Publication date: Apr 2013
Abstract:
Current-carrying string loops are adopted in astrophysics to model the
dynamics of isolated flux tubes of magnetized plasma expected to arise
in the gravitational field of compact objects, such as black holes.
Recent studies suggest that they could provide a framework for the
acceleration and collimation of jets of plasma observed in these
systems. However, the problem remains of the search of physical
mechanisms which can consistently explain the occurrence of such plasma
toroidal structures characterized by nonvanishing charge currents and
are able to self-generate magnetic loops. In this paper, the problem is
addressed in the context of Vlasov-Maxwell theory for nonrelativistic
collisionless plasmas subject to both gravitational and electromagnetic
fields. A kinetic treatment of quasistationary axisymmetric
configurations of charged particles exhibiting epicyclic motion is
obtained. Explicit solutions for the species equilibrium phase-space
distribution function are provided. These are shown to have generally a
non-Maxwellian character and to be characterized by nonuniform fluid
fields and temperature anisotropy. Calculation of the relevant fluid
fields and analysis of the Ampere equation then show the existence of
nonvanishing current densities. As a consequence, the occurrence of a
kinetic dynamo is proved, which can explain the self-generation of both
azimuthal and poloidal magnetic fields by the plasma itself. This
mechanism can operate in the absence of instabilities, turbulence, or
accretion phenomena and is intrinsically kinetic in character. In
particular, several kinetic effects contribute to it, identified here
with finite Larmor radius, diamagnetic and energy-correction effects
together with temperature anisotropy, and non-Maxwellian features of the
equilibrium distribution function.
Authors:
Cremaschini, Claudio; Stuchlík, Zdeněk;