Plasma Properties and Radiation Signatures of Current Driven Active Region Loops on the Sun

Abstract

Our present understanding of the various phenomena taking place in the Sun's atmosphere has radically changed over the past few years.Recent observations, primarily from Skylab, have confirmed the existence of small scale structures which in turn have put severe constraints on the homogeneous plain parallel models of the Sun's atmosphere traditionally used to analyze the observations. It is now generally accepted that the small scale magnetic fields are responsible for these structures. Generally speaking, three types of regions can be identified on the Sun depending on the dominant magnetic field structure, i.e. the coronal holes, where the magnetic field lines are open; the "quiet sun" regions where the magnetic field lines are diffuse, irregular and weak; and the active regions, which are characterized by strong closed magnetic field structures.We usually identify as loops the magnetic field structures connecting opposite magnetic polarities in the photosphere. The average plasma density in the coronal part of the loop is higher than in its surroundings but the ratio of the plasma kinetic pressure to magnetic pressure is less than one. These loop-like magnetic field lines may act as the energy transport lines from the internal layers of the Sun's photosphere to the chromosphere and the corona where the energy is stored and suddenly released or slowly dissipated.We focus our studies at the photospheric disturbances which are associated with steady currents along the potential magnetic field lines. CHAPTER I: ACTIVE REGION OVERVIEW 1 1. Introduction 2 2. The Origin and Role of the Small Scale Magnetic Fields on the Sun 9 3. Photospheric Motions and Current Generation Mechanisms in Closed Loops 4. A Magnetic Model for Complex Active Region