A heliospheric bridge within space physics connects the sun to planetary magnetospheres, Earth's in particular---which, with unconscious planetary chauvinism, we call `the' magnetosphere: it is the solar wind. The traffic crossing this solar-wind bridge flows outward. It consists of various kinds of solar-coronal weather, to which the magnetosphere responds like an active weather vane, by which we mean it has some of the rigid coherence of a weather vane, and like the external propellers on fancier ones, it has an internal, magnetic propeller, which---loosely speaking---goes around faster when the wind hits it harder. The magnetosphere adds its own kinds of weather to what it receives from the sun. The kind that gets the most attention is the substorm.
To predict the behavior of such a globally coherent, wind-propelled weather vane which generates internal weather---a compressible, deformable, and articulated weather vane made of magnetic fields, energetic particles, and plasmas---is a challenge, for the system being treated is spatially inhomogeneous to a high degree and intricately shaped. These make the problem inherently three-dimensional, while the weather aspect makes it inherently time-dependent. Further, the physics involved runs from microscale plasma kinetics to macroscale magnetohydrodynamics (MHD), all of which must be integrated into a single, self-consistent formalism.
The role that microscale plasma kinetics can play in structuring macroscale phenomena is well illustrated by the formation upwind of the magnetosphere of a high Mach number bow shock created entirely by non-collisional processes. In `How the bow shock does it,' Nick Omidi describes the inventive strategies that particles use to dissipate massive kinetic energy of flow without hitting anything.
The bow shock example reinforces the point that in the global context of the whole magnetosphere, the only hope for comprehending within one descriptive framework all aspects---great dynamic range of most parameters, inherent (read ineluctable) three dimensionality, inherent time dependence, and inherent cross-scale coupling---is to develop computer codes with the scope and power to incorporate all of them. As Ray Walker and Maha Ashour-Abdalla report in `Large scale theoretical models of the magnetosphere,' despite the difficulty of the problem, work on MHD codes, a complementary Large Scale Kinetic code, and others is well underway, and concrete results have begun to emerge. Furthermore, Surjalal Sharma's `Assessing the magnetosphere's nonlinear behavior' reviews work suggesting that in some respects the magnetosphere is quite predictable, at least during times dominated by internal weather, that is, substorms, when the magnetosphere viewed as a natural nonlinear dynamical system appears to have a low dimension. One facet regarding global magnetospheric behavior has reached a level of at least empirical understanding where it is now being incorporated into operational space weather forecasting. This is predicting the size and shape of the boundary of the magnetosphere, the `magnetopause,' as it changes in response to changes in solar wind conditions. Dave Sibeck's `Mapping the magnetopause' recounts the maturing of this subject made possible by the accumulation over several decades of adequate data bases.
The solar wind affects the internal state of the magnetosphere primarily through the action of the magnetic field it brings with it from the sun. The solar wind's magnetic field brushes against the magnetosphere's magnetic field all along the magnetopause. Here, over a finite volume, the fields can merge together, giving the solar wind access to the magnetosphere. One result is to direct part of the solar wind's momentum and energy onto the magnetic `blades'---to return to our analogy---of the internal rotor, setting it rotating in a circulating motion called magnetospheric convection. Magnetospheric convection is the main mode of energy transfer in global magnetospheric dynamics. Understanding the processes by which magnetic fields merge at the magnetopause is, therefore, a topic of central importance to magnetospheric physics. Unfortunately it is also one of the deepest, and accordingly, it is the object of intense theoretical studies. In `Magnetic reconnection at the magnetopause: A fundamental process and manifold properties,' Antonius Otto analyzes these studies, which, he concludes, have continued over the past four years to make steady and marked progress, yet definitive, that is, prediction-enabling, understanding still eludes us. To underscore the difficulty of the problem, Pat Newell's `Do the dayside cusps blink?' addresses an aspect of the time dependence that characterizes the merging process as inferred from spacecraft measurements taken near the magnetopause. At issue is an apparent conflict between the time dependence of field line merging seen at the magnetopause and the time independence of the effects of merging seen at the ionospheric footprint of the field lines that merge at the magnetopause. Newell surveys recent work to arrive at a reconciliation.
From the dayside magnetopause, which takes the brunt of the solar wind's weather and transmits it muted to the magnetosphere, the reports move nightward to the magnetotail where substorms are made---the site of the magnetosphere's own weather factory. The magnetotail's long, cylindrical shape is what makes our earlier weather vane analogy work: the tail swings to keep the magnetosphere pointing into the wind, the direction from which the solar wind blows. On its inside, the magnetotail owes its distinctive geometrical and dynamical properties to the equatorial current sheet, which bisects the magnetotail cylinder from east to west and divides it into a northern half where magnetic field lines point toward the earth and a southern half where they point the other way. As the sine qua non of the magnetotail and substorms, the equatorial current sheet is the subject of many studies to learn how the separate trajectories of individual particles organize themselves to create the largest scale coherent structure in the magnetosphere. The answer, as Jim Chen explains in his `Particle dynamics, chaos, and order in the magnetotail,' apparently involves the transfer of information across scales of organization. The constants of the motions of the particles generating the current sheet are fixed long before they enter the current sheet. Macroscale organization of energy flow is the topic of Michael Hesse's `The magnetotail's role in magnetospheric dynamics: Engine or exhaust pipe.' The question at issue separates schools of thought regarding the basic physical nature of the substorm. One school holds that the magnetotail merely vents some of the energy dissipated in substorms but is otherwise a minor player; another holds that the magnetotail is the main supplier of substorm energy. Pursuing the question lets Hesse survey the current literature and leads to the finding that the evidence favors the position that the magnetotail supplies the substorm with much of its energy. Coming to the topic of the substorm itself, we note that a recent count puts the number of distinct, currently competing substorm models at nine. Nonetheless, they have features in common, and in general each among them is better than the others in addressing some particular aspect of substorm phenomena. There is hope, therefore, that by starting from a base of the models' common features and selecting from their strengths, one might construct a viable syncretic model, superior to its parts. Such a project forms the background for Gary Erickson's report `Substorm theories: United they stand, divided they fall.'
In our progress inward from the tail, we regrettably pass over the ring current, the great energy reservoir that forms during magnetic storms, though many fine papers have appeared on this subject in the past four years. I hope they will be included in the next review, which will come after a current resurgence of interest in magnetic storms and the ring current will have left a large corpus of works to be organized and analyzed. Here, however, is an appropriate place to position Mark Engebretson's report `Catching the wave: ULF research in the U.S. since 1991.' Ultra low frequency (ULF) waves are ubiquitous; they fill the inner and outer magnetosphere and the magnetosheath. They have been called magnetospheric analogues of seismic waves, which enable remote sensing of volumes from point measurements. They are rich sources of information, as their properties---frequencies, wave forms, polarizations, amplitudes---encode the conditions that made them and the parameters of the medium where they were born and through which they propagate. Engebretson's review is correspondingly comprehensive.
With Jim Horwitz's report we encounter the ionosphere, though as his title, `The ionosphere's wild ride in outer space,' tells us, it has come to us, not we to it. The ionosphere constantly escapes into space and does so faster than first predicted. The fact has far-reaching consequences for ionospheric and magnetospheric dynamics, and as Horwitz notes, it relates to the charging of satellites in geosynchronous orbit. He recounts how, driven by the complexity of the escape processes as revealed through increasingly better data, theoretical sophistication has evolved until now it can account for the known phenomena. He also reviews some of the literature on the ring current from the viewpoints of its interaction with the extended ionosphere and of its having been revealed to be a reservoir of energized ionospheric ions. Ed Shelley's `The auroral acceleration region: The world of beams, conics, cavitons and other plasma exotica' covers a related topic. `Exotic' describes the auroral acceleration region itself: an otherwise nondescript region lying somewhere short of midway along magnetic field lines running from the auroral ionosphere to the equatorial outer magnetosphere, a region where, as reviewed here, electric structures self organize and accelerate electrons down into the ionosphere to stimulate auroral light while accelerating ions beams up into the magnetosphere. The emphasis, however, is on what happens beneath the acceleration region where, besides auroras, the accelerated electrons create a small zoo of plasma exotica. Shelley's discussion demonstrates that to enable an understanding of these complex, nonlinear phenomena, state-of-the-art diagnostic measurements from rockets and satellites are essential.