The effects of magnetic fields on oscillations in the solar atmosphere
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A study has been made of wave propagation in two regions of the solar atmosphere in which magnetic forces are significant. Sunspot observations indicate a rich variety of characteristic modes of oscillation roughly divided into three categories: three minute umbral oscillations, five minute umbral oscillations and penumbral waves. Outside of intense magnetic flux concentrations the oscillation spectrum is dominated by the five minute period. These waves are trapped in a cavity whose upper boundary may be affected by the magnetism of the chromosphere. A sunspot has been modelled by a uniform cylindrical flux tube. The allowable modes of oscillation are found to vary as the atmospheric parameters change with depth. Umbral three minute oscillations are interpreted as slow body modes. The umbral five minute oscillations arise through a complicated interaction with acoustic waves outside the sunspot. This drives fast body modes as well as waves simply passing through the flux tube. The former may propagate upwards and become fast surface waves. Fast and slow surface waves may explain some of the oscillations of the penumbra. The magnetic structure of the chromosphere has been modelled as an isothermal atmosphere permeated by a uniform and horizontal magnetic field. A dispersion relation for the trapped waves below such an atmosphere has been derived and both asymptotic and numerical solutions found. The effect of a uniform magnetic field is to increase the frequency of the trapped modes. A physical explanation for these changes in frequency has been put forward. Observational evidence may indicate that such effects are indeed seen. This model has been further generalised to take some account of the variation in canopy height which has been observed.
Thesis, PhD Doctor of Philosophy
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