Convection and dynamo action in astrophysical systems
Faculty: Banibrata Mukhopadhyay (Physics), Binod Sreenivasan (CEaS)
The magnetic fields of astronomical bodies are thought to be either generated by dynamo action in them, e.g. in the cores of stars and planets, or fossil fields which must have then been increasing by flux-freezing. However, there are some astronomical bodies with inferred high magnetic fields, e.g. magnetars, highly magnetized white dwarfs, whose strong fields cannot be explained solely based on the fossil field hypothesis. Numerical dynamo simulations performed on massively parallel computers have successfully captured several observable features of planetary magnetic fields as well as accretion disks which are necessarily turbulent. Research at the Centre for Earth Sciences (CEaS), IISc, led by Prof. Binod Sreenivasan, focuses on building Earth-like dynamo models. The structure of convection in the Earth’s core, which in turn gives rise to the observed geomagnetic field, is of particular interest. Research at the Department of Physics, IISc, led by Prof. Banibrata Mukhopadhyay, aims at understanding magnetized galactic and extra-galactic astronomical objects and resolving underlying astrophysical observations. Certain highly luminous astrophysical sources and jets from accretion disks around a black hole are expected to be involved with very strong magnetic fields. There is no concrete idea of the origin of these fields. Dynamo action in an astronomical body is determined by several factors such as the basic state temperature (buoyancy) profile, background rotation rate and the self-generated magnetic field itself. Whereas three- dimensional dynamo simulations attempt to capture the combined effect of all these factors, understanding their individual roles in isolation requires simpler linear models. A popular example is that of Rayleigh-Benard convection in a rotating plane layer with an imposed magnetic field. The effects of different buoyancy profiles and magnetic fields can be understood from such models. These problems are more complex to set up mathematically and their numerical solution is memory-intensive; nevertheless they are simpler to interpret than three-dimensional dynamo simulations.
The proposed Ph.D. project will aim to set up simplified models for field generation in the interiors of astrophysical systems. This will be attempted using the compressible magnetohydrodynamic (MHD) equations. In turn, three-dimensional simulations will also be performed using an existing code.
The Ph.D. student should have a good mathematical and preferably computational backgrounds. Knowledge of fluid dynamics, spherical harmonic expansions and numerical solutions of PDEs will be an advantage. The student will have access to the HPC machines in SERC, IISc.
Selected publications:
M Bhattacharya, A J Hackett, A Gupta, C A Tout and B Mukhopadhyay, Evolution of highly magnetic white dwarfs by field decay and cooling: Theory and simulations, ApJ, 925, 133, 2022.
A Gupta, B Mukhopadhyay and C A Tout, Suppression of luminosity and mass-radius relation of highly magnetized white dwarfs, MNRAS, 496, 894, 2020.
T Mondal and B Mukhopadhyay, Role of magnetically dominated disc-outflow symbiosis on bright hard-state black hole sources: ultra-luminous X-ray sources to quasars, MNRAS, 495, 350, 2020.
S K Nath and B Mukhopadhyay, Origin of nonlinearity and plausible turbulence by hydromagnetic transient growth in accretion disks: Faster growth rate than magnetorotational instability, Phys. Rev. E, 92, 023005, 2015.
B Sreenivasan, S Sahoo and G Dhama, The role of buoyancy in polarity reversals of the geodynamo, Geophys. J. Int., 199, 1698, 2014.
B Sreenivasan and C A Jones, Helicity generation and subcritical behaviour in rapidly rotating dynamos, J. Fluid Mech., 688, 5, 2011.