This chapter describes the magnetic activity of the Sun throughout most of its history. We begin with a summary of the activity of the Sun as described in historical records, i.e. going back at most a few centuries. The picture of the Sun in time can be completed by complementing solar observations with stellar data. This chapter focuses on the period from a solar age of roughly 0.1 Gyr onward. The formation of the Sun and its solar system and the very earliest phases of their joint evolution are discussed in Chapters 3 and 4.

The Sun's magnetic activity and the associated changes in its extended atmosphere evolve on time scales that range from minutes up to billions of years. The coupling of these scales requires that we describe the patterns of the solar magnetic field from the smallest currently observable structures to the global dipole field; for the longest time scales, we need to include a discussion of stellar evolution. This chapter therefore summarizes the observationally determined properties of the solar magnetic field before discussing the variations in solar spectral radiance and the coupling of the solar coronal field to the heliosphere. These descriptions are based on the historical records of this activity that extend over only a few decades for high-energy particle and electromagnetic radiation, over about a century for its magnetic field, and four centuries for sunspot records.

Over the past few centuries, our awareness of the couplings between the Sun's variability and the Earth's environment, and perhaps even its climate, has been advancing at an ever increasing rate. The Sun is a magnetically variable star and, for planets with intrinsic magnetic fields, planets with atmospheres, or planets like Earth with both, there are profound consequences and impacts. Today, the successful increase in knowledge of the workings of the Sun's magnetic activity, the recognition of the many physical processes that couple the realm of the Sun to our galaxy, and the insights into the interaction of the solar wind and radiation with the Earth's magnetic field, atmosphere, and climate system have tended to differentiate and insularize the solar, heliospheric, and geospace sub-disciplines of the physics of the local cosmos. In 2001, the NASA Living With a Star (LWS) program was initiated to reverse that trend.

The recognition that there are many connections within the Sun–Earth systems approach has led to the development of an integrated strategic mission plan and a comprehensive research program encompassing all branches of solar, heliospheric, and space physics, and aeronomy. In doing so, we have developed an interdisciplinary community to address this program. This has raised awareness and appreciation of the research priorities and challenges among the LWS scientists and has led to observational and modeling capabilities that span traditional discipline boundaries.

Introduction

Our understanding of the prehistory of geospace is largely addressed by a series of “what if” scenarios or thought experiments. Hence, this chapter will appear more qualitative than many of the other chapters. As the thought experiments will primarily be based on knowledge of the geospace presented quantitatively in other chapters, these chapters are referenced as the thought experiments are performed. Having freed ourselves from mathematical rigor, we can step backwards into earlier times. As we shall see, it is quite surprising that mankind's earliest records of the cosmos also detail geospace events. Geospace is a very modern term and represents the assimilation of knowledge gained by modern technologies, radio transmitters and receivers, optical and laser systems, and rocket and satellite instruments, whose measurements were made inside geospace. However, prior to this technological momentum of the last century, mankind was aware of geospace phenomena via observations of aurorae, sunspots, solar flares, magnetometer deflections, induced currents in telegraph lines, etc. (see also Vol. II, Chapter 2). This chapter begins by exploring the extent of this human awareness from a geospace climate perspective.

My own introduction to these phenomena occurred while I was a Scottish schoolboy in the Borders town of Gordon. As a 6-year old at a Christmas party on a cold, dark December afternoon, I heard a rendition of the song “The Northern Lights of Old Aberdeen”, the lyrics of which were written by Mel and Mary Webb in 1952.

Introduction

Solar photons are Earth's primary energy source: Earth is habitable only because the Sun shines, radiating energy throughout the entire heliosphere. The Sun shines because its surface, warmed by energy produced in its nuclear burning core (Bahcall, 2000), is hotter (5770 K) than the surrounding cosmos (4 K). Electromagnetic energy traveling radially outward from the Sun illuminates the heliosphere with a flux of photons that diminishes inversely with the square of increasing distance. Earth, which has only an insignificant internal energy source (see Section 7.4.1), intercepts solar radiant energy, collecting photons emitted from all locations of the solar disk. Unimpeded in their journey to Earth, solar photons take eight minutes to establish the fastest and most direct of all Sun–Earth connections.

The photon energy incident on the Earth at its average distance from the Sun of one astronomical unit (AU), and prior to absorption in the Earth's atmosphere, is called the solar irradiance (e.g. Fröhlich and Lean, 2004). When the energy of the photons is integrated over all wavelengths across the electromagnetic spectrum, the average total solar irradiance is 1361 ± 4W m−2. Solar photons at visible wavelengths have the largest flux (Fig. 10.1) because the emission spectrum of a blackbody near 5770 K peaks in this region. Although really a high temperature plasma, the Sun's “surface” is defined as that layer of the solar atmosphere for which optical depth is unity for photons near 500 nm (cf. Vol. I, chapter 8).

Introduction

The volumes on heliophysics, of which this is the third, emphasize universal processes for which some basic physical phenomenon manifests itself in a variety of circumstances throughout the local cosmos and beyond. The topics range from the variability of the star next to which we live to the distant interstellar medium, via planetary environments including, in particular, geospace in which a magnetic field and atmosphere shield us from most of the dangerous consequences of solar variability – taking us from solar flares, coronal mass ejections, and their associated energetic particles, via the dynamic interplanetary medium, to magnetospheric, ionospheric, and tropospheric consequences. This volume in particular emphasizes interconnectedness, which manifests itself in three different guises that appear, often implicitly, throughout the text.

First, there is the interconnectedness by the universal processes themselves: magnetohydrodynamics, radiative transfer, networks of chemical reactions, magnetic-field dynamics and topology, particle acceleration, shocks, turbulence, etc., pervade all three volumes. Second, we see the interconnectedness in the very evolution of the solar system, from the formation of the central star and its orbiting planets, to the impact of the star on planetary habitability, and the eventual demise of the solar system as we know it (Fig. 1.1). Third, there are many connections between a variety of research disciplines, each of which is advanced by what is learned from other disciplines, thus providing mutual support in their quest for deeper understanding.

Introduction

Parker (1958) showed that the natural state of a hot and extended stellar corona is one of supersonic, super-Alfvénic expansion. In the case of the Sun, the evolution of the strong magnetic field that permeates the corona modulates this expansion (e.g. Pneuman and Kopp, 1971). Indeed, it is the interplay between the coronal magnetic field and the expansion that produces both a highly structured solar corona and a spatially variable solar wind (see Vol. I, Chapter 9). For example, the combed-out appearance of the outer solar corona is a product of the coronal expansion. Because the solar wind plasma is an excellent electrical conductor, the coronal magnetic field is “frozen” into the solar wind flow (Vol. I, Chapter 3) as it expands away from the Sun, forming what is now commonly called the heliospheric magnetic field, HMF. A simple model of the HMF (Parker, 1958) predicts that solar rotation causes the HMF in the solar equatorial plane to be bent into Archimedean spirals (Vol. I, Section 9.2) whose inclinations relative to the radial direction depend on heliocentric distance and the speed of the wind.

The Sun's magnetic field evolves continually, the most pronounced changes being those associated with the advance of the ˜11-year solar activity (sunspot) cycle and the ˜22-year magnetic cycle.

The building blocks: elementary particles

2.1.1 Atoms and quanta

We are all familiar with the concept of atoms. We are made out of them, our surroundings are made out of them, and our Universe is made out of them. The name derives from the Greek, meaning “indivisible”, which conveys the idea that these are the smallest building blocks out of which the Universe is built. In the early 1900s the smallest units were indeed considered to be the atoms, consisting of a central more massive kernel, the nucleus, surrounded by a cloud of orbiting, much smaller and lighter particles called the electrons. The electrons were found to have negative electrical charge, while the much heavier nucleus had an equal amount of positive electrical charge, which was attributed to heavy particles called protons. Later, in the early 1930s, it was found that the nucleus contained other particles as well, slightly heavier than the protons but electrically neutral, which were consequently given the name of neutrons.

For a while these appeared to be all of the basic building blocks of matter. Different atoms, such as hydrogen, helium, carbon, iron, etc., consisted of a nucleus which differed by containing increasing amounts of protons, and except for hydrogen, a comparable or slightly larger number of neutrons, and around the nucleus a number of electrons matching the number of protons, so as to ensure electrical neutrality. This was thought to be what ordinary matter consists of, and in fact this picture continues to be basically correct to this day, except for the fact that it is not the complete picture.

This book provides an overview of topics in high energy, particle and gravitational astrophysics, aimed mainly at interested undergraduates and other readers with only a modest science background. Mathematics and equations have been kept to a minimum, emphasizing instead the main concepts by means of everyday examples where possible. I have tried to cover and discuss in some detail all the major areas in these topics where significant advances are being made or are expected in the near future, with discussions of the main theoretical ideas and descriptions of the principal experimental techniques and their results.

Cosmology, particle physics, high energy astrophysics and gravitational physics have, in the last two decades, become increasingly closely meshed, and it has become clear that thinking and experimenting within the isolated confines of each of these disciplines is no longer possible. The multi-channel approach to investigating nature has long been practiced in high energy accelerators involving the strong, the weak and the electromagnetic interactions, whereas astrophysics has long been possible only using electromagnetic signals. This situation, however, is rapidly changing, with the advent of major cosmic-ray, neutrino and gravitational wave observatories for studying cosmic sources, and the building of particle physics experiments using beams and signals of cosmic origin. At the same time, theoretical physics has increasingly concentrated efforts in attempts to unify gravity with the other three forces into an ultimate theory involving all four. The intense activity in these fields is beginning to open new vistas onto the Universe and our understanding of Nature's working on the very small and very large scales. In this book I have sought to convey not only the facts but also the challenges and the excitement in this quest.

Particles from Heaven

Cosmic rays are energetic particles that reach us from outer space, arriving from all directions. They are generally electrically charged particles, such as protons, heavy nuclei, electrons and positrons, but more broadly one includes among them also electrically neutral particles such as neutrons and neutrinos from outer space. If one subtracts those that arrive from the Sun, the rest arrive essentially isotropically, constituting a uniform background of cosmic-ray radiation, made up of particles with a finite mass. In addition to these, there is also a separate photon background, which includes the cosmic microwave background, the diffuse starlight optical-infrared background, and X-ray and gamma-ray backgrounds, all of which are also essentially isotropic, after subtraction of individual resolved sources.

A major difference between the cosmic-ray background and the photon background is that photons are massless and electrically neutral, so they travel essentially in straight lines from their sources, making it (at least at some wavelengths) easier to identify where they ultimately came from. The vast majority of cosmic rays, however, are electrically charged, and this makes it far harder to discern where they came from. This is because the interstellar and intergalactic space is woven through by random magnetic fields, and the Earth's atmosphere is permeated by an ordered magnetic field, so that as a result of propagating through these magnetic fields the cosmic ray path has little to do with the direction of whatever source they originated from [57].

The dynamics of the Universe

The present-day Universe appears to be expanding in all directions, as shown by the fact that all distant galaxies and clusters of galaxies appear to be receding from us. This was the first and most obvious piece of evidence indicating that our Universe was initially much denser, leading to the hypothesis of an origin in an initial “Big Bang”.

The recession velocities of the galaxies are measured by analyzing the light they emit, which in a spectrograph is seen to contain not only a continuum of frequencies but also discrete frequencies, due to electronic transitions between energy levels of atoms in these galaxies. Such lines have a well-determined laboratory frequency, and when we observe such well-known atomic lines but we see that their frequency is lower (or their wavelength is longer, since wavelength equals speed of light divided by frequency), we infer that the atoms and the galaxy are moving away from us. This effect is called the Doppler shift. A simple everyday acoustic analogy of this Doppler shift is provided by the pitch of an ambulance's siren, which gets lower as the ambulance speeds away from us: the motion away from us “stretches” out the wavelength.

The expansion velocities increase with the distance away from us at a rate which is proportional to the distance, as long as the galaxies are not too far away.

What are gamma-ray bursts?

Gamma-ray bursts are sudden, intense flashes of gamma-rays, detected mainly in the MeV gamma-ray band. When they occur, for a few seconds they completely overwhelm every other gamma-ray source in the sky, including the Sun.

GRBs were first discovered in 1967 by the Vela military satellites, although a public announcement was only made in 1973. These spacecraft carried omni-directional gamma-ray detectors, and were flown by the US Department of Defense to monitor for nuclear explosions which might violate the Nuclear Test Ban Treaty. When these mysterious gamma-ray flashes were first detected, it was determined that they did not come from the Earth's direction, and the first, quickly abandoned suspicion was that they might be the product of an advanced extraterrestrial civilization. However, it was soon realized that this was a new and extremely puzzling cosmic phenomenon. For the next 25 years, only these brief gamma-ray flashes were observed, which vanished quickly and left no traces, or so it seemed. Gamma-rays are notoriously hard to focus, so no sharp gamma-ray “images” exist to this day: the angular “error circle” within which the gamma-ray detectors can localize them is at best several degrees, which contains thousands of possible culprits. This mysterious phenomenon led to huge interest and to numerous conferences and publications, as well as to a proliferation of theories. In one famous review article at the 1975 Texas Symposium on Relativistic Astrophysics, no fewer than 100 different possible theoretical models of GRBs were listed, most of which could not be ruled out by the observations then available.