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[PAST EVENT] Physics Colloquium - John V. Shebalin
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Dr. John V. Shebalin, Affiliate Research Professor, George Mason University, Title of Talk: Statistical Solution of the Dynamo Problem
Abstract: The ‘dynamo problem’ asks the question: How do planets and stars produce a quasi-stationary, energetic dipole magnetic field? We will show that this problem is solved by applying statistical mechanics to magnetohydrodynamic (MHD) turbulence. Joseph Larmor hypothesized in 1919 that the solution lay in a ‘self-excited dynamo’ within the Earth or the Sun. He conjectured that the dynamo was not mechanical, like Faraday's dynamo, but was thought to arise from ‘convective circulation’ and ‘electric currents.’ In 1956, William Elsässer saw that for ‘the dynamo problem, that is ... the problem of generating and maintaining magnetic fields which draw their energy from the mechanical energy of the fluid, the nonlinear character of the equations is altogether essential’, as it produces ‘turbulence, the most conspicuous of the nonlinear phenomena of fluid dynamics.’ Elsässer realized that statistically stationary MHD turbulence was fundamental to the solution of the dynamo problem, rather than ‘rigorously stationary flow’ (i.e., a kinematic dynamo). He added that there were ‘qualitative conditions, three in number, requisite for the operation of ... dynamo models’; these conditions were (1) large linear dimensions, (2) rotation and (3) convection. The first condition implies that Reynolds numbers are sufficiently large; the second that rotation axis and magnetic dipole vector appear on average to be aligned in planets and stars; and the third, that convection is the dynamo’s source of energy. MHD turbulence, due to conditions (1) and (3) is expected to occur in planetary liquid cores and stellar interiors when global magnetism is observed, and must be an integral part of any dynamo model. Although condition (2), rotation, is prevalent in planets and stars and a close alignment often occurs between the dipole moment vector and the rotation axis, rotation is not essential for dynamo action. What is critical is magnetic helicity, which is found to be directly proportional to dipole energy. Thus, MHD turbulence, per se, is the dynamo. Here, we will review the mathematical model, as well as the theoretical and computational results that lead to this conclusion.
Bio: John V. Shebalin received his Ph.D. in Physics from W&M in 1982. Part of his dissertation entitled, “Anisotropy in MHD Turbulence Due to a Mean Magnetic Field,” appeared in a Journal of Plasma Physics (JPP) article of the same title in 1983; this appears to be the most citied paper ever published by JPP. He held positions in industry, academe and government, retiring from NASA as an Astrophysicist in 2017 after 30 years of service. He is currently an Affiliate Research Professor at the Space Weather Lab, Dept. of Physics & Astronomy, George Mason University.