by using a simple model allowing an analytical solution. The results are then confirmed by a more refined numerical investigation. It is shown that the propagation of moderate amplitude Alfven waves interacting with the background may result in the dissipation of the equilibrium energy much faster than its standard resistive timescale
Resistivity-independent dissipation of magnetohydrodynamic waves in an inhomogeneous plasma
No full textThe heating of high temperature plasmas by magnetohydrodynamic (MHD) waves is one of the most interesting and challenging problems of plasma physics especially when the energy is injected into the system at length scales much larger than the dissipative ones. It has been conjectured that in two-dimensional MHD systems the possibility exists of establishing a state in which energy is dissipated at a rate that is independent of the Ohmic resistivity and that the time needed to reach such a state is finite and independent of resistivity as well. In this paper we prove that this is actually possible as a. result of the nonlinear interaction of long-wavelength, "small" amplitude perturbations with a constant, inhomogeneous magnetic field, at least in the relatively moderate Lundquist number (magnetic Reynolds) range 100 less than or equal to S less than or equal to 3200. [S1063-651X(99)09410-6]
Asymptotic evolution of weakly collisional Vlasov-Poisson plasmas
No full textWe study the role of (weak) numerical diffusion on the long time evolution of the Vlasov-Poisson plasma. We consider the classical problem of phase space vortex formation by particle trapping. We show that the asymptotic macroscopic state is not independent of diffusion even if the dissipative length scale is much shorter than any characteristic physical length scale of the system
Fluid and kinetic (Vlasov) numerical simulations of the wave-plasma interaction in conditions of relevance for rf heating
No full textThe interaction of electrostatic waves of finite amplitude with the plasma which characterizes the edge region of a magnetic confinement device during radiofrequency (rf) heating experiments is investigated on the basis of fluid and kinetic models. In the former case, the time evolution of a two-dimensional initial distribution of the rf energy, coupled with the slow plasma density motion through the action of ponderomotive forces, is investigated. A fluid magnetized plasma is considered and the electric field evolution is treated in the frame of the slowly varying envelope approximation. In the latter case, the Vlasov equations for electrons and ions are integrated together with the Poisson equation in a one-dimensional geometry. An externally applied a.c. forcing term acts on both the species with given frequency and wavevector spectrum, which can be either monochromatic or broad. It is shown that, under conditions typical of the lower hybrid or ion Bernstein heating experiments of tokamak plasmas, numerous nonlinear effects are expected to accompany the wave-plasma interaction, as for example, the formation of strong plasma non-uniformities, the acceleration of charged particles, the nonlinear plasma heating
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