variability. The observation of the strongest flare in recorded history by Carrington in 1858 was another important milestone and the building block for solar activity and space weather. The exact nature of these phenomena remained hidden until the discovery of solar magnetism by George Hale (1908). He stated in his The Astrophysical Journal paper: “The present paper describes an attempt to enter one of the new fields of research opened by this recent work with the spectroheliograph,” which is a testament to the fact that scientific advances go hand in hand with technology. In the next half‐ century, three major phenomena were discovered: the coronal heating problem, the solar wind and its acceleration, and coronal mass ejections. All of these phenomena are driven by the magnetic field, which is generated deep in the convection zone
The Sun remains the most observed and studied star in the Universe. Yet many of the processes that govern its behavior are not fully understood. The solar dynamo and convection, coronal heating, the acceleration of the solar wind, flares, coronal mass ejections, and solar energetic particles are all outstanding examples of challenging solar and heliospheric problems. Solar and heliospheric research has undergone a renaissance in the last decades. Since the advent of the Space Age, significant strides have been made in our understanding of the Sun along with breakthrough discoveries. Important advances in theory and computing power are keeping pace, allowing access to fundamental physical processes that are otherwise impossible to explore because of the complexity of the underlying theories. These advances have helped us better understand the challenges facing us when we try to comprehend how the Sun and its corona work and also what it takes to obtain the measurements needed to make progress. Figure I.1 is an illustration of the integrated ground–space–theory system, which is a prerequisite to overcoming the hurdles we were facing for decades.
Understanding the Sun is essential not only because we live in its extended atmosphere (i.e., the corona and the solar wind) but also because it is the only star we can study in detail. The knowledge we gain from observing the Sun and its environment provides insights into other worlds that may harbor life like our Earth.
This book provides an overview of solar physics and the advances made over the past few decades as well as the challenging problems that remain. The seven chapters cover the solar interior, the atmosphere, magnetism and radiation, plasma heating and acceleration, and solar activity. It is a comprehensive view of how our star works and what is needed to understand it better. We hope that this reference work will help researchers in other fields, young scientists, teachers, students, and the public to familiarize themselves with the status of the field as of 2019.
Nour E. Raouafi and Angelos Vourlidas Johns Hopkins University Applied Physics Laboratory USA
Figure I.1 Illustration of the complex solar environment and the recent observational and modeling advances that will lead to breakthrough insights into challenging phenomena that have been puzzling scientists for decades. Top left: Slice view showing the Sun at different wavelengths. Top right: The fleet of heliospheric space missions. Bottom left: Image of the 4‐m DKIST solar telescope. Bottom right: Simulation of the structure of the solar corona for the 2017 eclipse.
(Source: Reproduced with permission from Predictive Science Inc.)
1 The Solar Wind
Alexis P. Rouillard1, Nicholeen Viall2, Viviane Pierrard3, Christian Vocks4, Lorenzo Matteini5, Olga Alexandrova6, Aleida K. Higginson2, Benoit Lavraud7, Michael Lavarra1, Yihong Wu4, Rui Pinto1,8, Alessandro Bemporad9, and Eduardo Sanchez‐Diaz1
1 Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
2 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
3 Belgian Institute for Space Aeronomy, Brussels, Belgium
4 Leibniz Institute for Astrophysics Potsdam, Potsdam, Germany
5 Department of Physics, Imperial College London, London, UK
6 Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, Université PSL, CNRS Sorbonne Université, Université de Paris, Meudon, France
7 Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, Pessac, France
8 Laboratoire Dynamique des Etoiles, des (Exo)planètes et de leur Environnement (LDE3), Astrophysics Division (DAp/ AIM), Saclay Nuclear Research Centre (CEA Saclay), Gif‐sur‐Yvette, France
9 INAF Osservatorio Astrofisico di Torino, Turin, Italy
1.1. INTRODUCTION
One of the current mysteries in heliophysics is the heating of the solar atmosphere to temperatures that are orders of magnitude hotter than the solar surface. As a result of this heating, the Sun cannot contain its atmosphere, and a continual outflow of plasma streams out from the solar corona to interplanetary space and beyond. For the debate surrounding the exact physical mechanisms of the heating of the corona, we direct the reader to Chapter 2. We here discuss the physical mechanisms behind the formation and propagation of the solar wind that are not yet well understood.
The whole volume of space influenced by the solar wind is called the heliosphere, and its size is in part modulated by the solar wind ram pressure. The solar wind extends from the corona to well beyond a hundred astronomical units (AU) to a termination shock. Beyond that shock, the solar wind slows down abruptly in response to the pressure of the interstellar medium, and the plasma becomes compressed and more turbulent until it reaches a zone where it can no longer push back the interstellar plasma. In situ measurements of these outer regions are sparse, and our exploration of this boundary layer has only just begun with the Voyager spacecraft. In particular, the global shape of the heliosphere is still not known because the Voyager has only measured a very small region of the heliospheric boundary.
Birkeland (1908) argued very early that a corpuscular emission from sunspots consisting of relativistic electrons must impact Earth’s magnetic field and be deflected to the polar regions to create the aurora. For several decades, it was realized that particles could be emitted from the Sun during flares, but it was generally thought that the space around Earth was mostly empty or perhaps traversed by occasional streams of particles from the Sun (Chapman & Ferraro, 1931). The prevailing view was that the solar corona consists of a hot gas (possibly extending to 1 AU), in thermal and hydrostatic equilibrium, pulled back by the solar gravitational field (Chapman & Zirin, 1957). Detailed observational studies of comets by Biermann (1951) showed that a subset of their tails cannot be accelerated by radiation pressure alone but may also respond to material flowing out from the Sun’s atmosphere. He suggested that the passage of solar particles at the comet formed an ion tail and that these particles must have a very high speed relative to the comet in order to align the tail in the Sun’s direction. Parker (1958) built on these observations and realized that the high temperature of the corona can provide enough energy to force coronal plasma to accelerate from subsonic to supersonic speeds. He demonstrated that the hydrostatic approach predicted too high kinetic pressure at infinity and that a continuous radial expansion of solar gas must act to reduce the coronal pressure. This was the first theory describing the continual expansion of what we now call the solar wind.
In this model, a dominant force affecting coronal particles and pushing them outward is induced by the thermal