Self-similar structure of magnetized advection-dominated accretion flows and convection-dominated accretion flows

Abstract

We study the effects of a global magnetic field on viscously rotating and vertically integrated accretion discs around compact objects using a self‐similar treatment. We extend Akizuki & Fukue's work by discussing a general magnetic field with three components (r, ϕ, z) in advection‐dominated accretion flows (ADAFs). We also investigate the effects of a global magnetic field on flows with convection. For these purposes, we first adopt a simple form of the kinematic viscosity ν=αc2s/ΩK to study magnetized ADAFs: a vertical and a strong magnetic field, for instance, not only prevents the disc from being accreted but also decreases the isothermal sound speed. Then, we consider a more realistic model of the kinematic viscosity ν=αcsH, which makes the infall velocity increase but the sound speed and toroidal velocity decrease. We next use two methods to study magnetized flows with convection, i.e. we take the convective coefficient αc as a free parameter to discuss the effects of convection for simplicity. We establish the αc–α relation for magnetized flows using the mixing‐length theory and compare this relation with the non‐magnetized case. If αc is set as a free parameter, then |vr| and cs increase for a large toroidal magnetic field, while |vr| decreases but |vϕ| increases (or decreases) for a strong and dominated radial (or vertical) magnetic field with increasing αc. In addition, the magnetic field makes the αc–α relation be distinct from that of non‐magnetized flows, and allows the ρ∝r−1 or ρ∝r−2 structure for magnetized non‐accreting convection‐dominated accretion flows with α+gαc < 0 (where g is the parameter to determine the condition of convective angular momentum transport).

Resistive Hot Accretion Flows with Anisotropic Pressure

“…By reducing α 2 , the modified viscosity causes the rotational velocity and the sound speed to reduce. Our results in the low-α 2 limit are in good agreement with the findings of Zhang and Dai (2008). They also pointed out that the modified viscosity decreases the radial velocity and the sound speed.…”

“…( 19)). These results are in excellent agreement with previous work (e.g., Zhang and Dai 2008;Ghoreyshi and Shadmehri 2020;Ghoreyshi 2020). Now, we explore the effects of s on the physical variables of hot accretion flows when η 0 = 0.1.…”

“…We find that the modified viscosity ν = α 1 + β φ c s 2 /Ω K increases the infall velocity. One can expect that the value of β φ plays a key role in these changes (see also Zhang and Dai 2008). Although this increase occurs within all α 2 , the strength of the anisotropic pressure affects these changes.…”

Self-similar structure of resistive ADAFs with outflow and large-scale magnetic field

“…Thus, the effect of other components of the field on the structure of ADAFs must be investigated. The influence of poloidal field component in an ADAF in the absence of outflows has been examined by Zhang & Dai (2008). Adopting the ideal magnetohydrodynamic (MHD) approximation, they concluded that the poloidal field component can reduce the temperature (see also Ghasemnezhad 2017).…”