Introduction

The Sun is an average main-sequence star in middle age and of spectral class G2 v. The Sun has mass M⊙ = 2 × 1030 kg, radius R⊙ = 700 Mm, and effective temperature Teff = 5780 K. The spectral class G2 v implies that the energy generated by hydrogen fusing into helium in the solar core is carried by convection in the outer one-third of the Sun's envelope. Lighter main-sequence stars, of spectral classes K and M, have convection zones that extend steadily more deeply as the stars become less massive. Conversely, main-sequence stars heavier than the Sun, of spectral classes F and A, are convective only in a thin layer near their surface. The heaviest main-sequence stars, of spectral classes B and O, have convective cores but no convection near the surface. It is the convective motions in a stratified environment, coupled with differential rotation, that are ultimately responsible for solar and stellar dynamos and thus solar magnetic fields and solar and stellar activity. As the Sun is a middle-aged star, the solar wind has had time to carry away a large fraction of the initial angular momentum of the proto-solar cloud (see Vol. III). At present the Sun's rotation period ranges from 35 days at the poles to 25 days at the equator, but it was presumably much shorter early in the Sun's life, 4.5 Gyr ago.

Introduction

The comparative study of the magnetospheres associated with various planets (and with some other objects within the solar system) aims to develop a unified general description of magnetospheric phenomenology and physics that is applicable to a variety of different systems. In this way concepts and theories, often developed in the first instance to fit specific phenomena in a particular magnetosphere, can be tested for correctness and applicability in a more general context.

The subject matter of magnetospheric physics, in general terms, deals with the configuration and dynamics of systems that result from the magnetic field interaction between isolated objects and their environments. Such systems exhibit a variety of fascinating physical phenomena, both visible (e.g. auroral light emissions) and invisible (e.g. charged-particle radiation belts), some of which have significant aspects of practical application (e.g. space weather effects at Earth, radiation dosage problems for spacecraft near Jupiter or Saturn). They also present a special challenge for physical understanding: as regions of transition between an object and its environment, magnetospheres are by their very nature spatially inhomogeneous systems, characterized by the inclusion of very different physical regimes, large ranges of parameter values, and the overwhelmingly important role of gradients.

The aim of this chapter is to present magnetospheric physics, at least in outline, as a self-contained discipline – a branch of physics described in logical sequence, as distinct from a study of individual objects (often presented in historical sequence).

Over the past few centuries, our awareness of the coupling 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 geo-space 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 LWS scientists and has led to observational and modeling capabilities that span traditional discipline boundaries. The successful initial integration of the LWS sub-disciplines, under the newly coined term “heliophysics”, needed to be expanded into the early education of scientists.

Introduction

In this chapter we review some basic physical processes in the upper atmospheres of planets, with a particular focus on Earth with its strong internal magnetic field. Less attention will be given to the upper atmospheres of planets with little or no internal magnetic field. The same basic physical processes operate in all planetary atmospheres. The main distinctions are in the species present (see the review by Bougher et al., 2002), the rotation rate and gravitational force, and whether the planet has a significant intrinsic magnetic field. The vertical extent, absorption of radiation, interaction with the solar wind, and dynamical balances will clearly differ. Planets with a small magnetic field will also have quite different electrodynamic properties.

Earth's upper atmosphere can be categorized as a gravitationally bound, partially ionized, fluid. The fluid properties of the gas result from the frequent collisions between the atoms and molecules of the medium. Rather than having to accommodate the random nature of the forces exerted on individual gas particles the principles of kinetic theory can be invoked, so that the medium can be described by the bulk properties of the fluid such as pressure, density, temperature, and velocity. The vertical extent of the sensible atmosphere is usually defined by the altitude at which the fluid approximation is no longer valid, referred to as the exobase. Below the exobase, i.e. the topmost extent of an atmosphere, the distance and time between collisions is short compared with the scale sizes of interest in the dynamics and energetics of the fluid.

The solar wind is responsible for maintaining the heliosphere and is the driving agent in the magnetospheres of the planets; furthermore, it provides the mechanism by which the Sun has shed angular momentum during the aeons since its formation. We can assume that other cool stars with active coronae have similar winds, which play the same roles for those stars (Wood et al., 2005). The decade since the launch of SOHO has seen considerable changes in our understanding of thermally driven winds such as the solar wind, owing to theoretical, computational, and observational advances. Recent solar wind models are characterized by low coronal electron temperatures while proton, α-particle, and minor-ion temperatures are expected to be quite high and perhaps anisotropic. This entails an assumption that the electric field is relatively unimportant and that one is able to obtain in a quite natural way a solar wind outflow that has a high asymptotic flow speed while maintaining a low mass flux. In this chapter we will explain why these changes have come about and outline the questions now facing thermal wind astrophysicists.

The progress we have seen in the last decade is largely due to observations made with instruments on board Ulysses (McComas et al., 1995) and SOHO (Fleck et al., 1995). These observations have spawned a new understanding of solar wind energetics and the consideration of the chromosphere, corona, and solar wind as a unified system.

Preamble

In this chapter, we examine the concepts of hydrodynamic turbulence as they apply to space plasmas. We focus primarily on the solar wind at 1AU in order to illustrate the basic ideas of magnetohydrodynamic fluid turbulence in a unified and coherent fashion, but we extend these lessons to other space plasma systems. Of particular interest is the energy cascade that leads to heating of the background plasma. There is growing evidence that a turbulent dynamic does account for the in situ heating of the solar wind from 0.3 to 100AU. Turbulence may also provide an explanation for the coronal heating that accelerates the solar wind at the source. The discussion offered here is an attempt to explain how the fundamental ideas of turbulence apply to plasma dynamics. These thoughts are only a beginning, an introduction to the subject, but will lead the reader to the most recent and sometimes controversial ideas being discussed by the space physics and astrophysics communities.

Turbulence, by its rightful definition, is a branch of fluid dynamics. The term “plasma turbulence” has been used to denote the nonlinear evolution of dynamically coupled wave and particle populations, but this is something different. However, this raises an important point: the magnetohydrodynamic (MHD) systems of space physics are not true fluids but are plasmas that mimic magneto-fluid behavior at low frequencies. They retain their ability to exhibit purely plasma kinetic behavior such as the resonance of plasma waves with charged particle populations.

Introduction – magnetic fields in the universe

Objects in the universe from the scale of a planet to the size of a galaxy show evidence of large-scale magnetic fields. Despite the fact that the physical conditions in such objects are quite different, the creation and destruction of magnetic fields is closely linked to turbulent motions of a highly conducting fluid within these bodies. Dynamo theory focuses on the characterization of conditions under which a flow of highly conducting fluid can sustain a magnetic field against resistive decay. This chapter is an introduction to dynamo theory with a primary focus on general concepts rather than detailed applications; the latter are discussed in Vol. III of this series. We start this introduction with a brief overview of the properties of objects with large-scale magnetic fields in the universe.

The Earth and other planets

The magnetic field of the Earth has a strength of about 0.5 gauss and a mainly dipolar character. Currently the dipole axis is tilted by about 11° with respect to the axis of rotation. From studies of rock magnetism (when rocks cool below the Curie point they preserve the magnetic field that was present in them at that time) it is known that the Earth has had a magnetic field over the past 3.5 × 109 years and that the strength and orientation of the field has varied significantly on time scales of 103 to 104 years.

A voyage through the local cosmos

The place that we call home, the surface of the planet Earth, presents us with an environment in which temperatures range over perhaps 80 kelvins from the cool arctic regions or mountain tops to the hottest deserts or jungles. We are composed largely of liquid water with a density of 1 gram per cubic centimeter; we walk on solid rock with a density that is about five times higher than this and breathe a gas with a density that is 1000 times lower. These conditions are such that chemical reactions and phase transitions between solids, liquids, and gases are the processes that dominate our everyday experience.

When we move away from the Earth's surface, conditions change markedly. Deep in the Earth, for example, where densities are still only a few times higher than those at the surface, the pressure rapidly increases and temperatures reach up to some 20 times those characteristic of the range that is comfortable to mammals. In the Sun's core densities are larger still, almost a hundred times that of liquid water, at temperatures that exceed ten million kelvins. Those same temperatures may be found again in the hottest, flaring parts of the Sun's outermost atmosphere, called the corona, and furthermore are often characteristic of the ion energies high above the Earth around the altitudes where geosynchronous satellites orbit.

Introduction

In Chapter 10 the basic principles that govern magnetospheric structure and dynamics in general were explained. There it was made clear that the magnetosphere is a time-dependent, three-dimensional, multi-component, interacting system. The point of the present chapter is that to obtain a quantitative description of so complex a system, one must resort to numerical models. Moreover, to check the self-consistency of the assumptions behind a conceptual model, one must rely on global numerical simulations. In this chapter we use numerical simulation models that are generally available to illustrate their use in giving global-scale depictions of the structure and dynamics of the terrestrial magnetosphere and we compare the results with our understanding of these subjects developed by other means.

We consider the flow of mass, momentum, energy, and magnetic flux through and around the magnetosphere. These topics involve the macro-scale coupling between the solar wind and the magnetosphere and so are well suited for treatment by global MHD simulations. We start by explaining where numerical models are positioned in the magnetospheric scientist's armory and by briefly describing the illustrative models that we will use.

It comes as no surprise that of the known magnetospheres Earth's is the best observed and understood. What we know about its structure, dynamics, and interaction with the solar wind has been gleaned from decades of ground- and space-based observations, each of which gives us a snapshot of one or more physical processes in the magnetosphere.

Introduction

The nature of the interaction between a planetary object and the surrounding plasma depends on the properties of both the object and the plasma flow in which it is embedded. A planet with a significant internal magnetic field forms a magnetosphere that extends the planet's influence beyond its surface or cloud tops. A planetary object without a significant internal dynamo can interact with any surrounding plasma via currents induced in an electrically conducting ionosphere.

All the solar system planets are embedded in the wind that streams radially away from the Sun. The flow speed of the solar wind exceeds the speed of the fastest wave mode that can propagate in the interplanetary plasma. The interaction of the supersonic solar wind with a planetary magnetic field (either generated by an internal dynamo or induced externally) produces a bow shock upstream of the planet. Objects such as the Earth's Moon that have no appreciable atmosphere and a low-conductivity surface have minimal electrodynamic interaction with the surrounding plasma and just absorb the impinging solar wind with no upstream shock. Interactions between planetary satellites and magnetospheric plasmas are as varied as the moons themselves: Ganymede's significant dynamo produces a mini-magnetosphere within the giant magnetosphere of Jupiter; the electrodynamic interactions of magnetospheric plasma flowing past the atmospheres of volcanically active Io (Jupiter) and Enceladus (Saturn) generate substantial currents and supply more plasma to the system; moons without significant atmospheres (e.g. Callisto at Jupiter) absorb the impinging plasma.