Origin of Geomagnet 1.Introduction to MHD 2.Dynamo theory 3.Geodynamo 3.5 Secular variation&Field reversals 4.Reference 김희준.

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Origin of GeomagnetOrigin of Geomagnet1.Introduction to MHD2.Dynamo theory3.Geodynamo3.5 Secular variation&Field reversals4.Reference1MagnetohydrodynamicsFundamental eqs.Maxwells eqs. (continuity eq)Navier-Stokes eq.Ohms lawIncompressible flowDel dot u=0divMHD approximationE is gone!Induction equationcf)vorticity eqMagnetic Reynolds NumberNon-dimensionalisation of induction eq.Measures relative strengthTwo limiting casesDiffusive limit Rm1Alfvens theorem:The magnetic flux through any material surface of a perfectly conducting fluidis a constant.Lorentz ForceMagnetic pressureMagnetic tensionDynamo theoryCan we construct a steady velocity field leading to dynamo action? yesCan the velocity field be maintained by some force, say Coriolis or buoyancy force, in the face of lorentz force which tends to slow the fluid down? maybe=>no self-consistent model of geo- dynamo yet!EnergeticsAntidynamo theoryCowlings theoremMean-field dynamo theoryRecallcowlings theoremPerturbation is relative to mean feld there.18Geodynamo - model -effectDifferential rotation -effect: possibility for ^2 dynamoSmall scale random magnetic field is thought to organise it self in such a way as to reinforce the large scale dipole field. Ex)ferromagnetism20Axisymmetric field decompositionAxisymmetric mean field dynamoDecaying of poloidal fieldSecular variation & Field reversalsSecular variation is thought to be caused by outer part of the core moving slower than the inner part. Convection!Free-decay models vs Dynamic-reversing modelsDeterministic chaotic vs nonchaoticFree decay is not seemingly true.23ReferenceDavidson, An introduction to MagnetohydrodynamicsMerrill, Mcelhinny, Mcfadden, The Magnetic Field of The Earthhttp://www.maths.gla.ac.uk/~drf/courses/mhd/lect1.pdf


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