Most of the Universe is made of plasma. And yet, plasmas are very rare on the Earth, where solids, liquids and gases – the three primary states of matter – are ubiquitous (Fig. 2.1). These states are the result of a competition between thermal energy and intermolecular forces. In solids, the latter win, maintaining the atoms and/or molecules at nearly fixed positions, whereas thermal energy merely produces vibrations around these positions. In gases on the contrary, thermal energy wins, making the particles almost completely free. Liquids are in between: the intermolecular forces are sufficiently strong to resist compression, but sufficiently weak to enable deformation and flow; it is not surprising that this intermediate state is less well understood than the other two.

Common experience and elementary physics tell us that we may transform a solid into a liquid by heating it; this weakens the bonds between molecules so that they may move slightly, enabling matter to change shape. This requires an amount of energy per molecule somewhat smaller than the binding energy. If the energy furnished exceeds the binding energy, the bonds break out completely, producing a gas of free atoms and/or molecules.

The plasma is the next state: the fourth, reached by furnishing enough energy to break the atoms themselves, or rather to kick off at least the outer atomic electron, producing a mixture of electrons and ions.

Not only does the Sun radiate the light we see – and that we do not see – but it also continually ejects into space 1 million tonnes of hydrogen per second. This wind is minute by astronomical standards; it carries a very small fraction of the solar energy output, and compared to the violent explosions pervading the universe it blows rather gently. Yet it has amazing effects on the solar surroundings. It blows a huge bubble of supersonic plasma – the heliosphere – which engulfs the planets and a host of smaller bodies, shaping their environments. It also conveys perturbations that can be seen in our daily life.

The object of this chapter is twofold. To give a concise historical account of the key ideas and observations that made our modern view of the solar wind emerge; and introduce the main properties of the Sun and of its wind, and their interpretation in terms of basic physics. The latter goal requires some tools of plasma physics, and will be developed in the rest of the book.

A brief history of ideas

The idea that planets are not moving in a vacuum is very old.

The light of the Sun heats the bodies of the Solar System, making them evaporate, and driving their atmospheres, while photons of adequate energy ionise the surfaces and atmospheres. But the wind of the Sun does much more: it carves these environments, producing elaborate structures and powering mighty engines.

The Solar System bodies are extremely diverse, from dust grains to asteroids, comets, planets – and space probes. Even though the interactions are extremely diverse, too, depending on the flow properties, on the nature of the object, and on its size relative to the basic plasma scales, they illustrate similar basic processes. We survey in this chapter the different kinds of bodies and the basic physics of their interaction with the solar wind.

The range of sizes is very large: from small dust grains made of minute assemblages of atoms, to the largest planet, Jupiter. The small dust grains respond to a number of forces: the solar gravitational and radiative forces, electrostatic and Lorentz forces (since they carry an electric charge), and the ram pressure of the solar wind. Because these objects are much smaller than the basic scales of the medium, they do not perturb it significantly, except through the exchange of particles that are absorbed on their surface or emitted.

What is the solar wind to an astronomer? I shall not address this question in detail, but only briefly examine how the solar wind interacts with its cosmic environment, and how it compares with some other cosmic winds. I also address briefly the physics of cosmic rays, their acceleration and their interaction with the solar wind.

The frontier of the heliosphere

In studying the interaction of the solar wind with objects in Chapter 7, we omitted one major obstacle: the interstellar medium.

In the long term, the interstellar medium is the direct partner of the Sun in the galactic system; it served as a source of matter for the formation of the Sun and the Solar System; in turn the Sun returns matter to the interstellar medium via the solar wind at a rate of about 106 tonnes per second, a figure that will change dramatically in the final phase of its life.

In the short term, we saw in Section 5.2 that the interstellar medium plays a major role in the solar mass ejection, by providing the low-pressure exit of the solar wind nozzle. We address in this section this aspect of the interaction, studying the large-scale structure of the cavity – the heliosphere – that the solar wind carves in the interstellar medium.

In order to concentrate on the basic physics of the solar wind acceleration, we considered in Chapter 5 a spherically symmetric and stationary problem, with a radial magnetic field. Unfortunately, the solar wind is more complicated. We now introduce some of these complications, trying however to keep a bias towards basics. More details may be found in the books, with some updates in and. Most of this chapter considers the wind having already been accelerated to a large velocity, which we note vw.

Basic large-scale magnetic field

Parker's spiral

Figure 6.1 (left) reminds us of the geometry considered in the previous chapter: a radially expanding solar wind with a radial magnetic field. This is an application of the frozen-in magnetic field concept (see Section 2.3). Since the magnetic Reynolds number is extremely large, any magnetic flux tube in the steady plasma flow will hold the same fluid parcels later on, so that the magnetic field lines are dragged by the flow and tend to be aligned with the radial flow lines. Conservation of the magnetic flux then yields a radial magnetic field varying as B ∝ r-2.

For science-fiction writers and some space engineers, the ‘wind from the Sun’ is a wind of photons – the light we see, whose pressure might allow solar sailing and drive space windjammers through the solar system. Yet the Sun blows another kind of wind, made of material particles, whose importance is considerable since it bathes the whole Solar System and shapes all planetary environments.

This wind has many faces. To the layman, it sounds rather mysterious, being made of a strange medium, a plasma: the fourth state of matter. Not only do its tempests affect our everyday technology by disrupting communications and power stations, but it drives two bewildering sky displays: comet tails and auroras. To the space scientist, in contrast, the solar wind is a close companion, and the challenge is to explore and tame a jungle where his or her instruments reveal a strange fauna. The plasma physicist is delighted to find there a number of stunning surprises and extreme properties which are virtually impossible to simulate in the laboratory. And to the astronomer who is trying to understand how cosmic bodies – from planets and comets to stars and galaxies – eject particles into space, it is the only stellar wind that can be studied in detail.

The solar wind has been explored in situ by numerous space probes, from inside Mercury's orbit to far beyond the distance of Neptune, and, quite recently, at virtually all heliocentric latitudes.

The Sun and the Earth's magnetosphere (hereafter simply called the magnetosphere) play particular roles in the history of reconnection, frequently also referred to as magnetic merging, as well as in the most recent advances in understanding the spatial structure and the physics of reconnection sites. The concept of reconnection (although not the term) was first suggested by Giovanelli (1946) as a mechanism for particle acceleration in solar flares. This proposed mechanism was extremely modern, as it considered not only electric fields along magnetic neutral lines (as in standard two-dimensional (2D) models; Section 2.1) but also electric fields with a local component along the magnetic field, which is the basic concept of general three-dimensional (3D) magnetic reconnection (Sections 2.2 and 2.3).

Investigations of reconnection in the magnetosphere have stimulated the development of the concept of 3D reconnection in the absence of magnetic nulls (Hesse and Schindler, 1988; Schindler et al., 1988) and considerations of the interaction between a dipole (magnetosphere) and the surrounding (interplanetary) magnetic field have stimulated investigations of the topology of the magnetic field and topological changes associated with reconnection (Dungey, 1961, 1963). They have also motivated the first investigations into the 3D magnetic structure of magnetic null points (Greene, 1988; Lau and Finn, 1990). More recently, the complex structures of the coronal magnetic field, inferred from astonishing X-ray and EUV pictures, have motivated the further detailed exploration of the 3D topology of magnetic fields and its role in reconnection (Chapters 2 and 5).

In this chapter we focus particularly on recent advances in observations and simulations of reconnection at the magnetopause and in the near magnetotail, as these are the sites most heavily investigated by observations and simulations. The scenarios at the two sites have characteristic differences. At the magnetopause, reconnection occurs between two topologically distinct regions, the shocked solar wind and the magnetosphere, which also have quite different plasma properties. Reconnection generates a magnetic field component normal to the magnetopause and thereby leads to an interconnection between the two regions. As discussed in Section 1.2 and further in 4.5, magnetopause reconnection may have quasi-stationary features (as indicated in Fig. 1.6) as well as features that indicate localized, temporally limited reconnection (FTEs; Russell and Elphic, 1978; Elphic, 1995; Fig. 1.9). Critical parameters in reconnection at the magnetopause are the magnitude of the magnetic field component in the direction of the magnetopause current (guide field), the angle between the magnetic fields on either side of the current sheet, and the plasma properties, all of which may play a role in when and where and how reconnection takes place. Major questions of magnetopause reconnection, to be addressed in Sections 4.1 and 4.2, concern the location of reconnection sites and the temporal variability of the process under different solar wind conditions, both of which may be related to the role of a guide field.

This book grew out of a month-long workshop on Magnetic Reconnection Theory held in 2004 at the Isaac Newton Institute, Cambridge, UK, organized by E. R. Priest, T. G. Forbes, and J. Birn. The focus of this workshop was on the most recent advances in understanding reconnection, particularly its three-dimensional aspects and the physics of collisionless reconnection. These are the two areas where the most rapid development beyond the classical theory of reconnection has taken place in recent years. In addition, it was found desirable to include new observational aspects from the two areas that have initiated the concept of reconnection as well as provided new, unprecedented details in remote and in situ observations, the Sun and the Earth's magnetosphere.

This book highlights recent progress and thus it is not a comprehensive overview. Rather it is complementary to recent reviews by Priest and Forbes (2000) and Biskamp (2000), which cover more of the traditional approaches to reconnection. Due to the focus on new results, rather than the classical concepts, about one-third of the citations in this book are from the new millenium, years 2001 to 2005. This makes it plausible that the latest developments have not led to a settled, unified, well-accepted picture, and that some topics are still controversial, even between different authors contributing to this book. We did not try to hide those controversies. Also, we did not try to consolidate various discussions of related topics into single sections or subsections.