DR. UPASANA DAS

Stability is a fundamental issue in the study of accretion disks and jets, both of which are strongly governed by magnetic fields. First, we examine the stability properties of strongly magnetized accretion disks. These disks have a subthermal (plasma-beta >> 1) vertical magnetic flux, but a large-scale, suprathermal (plasma-beta < 1) toroidal field. Such strongly magnetized disks have the potential to resolve several observational shortcomings of the standard accretion disk model. The strong field makes the curvature effects due to the disk geometry non-negligible, which are otherwise ignored in weak-field, local studies. In order to self consistently account for these effects, we perform a global stability analysis of the linearized MHD equations for a compressible, differentially rotating flow in cylindrical geometry. We find that the magnetorotational instability or MRI, which is believed to trigger angular momentum transport in accretion flows, gets highly suppressed at a critical suprathermal toroidal field in these disks; along with the appearance of two new instabilities. Next, we carry out a global linear stability analysis of magnetized cylindrical jets. We focus on characterizing the small-scale, internal instabilities that are confined deep within the jet interior. We analyze the importance of the often overlooked thermal pressure gradient for triggering instabilities in a region of the jet dominated by a toroidal magnetic field and a weak vertical field. Such regions are likely to occur far from the jet source and boundaries, and are potential sites of magnetic energy dissipation that is essential to explain the particle acceleration and radiation observed from astrophysical jets. We find that the most unstable modes are radially localized, which allows us to propose a generic instability criterion that transcends the complex nature of the magnetic field inside jets.