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Summer Research Fellowship Programme of India's Science Academies

Basics of Dynamo Theory

Md Ekramul Haque

Department of Physics, Patna University, Patna 800005

Dr. Binod Sreenivasan

Centre of Earth Science, Indian Institute of Science, Bengaluru 560012

Abstract

Dynamo action is the process of magnetic field generation by induction in an electrically conducting fluid. The mechanical energy of the fluid is converted to magnetic energy through the stretching and twisting of the magnetic field lines. It is generally agreed that the above mechanism is the source of the magnetic field in planets such as Earth. The temperature of Earth's core is well above the Curie point at which ferromagnetic materials lose their permanent magnetism. Hence, field generation due to dynamo action is likely. Larmor (1919) postulated that dynamo action produced the magnetic field in sunspots. The growth of the magnetic field relies on the advection term in the induction equation being larger than the diffusion term, which in turn means that the magnetic Reynolds number Rm is large. Differential rotation in planets and stars spiral out an azimuthal magnetic field from the axial dipole and this azimuthal field could be well be the dominant field in the interior of Earth. This field is supported by poloidal currents, which are confined to the core of the Earth. The helical motions in the fluid lift and twist the toroidal field to give the poloidal field, a mechanism known as the α-effect. This effect was put forward by Bullard and Gellman (1954) and Parker (1955) in the contexts of geophysics and astrophysics respectively.

Keywords: Curie point, Differential Rotation, Magnetic Reynolds no, Helical motion,α-effect,turbulence

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