Physics of the Sun and Its Atmosphere

The following sections are included:

• PREFACE

• ACKNOWLEDGEMENTS

• CONTENTS

For millennia, the Sun (and the universe) has been viewed in the visual light. As the bestower of light and life, the ancients made God out of the Sun. With the Babylonians, or with the multiple origins with the Chinese, Egyptians and Indians, quoting the Rig Veda:"All that exists was born from Sūrya, the God of gods.", we have come a long way to understanding the Sun. In the early seventeenth century, however, Galileo showed that the Sun was not an immaculate object. Thus began our scientific interests in our nearest stellar neighbour, the Sun (cf., Figure 1.1.), with its sunspots and the related solar activity. The observations of the Sun and their interpretations are of universal importance for at least two reasons: First, the Sun is the source of energy for the entire planetary system and all aspects of our life have direct impact on what happens on the Sun; and second, the Sun's proximity makes it unique among the billions of stars in the sky of which we can resolve its surface features and study physical processes at work…

The Sun plays a central role in two important respects: firstly, it provides a cosmic laboratory for investigating processes that cannot be simulated in the terrestrial environment and secondly, because of its relative closeness it serves as a proxy for understanding conditions in other stars. Formed about 4.6 billion years from a cloud of gas dust and frozen ice, the Sun at the current epoch in its life is a normal main-sequence star of spectral classification G2 with an average surface temperature of around 5700 K. It displays an astounding range of phenomena on myriad spatial and temporal scales that have traditionally defied comprehension. The Sun's magnetic field, that varies on a 22-year cycle, triggers activity and powerful eruptions that affect regions extending from the Earth's atmosphere to the distant edges of the solar system. Despite the inherent complexities of these processes, some progress has been achieved in understanding them through recent spectacular advances in observational techniques coupled with theoretical modelling…

The Sun has played a major role in the development of mathematics and physics over the past centuries and has been widely described as the "Rosetta Stone" of Astronomy. This is undoubtebly an apt description of a celestial object whose internal and external layers provide an ideal laboratory for testing atomic and nuclear physics, high-temperature plasma physics and magnetohydrodynamics, neutrino physics and general theory of relativity. The proximity of our star to Earth has enabled us to make a close scrutiny of its surface regions and the overlying atmosphere. It has provided a wealth of information of high spatial resolution about its surface features which is evidently not possible for other stars. Indeed, from very ancient times the Chinese and Greek astronomers had not failed to notice the dark spots on the otherwise immaculate surface of the Sun. Solar astronomers have, in fact, maintained systematic records of the appearance of these vivid regions on the visible disk of the Sun, hoping to understand the processes that drive the solar cycle and possibly shed some light on its role in influencing the terrestrial climate.…

This chapter is basically divided into 2 parts. In the first part, the important properties of the solar magnetic field are summarized. The discussion begins with a simple introduction to solar magneto hydrodynamics. This introduction will be sufficient to understand the current status of the solar dynamo theory that follows. Some very curious and interesting results on force free fields are then presented in very basic terms. Finally, the application of this theoretical framework to the problems of coronal heating, solar flares and coronal mass ejections are developed in a simple unified scheme, based on a hierarchy of physical conditions. The second part consists of a tutorial on magnetographs. It begins with a description of polarization of light from very fundamental notions of coherence of light. This is followed by simple but comprehensive explanations of the Zeeman and Hanle effects along with the necessary basic ideas of quantum physics of scattering of light. Then the working of a few important magnetographs is outlined, with special emphasis on a solar vector magnetograph developed for USO, to provide a ''hands on" perspective. The article concludes with a few brief remarks on the possible future directions for research in the domain of solar magnetism…

The actual operating heating process that generates and sustains the hot solar corona has so far defied a quantitative understanding despite efforts spanning over half a century. Particular attention is paid here towards the exploration of the coronal heating problem from the perspectives of MHD waves and oscillations. Do MHD waves play any role in the heating of the solar atmosphere? In order to attempt answering this question, first we need do embark on the key properties of the heating of the solar atmosphere…

VUV spectroscopic diagnostics make use of information contained in the measured intensities and profiles of the spectral lines. Access to images and spectra of hot plasma in UV, EUV, and X- ray regions have provided basic tools to address the fundamental problems in the outer solar atmosphere. Observations of coronal lines have been recognized as important means of investigating the physical properties of coronal plasma and addressing the questions, which are in the forefront of space physics, such as coronal heating and solar wind acceleration…

Active regions present us with complex and dynamic structures, which look relatively simple, but are in fact very difficult to model and explain. An active region is a region on the Sun where the magnetic field breaks through the surface (photosphere and chromosphere) to form giant arches of hot plasma. The magnetic field is stronger than in surrounding regions and the plasma is hotter, so emits strongly in UV and X-ray radiation. Figures 7.1 and 7.2 show UV images of the Sun taken with the Transition Region and Coronal Explorer (TRACE) spacecraft and the Extreme Ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. The hot plasma (around a million degree) traces out the magnetic field structures…

The sun is endowed with a variety of temperature variations resulting from a combination of thermal and radiative equilibria and departures therefrom. The thermal and the nonthermal nature of processes then translates into a plasma with varying degrees of ionization…

It seems worthwhile to explain some basic concepts and terms, in brief, before describing the coronal heating problem and various efforts made in this direction so far…

Coronal mass ejections or CMEs were first classified as a distinct class of solar phenomena soon after the launch of Skylab mission in space in 1973 which carried a coronagraph onboard…

The Sun is the most powerful radio waves emitting object in the sky. The first documented recognition of the reception of radio waves from the Sun was made in 1942 by Hey.¹⁵ Since then solar radio observations, from ground-based and space-based instruments, have played a major role in understanding the physics of the Sun and fundamental physical processes of the solar radio emitting phenomena…

The solar atmosphere consists of three main regions, the photosphere, the chromosphere and the corona. The photosphere is a thin layer of low-density gas and it allows visible photons to escape into space. The current of gas rising from the beneath the surface of the Sun causes the formation of granulations at the photosphere. In addition, the magnetic fields threading outward from the solar interior give rise to active regions, sunspots, coronal holes, and various structures at the solar atmosphere. These largescale magnetic structures can store huge amounts of energy, which may be released gradually or explosively…

Although separated by 150 million km, the Earth is inextricably linked to the Sun by an energy flow that both sustains and imperils life. A 15.8 million Kelvin nuclear furnace in the innermost one-third of the Sun is the source of this energy, which photons, particles and fields disseminate throughout the solar system. Figure 13.1 depicts major components of the Sun-Earth system, and Figure 13.2 summarizes the energy flow…