An observational study of the formation and evolution of sunspots

Ph.D. University of Hawaii at Manoa 2011.Includes bibliographical references.This dissertation focuses on the problem of molecules and the horizontal balance of forces in sunspots. Sunspots are quasi-static features on the solar surface and can be considered to be in a state of equilibrium. The weaker gas pressure of the cool sunspot interior is horizontally supported against the higher pressure of the hotter quiet-Sun by a strong vertical magnetic field. However, some sunspots show a rapid increase in magnetic pressure relative to the temperature of the gas in the coolest regions of the sunspot, implying that an isothermal decrease in the gas pressure must have occurred. The current model of sunspots is unable to describe this deviation from the assumed equilibrium state of the magnetic field and thermal gas pressure observed in these sunspots. Another method of altering the pressure of the gas must be occurring. The formation of molecules in sunspots may be the key to solving this puzzle. The sunspot interior provides a cool environment where molecules can form in abundance. As atoms become bound into molecules the total particle number of the gas is decreased. A sufficiently large molecular fraction could significantly alter the properties of the sunspot plasma, and specifically provide a mechanism for concentrating the magnetic field by non-thermally lowering the gas pressure. I have investigated the equilibrium condition of sunspots of different sizes and in a variety of evolutionary phases through a Milne-Eddington inversion of spectropolarimetric observations of the Zeeman-split Fe I lines at 6302 and 15650 A to obtain their thermal and magnetic topology. I carried out a calculation of the detailed radiative transfer and chemical equilibrium of model sunspot atmospheres to determine the molecular gas fraction. Several sunspots show unambiguous cases of isothermal magnetic field intensification, which can only be explained by the formation or destruction of a large molecular population. All sunspots with magnetic fields stronger than 2500 G and temperatures cooler than 5800 K consistently show a signature of magnetic field over-concentration, consistent with molecular hydrogen formation of a few percent of the total gas fraction. The formation of this large molecular population has widespread implications for sunspot physics