Magnetohydrodynamic waves in structured atmospheres

Wave-motion, Theory of;Nonlinear waves;Waves

Edwin, Patricia Mary ; Roberts, Bernard ; Roberts, Bernard

University:University of St Andrews

Department:Physics & Astronomy (School of)

关键词: Wave-motion, Theory of;    Nonlinear waves;    Waves;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/2701/PatriciaMEdwinPhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

The effect of structuring, in the form of magnetic or densityinhomogeneities, on the magnetohydrodynamic (mhd) waves of aninfinite plasma is investigated. The appropriate dispersion formulae,in both Cartesian and cylindrical polar coordinate geometries, arederived. The main properties of the allowable modes in structuredplasmas are described, particularly those featuring in a slenderinhomogeneity.The inclusion of non-adiabatic effects is examined, specificallyfor a thermally dissipative, unstratified, finite structure and fora slender inhomogeneity in a stratified medium. The dissipative timescales of slender structures are shown to have a dependence on thePeclet number. Growth factors appropriate to these time scales forthe overstable motions of a thermally dissipative, Boussinesq fluidare derived.For the linear analysis of a slender structure it is shownthat the dispersive nature of the waves is deducible from thesimplified one-dimensional equations. The analysis is extended,for slender structures, to nonlinear motions and the governingequation representing an effective balance between nonlinear,dispersive and dissipative effects, the Benjamin-Ono-Burgersequation, is established. The solutions of this equation areconsidered and, for weakly-dissipative systems, are shown to beslowly decaying solitons.The importance, in the context of group velocity, of thedispersive nature of waves in ducted structures is discussed andanalogies are made with other ducted waves, for example, the Lovewaves of seismology. It is suggested that the behaviour of suchwaves, following an impulse, may account for the range ofoscillatory behaviour, the quasi-periodic and short time scales,observed in both the solar corona and Earth's magnetosphere.Density variations across a structure and the structure'scurvature, with possible applications to coronal loops, are alsoconsidered.Further suggestions for possibly identifying some of thetheoretical results with observed behaviour in sunspots, chromosphericfibrils and spicules are also made.