Development and Evolution of Magnetic Fields in Active Regions on the Sun.

The purpose of this thesis is to investigate the development and evolution of magnetic fields in new active regions on the sun. The major observations are digital magnetograms of the line-of-sight component of magnetic fields made with the high sensitivity videomagnetograph at the Big Bear Solar Observatory, and H(alpha) filtergrams with the 1/4 (ANGSTROM) H(alpha) Zeiss filter. This thesis consists of three themes. First, the separation velocity of emerging magnetic flux is investigated. I measure the separation velocities of opposite polarities of 24 new bipoles, and compare them with the theoretical values estimated by the present theory of magnetic buoyancy. The predicted velocities are higher than those observed. Second, the cooling time scale of growing sunspots is studied. I define the cooling time scale and derive it from the measurements of intensity and magnetic field strength of sunspots. The cooling time scales of the ten growing sunspots studied range from 0.5 to 9 hr. I also estimate the cooling time scale from two models, the Inhibition Model and the Alfven Wave Model, based on linear theory. Both models give cooling times of about 0.05 hr. Third, nonadiabatic effects in convective instabilities in thin flux tubes are examined. I study the convective instabilities in thin flux tubes by including a nonadiabatic term. I find that a flux tube is convectively stable for any field strength.