Abstract. This paper attempts to summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms. (1) Four mechanisms are identified and discussed as the primary causes of enhanced electric fields in the interplanetary medium responsible for geomagnetic storms. It is pointed out that in reality, these four mechanisms, which are not mutually exclusive, but interdependent, interact differently from event to event. Interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagnetic storms. The other two mechanisms, i.e., HILDCAA (high-intensity, long-duration, continuous auroral electrojet activity) and the so-called RussellMcPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in creating magnetic storms has yet to be quantitatively understood.

Abstract. Thin current sheets represent important and puzzling sites of magnetic energy storage and subsequent fast release. Such structures are observed in planetary magnetospheres, solar atmosphere and are expected to be widespread in nature. The thin current sheet structure resembles a collapsing MHD solution with a plane singularity. Being potential sites of effective energy accumulation, these structures have received a good deal of attention during the last decade, especially after the launch of the multiprobe CLUS-TER mission which is capable of resolving their 3D features. Many theoretical models of thin current sheet dynamics, including the well-known current sheet bifurcation, have been developed recently. A self-consistent 1D analytical model of thin current sheets in which the tension of the magnetic field lines is balanced by the ion inertia rather than by the plasma pressure gradients was developed earlier. The influence of the anisotropic electron population and of the corresponding electrostatic field that acts to restore quasi-neutrality of the plasma is taken into account. It is assumed that the electron motion is fluid-like in the direction perpendicular to the magnetic field and fast enough to support quasi-equilibrium Boltzmann distribution along the field lines. Electrostatic effects lead to an interesting feature of the current density profile inside the current sheet, i.e. a narrow sharp peak of electron current in the very center of the sheet due to fast curvature drift of the particles in this region. The corresponding magnetic field profile becomes much steeper near the neutral plane although the total cross-tail current is in all cases dominated by the ion contribution. The dependence of electrostatic effects on the ion to electron temperature ratio, the curvature of the magnetic field lines, and the average electron magnetic moment is also analyzed. The implications of these effects on the fine structure of thin current sheets and their potential impact on substorm dynamics are presented.

Abstract. A self-consistent theory of thin current sheets, where the magnetic field line tension is balanced by the ion inertia rather than by the pressure gradient, is presented. Assuming that ions are the main current carriers and their dynamics is quasi-adiabatic, the Maxwell-Vlasov equations are reduced to the nonlocal analogue of the Grad-Shafranov equation using a new set of integrals of motion, namely, the particle energy •

Abstract. Thin anisotropic current sheets (CSs) are phenomena of the general occurrence in the magnetospheric tail. We develop an analytical theory of the self-consistent thin CSs. General solitions of the Grad-Shafranov equation are obtained in a quasi-adiabatic approximation which neglects the jumps of the sheet adiabatic invariant Iz This is possible if the anisotropy of the initial distribution function is not too strong. The resulting structure of the thin CSs is interpreted as a sum of negative dia- and positive paramagnetic currents flowing near the neutral plane. In the immediate vicinity of the magnetic field reversal region the paramagnetic current arising from the meandering motion of the ions on Speiser orbits dominates. The maximum CS thick-ness is achieved in the case of weak plasma anisotropy and is of the order of the thermal ion gyroradius outside the sheet. A unified picture of thin CS scalings includes both the quasi-adiabatic regimes of weak and strong anisotropies and the nonadiabatic limit of super-strong anisotropy of the source ion distribution. The later limit corresponds to the case of almost field-aligned initial distribution, when the ratio of the drift velocity outside the CS to the thermal ion velocity exceeds the ratio of the magnetic field outside the CS to its value in-side the CS (vD/vT> B0/Bn). In this regime the jumps of Iz, become essential, and the current sheet thickness is approaching to some small but finite value, which depends upon the parameter Bn /B0. Convective electric field increases the effective anisotropy of the source distribution and might produce the essential CS thinning which could have important implications for the sub-storm dynamics.

The magnetospheric response to the solar wind input, as represented by the time series measurements of the AE index, has been examined using phase space reconstruction techniques. The system was found to behave as a low‐dimensional chaotic system with a fractal dimension of 3.6 and has a Kolmogorov entropy <0.2/min. These are indicative that the dynamics of the system can be adequately described by four independent variables and a corresponding intrinsic time scale is of the order of 5 min. The relevance of the results to magnetospheric modelling is discussed.

Abstract. Many phenomena in the Earth's magnetotail have characteristic temporal scales of several minutes and spatial scales of a few Earth radii (R E ). Examples of such transient and localized mesoscale phenomena are bursty bulk flows, beamlets, energy dispersed ion beams, flux ropes, traveling compression regions, night-side flux transfer events, and rapid flappings of the current sheet. Although most of these observations are linked to specific interpretations or theoretical models they are inter-related and can be the different aspects of a physical process or origin. Recognizing the inter-connected nature of the different transient and localized phenomena in the magnetotail, this paper reviews their observations by highlighting their important characteristics, with emphasis on the new results from Cluster multipoint observations. The multi-point Cluster measurements have provided, for the first time, the ability to distinguish between temporal and spatial variations, and to resolve spatial structures. Some examples of the new results are: flux ropes with widths of 0.3 R E , transient field aligned currents associated with bursty bulk flows and connected to the Hall current at the magnetic reconnection, flappings of the magnetotail current sheet with time scales of 100 s-10 min and thickness of few thousand km, and particle energization including velocity and time dispersed ion structures with the Correspondence to: A. S. Sharma (ssh@astro.umd.edu) latter having durations of 1-3 min. The current theories of these transient and localized processes are based largely on magnetic reconnection, although the important role of the interchange and other plasma modes are now well recognized. On the kinetic scale, the energization of particles takes place near the magnetic X-point by non-adiabatic processes and wave-particle interactions. The theory, modeling and simulations of the plasma and field signatures are reviewed and the links among the different observational concepts and the theoretical frameworks are discussed. The mesoscale processes in the magnetotail and the strong coupling among them are crucial in developing a comprehensive understanding of the multiscale phenomena of the magnetosphere.

The strong correlation between magnetic storms and southward interplanetary magnetic field (IMF) is well known from linear prediction filter studies using the Dst and IMF data. However, the linear filters change significantly from one storm to another and thus are limited in their predicting ability. Previous studies have indicated nonlinearity in the magnetospheric response as the ring current decay rate varies with the Dst value during storms. We present in this letter nonlinear models for the evolution of the Dst based on the OMNI database for 1964–1990. When solar wind data are available in advance, the evolution of storms can be predicted from the Dst and IMF data. Solar wind data, however, are not available most of the time or are available typically an hour or less in advance. Therefore, we have developed nonlinear predictive models based on the Dst data alone. In the absence of solar wind data, these models cannot predict the storm onset, but can predict the storm evolution, and may identify intense storms from moderate ones. The input‐output model based on IMF and Dst data, the autonomous model based on Dst alone, and a combination of the two can be used as forecasting tools for space weather.

[1] This paper addresses the question of particular characteristics and causes of intense space storms. We focus on several unresolved issues, which are critical to storm research and often nourish open disputes: the extent of interplanetary driving, the role of substorms in storm dynamics through the acceleration of particles to ring current energies, the identity of the lead agent of fast ring current decay right after storm maximum and the cause of the two-phase recovery of intense storms, the global morphology of the stormtime ring current, and the predictability of intense storms. Space storm physics has been driven by several more or less successful paradigms during the four decades following the dawn of the space flight era. However, recent suggestions and conclusions resulting from a number of observational and modeling studies have brought significant constraints to several of these paradigms. Thus for example, interplanetary driving through southward oriented magnetic fields is not always by itself sufficient to drive intense space storms because it is conditioned by internal magnetospheric conditions, the MLT distribution of storm-time magnetic disturbances is often asymmetric during the storm main phase, and charge exchange is not the lead agent of ring current decay at all times.