Date of Award
Program or Major
Doctor of Philosophy
J V Hollweg
Numerical and analytical studies of MHD waves on magnetic flux tubes are applied to problems of the solar atmosphere. In particular, theoretical analysis of the chromospheric features known as spicules and fibrils are undertaken. The thesis consists of three principal segments:
i. A preexisting spicule model is extended and developed. In the model, a series of rebound shocks propagating on a vertical magnetic flux tube results in chromospheric material with spicule-like properties below a raised transition region. The model emphasizes dynamic motions and shock heating, but excludes radiative and ionization losses. At long times, the model approaches a new hydrostatic equilibrium with the transition region remaining raised, and with a region of shock-heated chromosphere below it. The variation of the model properties in response to different initial parameters is investigated. One conclusion is that the model is capable of generating structures with properties consistent with observations of spicules (with the exception of temperature) when only the dynamics is considered.
ii. An analytical study is performed using linearized MHD equations to demonstrate that spicules may act as resonance cavities for MHD Alfven waves propagating along a vertical magnetic flux tube. When the resonances are excited, large amounts of wave energy from the photosphere and lower chromosphere can propagate into the spicule. This may result in the observed heating, fading, and twisting motions of spicules. It is assumed that the wave energy can be dissipated as heat via a turbulent cascade which follows a Kolmogorov.
iii. The spicule model used in the first segment of the thesis is applied to a magnetic field geometry which is vertical through the photosphere and chromosphere, turns horizontal in the low corona, and eventually turns vertical again and extends into the outer corona. Radiative and ionization losses are again omitted. A structure develops on the horizontal segment which may be identifiable with a fibril, but a full spicule does not develop. At long times, the fibril and short spicule remain extended, and a standing wave develops on the flux tube.
Sterling, Alphonse Christopher, "Dynamics of the solar atmosphere: Spicules and fibrils" (1988). Doctoral Dissertations. 1545.