MR. PRASUN DHANG

Accretion powers most of the energetic sources (e.g., XRBs, AGNs) in the universe. The most successful model of accretion disc proposed by Shakura and Sunyaev (1973) assume that an emergent turbulent viscosity is responsible for the outward angular momentum transport. However, a convincing source of turbulence was unknown until Balbus and Hawley (1991) drew attention to a weak field instability, namely, magneto-rotational instability (MRI). While linear MRI guarantees outward angular momentum transport, its study in the non-linear regime is essential to explain observed luminosity, time variability, jets etc. In this talk, I will present the study of MRI in geometrically thick (H/R ~ 0.5) radiatively inefficient accretion flows (RIAFs) using 3D global ideal MHD simulations and a pseudo-Newtonian gravity. In saturation, we observe dynamo-generated large-scale magnetic fields, a necessary component to produce jets. The dynamo cycles observed in the geometrically thick RIAFs are intermittent, unlike the very regular cycles seen in the global thin disc (H/R << 1) simulations. The irregularity is due to the sub-Keplerian nature of the angular velocity (for which the shear parameter q = 1.7). We find signatures of two kinds of dynamos– one is the direct dynamo close to the mid-plane, and another being a Parker-type dynamo away from the mid-plane. Away from the mid-plane, the back reaction of the Lorentz force plays an important role in causing the suppression of kinetic helicity by the magnetic helicity of a similar magnitude.