Theory of quasi-stationary kinetic dynamos in magnetized accretion discs

Abstract

Abstract. Magnetic fields are a distinctive feature of accretion disc plasmas around compact objects (i.e., black holes and neutron stars) and they play a decisive role in their dynamical evolution. A fundamental theoretical question related with this concerns investigation of the socalled gravitational MHD dynamo effect, responsible for the self-generation of magnetic fields in these systems. Experimental observations and theoretical models, based on fluid MHD descriptions of various types support the conjecture that accretion discs should be characterized by coherent and slowly time-varying magnetic fields with both poloidal and toroidal components. However, the precise origin of these magnetic structures and their interaction with the disc plasmas is currently unclear. The aim of this paper is to address this problem in the context of kinetic theory. The starting point is the investigation of a general class of Vlasov-Maxwell kinetic equilibria for axi-symmetric collisionless magnetized plasmas characterized by temperature anisotropy and mainly toroidal flow velocity. Retaining finite Larmor-radius effects in the calculation of the fluid fields, we show how these configurations are capable of sustaining both toroidal and poloidal current densities. As a result, we suggest the possible existence of a kinetic dynamo effect, which can generate a stationary toroidal magnetic field in the disc even without any net radial accretion flow. The results presented may have important implications for equilibrium solutions and stability analysis of accretion disc dynamics.

Kinetic description of quasi-stationary axisymmetric collisionless accretion disk plasmas with arbitrary magnetic field configurations

“…In particular, in Paper I and in Ref. [15], it was proved that these equilibria can sustain a stationary kinetic dynamo. As a basic consequence, it was found that both toroidal and poloidal equilibrium magnetic fields can be self generated for quasi-neutral plasmas, without ongoing instabilities and/or turbulence phenomena.…”

Collisionless kinetic regimes for quasi-stationary axisymmetric accretion disc plasmas

“…In the present paper, we are focusing on equilibrium or quasi-stationary configurations as a preliminary to subsequently studying perturbations around these solutions. Note that what is meant here by the term "equilibrium" is in general a quasi-stationary KDF, expressed in terms of the relevant first integrals of motion and adiabatic invariants (see Papers I and II and [10][11][12][13]), which can also include a non-vanishing stationary radial accretion flow ( [14]). One of our main results is the demonstration that for strongly-magnetized collisionless plasmas, kinetic theory gives the possibility of having quasi-stationary accretion flows even in the absence of any additional forms of effective viscosity, such as those arising from turbulence phenomena (which lie beyond the equilibrium solutions).…”

Absolute Stability of Axisymmetric Perturbations in Strongly Magnetized Collisionless Axisymmetric Accretion Disk Plasmas

“…[12][13][14], where a perturbative kinetic theory for collisionless plasmas has been developed and the existence of asymptotic kinetic equilibria has been demonstrated for axisymmetric magnetized plasmas. In AD plasmas, in particular, they are characterized by the presence of stationary azimuthal and poloidal species-dependent flows and can support stationary kinetic dynamo effects, responsible for the self-generation of azimuthal and poloidal magnetic fields [15], together with stationary accretion flows. This provides the basis for a systematic stability analysis of such systems.…”

“…The existence of these equilibria is warranted by the validity of suitable kinetic constraints (see the discussion in Ref. [13,15]). As a consequence, the same equilibria are characterized by the presence of fluid fields (number density, flow velocity, pressure tensor, etc.)…”