10 research outputs found
Particle Acceleration in three dimensional Reconnection Regions: A New Test Particle Approach
Magnetic Reconnection is an efficient and fast acceleration mechanism by
means of direct electric field acceleration parallel to the magnetic field.
Thus, acceleration of particles in reconnection regions is a very important
topic in plasma astrophysics. This paper shows that the conventional analytical
models and numerical test particle investigations can be misleading concerning
the energy distribution of the accelerated particles, since they oversimplify
the electric field structure by the assumption that the field is homogeneous.
These investigations of the acceleration of charged test particles are extended
by considering three-dimensional field configurations characterized by
localized field-aligned electric fields. Moreover, effects of radiative losses
are discussed. The comparison between homogeneous and inhomogeneous electric
field acceleration in reconnection regions shows dramatic differences
concerning both, the maximum particle energy and the form of the energy
distribution.Comment: 11 pages, 21 figure
Particle acceleration in three-dimensional tearing configurations
In three-dimensional electromagnetic configurations that result from unstable
resistive tearing modes particles can efficiently be accelerated to
relativistic energies. To prove this resistive magnetohydrodynamic simulations
are used as input configurations for successive test particle simulations. The
simulations show the capability of three-dimensional non-linearly evolved
tearing modes to accelerate particles perpendicular to the plane of the
reconnecting magnetic field components. The simulations differ considerably
from analytical approaches by involving a realistic three-dimensional electric
field with a non-homogenous component parallel to the current direction. The
resulting particle spectra exhibit strong pitch-angle anisotropies. Typically,
about 5-8 % of an initially Maxwellian distribution is accelerated to the
maximum energy levels given by the macroscopic generalized electric potential
structure. Results are shown for both, non-relativistic particle acceleration
that is of interest, e.g., in the context of auroral arcs and solar flares, and
relativistic particle energization that is relevant, e.g., in the context of
active galactic nuclei.Comment: Physics of Plasmas, in prin
Shear flow instabilities in magnetized partially ionized dense dusty plasmas
Shear flow instabilities may play an important role in the dynamics of partially ionized dusty plasmas. Within a multifluid approach the onset criteria with and without electrical resistivity are derived. Long wave length modes can be stabilized by dust-neutral gas collisional momentum transport. Self-consistent numerical simulations of the multifluid plasma dynamics illustrate the nonlinear development of vortical structures in the dust, ion, and neutral velocity as well as in the magnetic field. The unstable modes lead to a significant local amplification of the magnetic field strength. Moreover, electric current sheets form that also show vortices. (C) 2002 American Institute of Physics
Self-magnetization of protoplanetary accretion disk matter
Shear-flow induced friction between neutral gas and charged particle components of different mass can yield a significant self-magnetization of matter. In this paper this process is discussed with respect to protoplanetary disk matter consisting of charged massive dust grains, neutral gas, ions and electrons. Self-consistent three-dimensional multi-fluid plasma-neutral gas-dust simulations are presented taking into account typical parameters for protoplanetary accretion disk matter like that of our early solar system. The results of the simulations show that self induced magnetic fields of 10(-5) Tesla up to 10(-3) Tesla can be expected in a protoplanetary accretion disk on very short time scales of a few years. Thus, shear-flow induced self-magnetization can yield a significant contribution to the magnetization of the early solar system. (C) 2002 American Institute of Physics
Noise of a quantum dot system in the cotunneling regime
We study the noise of the cotunneling current through one or several tunnel-coupled quantum dots in the Coulomb blockade regime. The various regimes of weak and strong, elastic and inelastic cotunneling are analyzed for quantum dot systems (QDS) with few-level, nearly degenerate, and continuous electronic spectra. We iind that in contrast to sequential tunneling, where the noise is either Poissonian (due to uncorrelated tunneling events) or sub-Poissonian (suppressed by charge conservation on the QDS), the noise in inelastic cotunneling can be super-Poissonian due to switching between QDS states carrying currents of different strengths. In the case of weak cotunneling we prove a nonequilibrium fluctuation-dissipation theorem: which leads to a universal expression for the noise-to-current ratio (Fano factor). In order to investigate strong cotunneling we develop a microscopic theory of cotunneling based on the density-operator formalism and using the projection operator technique. The master equation for the QDS and the expressions for current and noise in cotunneling in terms of the stationary state of the QDS are derived and applied to QDS with a nearly degenerate and continuous spectrum