Magnetosphere–Ionosphere Convection as a Compound System

Convection is the most fundamental process in understanding the structure of geospace and disturbances observed in the magnetosphere–ionosphere (M–I) system. In this paper, a self-consistent configuration of the global convection system is considered under the real topology as a compound system. Investigations are made based on the M–I coupling scheme by analyzing numerical results obtained from magnetohydrodynamic (MHD) simulations which guarantee the self-consistency in the whole system under the Bv (magnetic field and velocity) paradigm. It is emphasized in the M–I coupling scheme that convection and field-aligned current (FAC) are different aspects of same physical process characterizing the open magnetosphere. Special attention is given in this paper to the energy supplying (dynamo) process that drives the FAC system. In the convection system, the dynamo must be constructed from shear motion together with plasma population regimes to steadily drive the convection. Convection patterns observed in the ionosphere are also the manifestation of achievement in global self-consistency. A primary approach to apply these concepts to the study of geospace is to consider how the M–I system adjusts the relative motion between the compressible magnetosphere and the incompressible ionosphere when responding to given solar-wind conditions. The above principle is also applicable for the study of disturbance phenomena such as the substorm as well as for the study of apparently unique processes such as the flux transfer event (FTE), the sudden commencement (SC), and the theta aurora. Finally, an attempt is made to understand the substorm as the extension of enhanced convection under the southward interplanetary magnetic field (IMF) condition.     

MESSENGER: Exploring Mercury’s Magnetosphere

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ∼1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ∼1–2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3–0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind–Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multi-polar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’s highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.     

Theory and Simulation of Reconnection

Reconnection is a major commonality of solar and magnetospheric physics. It was conjectured by Giovanelli in 1946 to explain particle acceleration in solar flares near magnetic neutral points. Since than it has been broadly applied in space physics including magnetospheric physics. In a special way this is due to Harry Petschek, who in 1994 published his ground breaking solution for a 2D magnetized plasma flow in regions containing singularities of vanishing magnetic field. Petschek’s reconnection theory was questioned in endless disputes and arguments, but his work stimulated the further investigation of this phenomenon like no other. However, there are questions left open. We consider two of them – “anomalous” resistivity in collisionless space plasma and the nature of reconnection in three dimensions. The CLUSTER and SOHO missions address these two aspects of reconnection in a complementary way -- the resistivity problem in situ in the magnetosphere and the 3D aspect by remote sensing of the Sun. We demonstrate that the search for answers to both questions leads beyond the applicability of analytical theories and that appropriate numerical approaches are necessary to investigate the essentially nonlinear and nonlocal processes involved. Necessary are both micro-physical, kinetic Vlasov-equation based methods of investigation as well as large scale (MHD) simulations to obtain the geometry and topology of the acting fields and flows.     

Magnetospheric and Plasma Science with Cassini-Huygens

Magnetospheric and plasma science studies at Saturn offer a unique opportunity to explore in-depth two types of magnetospheres. These are an ‘induced’ magnetosphere generated by the interaction of Titan with the surrounding plasma flow and Saturn's ‘intrinsic’ magnetosphere, the magnetic cavity Saturn's planetary magnetic field creates inside the solar wind flow. These two objects will be explored using the most advanced and diverse package of instruments for the analysis of plasmas, energetic particles and fields ever flown to a planet. These instruments will make it possible to address and solve a series of key scientific questions concerning the interaction of these two magnetospheres with their environment. The flow of magnetospheric plasma around the obstacle, caused by Titan's atmosphere/ionosphere, produces an elongated cavity and wake, which we call an ‘induced magnetosphere’. The Mach number characteristics of this interaction make it unique in the solar system. We first describe Titan's ionosphere, which is the obstacle to the external plasma flow. We then study Titan's induced magnetosphere, its structure, dynamics and variability, and discuss the possible existence of a small intrinsic magnetic field of Titan. Saturn's magnetosphere, which is dynamically and chemically coupled to all other components of Saturn's environment in addition to Titan, is then described. We start with a summary of the morphology of magnetospheric plasma and fields. Then we discuss what we know of the magnetospheric interactions in each region. Beginning with the innermost regions and moving outwards, we first describe the region of the main rings and their connection to the low-latitude ionosphere. Next the icy satellites, which develop specific magnetospheric interactions, are imbedded in a relatively dense neutral gas cloud which also overlaps the spatial extent of the diffuse E ring. This region constitutes a very interesting case of direct and mutual coupling between dust, neutral gas and plasma populations. Beyond about twelve Saturn radii is the outer magnetosphere, where the dynamics is dominated by its coupling with the solar wind and a large hydrogen torus. It is a region of intense coupling between the magnetosphere and Saturn's upper atmosphere, and the source of Saturn's auroral emissions, including the kilometric radiation. For each of these regions we identify the key scientific questions and propose an investigation strategy to address them. Finally, we show how the unique characteristics of the CASSINI spacecraft, instruments and mission profile make it possible to address, and hopefully solve, many of these questions. While the CASSINI orbital tour gives access to most, if not all, of the regions that need to be explored, the unique capabilities of the MAPS instrument suite make it possible to define an efficient strategy in which in situ measurements and remote sensing observations complement each other. Saturn's magnetosphere will be extensively studied from the microphysical to the global scale over the four years of the mission. All phases present in this unique environment — extended solid surfaces, dust and gas clouds, plasma and energetic particles — are coupled in an intricate way, very much as they are in planetary formation environments. This is one of the most interesting aspects of Magnetospheric and Plasma Science studies at Saturn. It provides us with a unique opportunity to conduct an in situ investigation of a dynamical system that is in some ways analogous to the dusty plasma environments in which planetary systems form. This revised version was published online in August 2006 with corrections to the Cover Date.     

Giant Planet Ionospheres and Thermospheres: The Importance of Ion-Neutral Coupling

Planetary upper atmospheres-coexisting thermospheres and ionospheres-form an important boundary between the planet itself and interplanetary space. The solar wind and radiation from the Sun may react with the upper atmosphere directly, as in the case of Venus. If the planet has a magnetic field, however, such interactions are mediated by the magnetosphere, as in the case of the Earth. All of the Solar System’s giant planets have magnetic fields of various strengths, and interactions with their space environments are thus mediated by their respective magnetospheres. This article concentrates on the consequences of magnetosphere-atmosphere interactions for the physical conditions of the thermosphere and ionosphere. In particular, we wish to highlight important new considerations concerning the energy balance in the upper atmosphere of Jupiter and Saturn, and the role that coupling between the ionosphere and thermosphere may play in establishing and regulating energy flows and temperatures there. This article also compares the auroral activity of Earth, Jupiter, Saturn and Uranus. The Earth’s behaviour is controlled, externally, by the solar wind. But Jupiter’s is determined by the co-rotation or otherwise of the equatorial plasmasheet, which is internal to the planet’s magnetosphere. Despite being rapid rotators, like Jupiter, Saturn and Uranus appear to have auroral emissions that are mainly under solar (wind) control. For Jupiter and Saturn, it is shown that Joule heating and “frictional” effects, due to ion-neutral coupling can produce large amounts of energy that may account for their high exospheric temperatures.     

Modelling of the electromagnetic field in the interplanetary space and in the Earth's magnetosphere

A magnetohydrodynamic model of the solar wind flow is constructed using a kinematic approach. It is shown that a phenomenological conductivity of the solar wind plasma plays a key role in the forming of the interplanetary magnetic field (IMF) component normal to the ecliptic plane. This component is mostly important for the magnetospheric dynamics which is controlled by the solar wind electric field. A simple analytical solution for the problem of the solar wind flow past the magnetosphere is presented. In this approach the magnetopause and the Earth's bow shock are approximated by the paraboloids of revolution. Superposition of the effects of the bulk solar wind plasma motion and the magnetic field diffusion results in an incomplete screening of the IMF by the magnetopause. It is shown that the normal to the magnetopause component of the solar wind magnetic field and the tangential component of the electric field penetrated into the magnetosphere are determined by the quarter square of the magnetic Reynolds number. In final, a dynamic model of the magnetospheric magnetic field is constructed. This model can describe the magnetosphere in the course of the severe magnetic storm. The conditions under which the magnetospheric magnetic flux structure is unstable and can drive the magnetospheric substorm are discussed. The model calculations are compared with the observational data for September 24–26, 1998 magnetic storm (Dst _min=−205 nT) and substorm occurred at 02:30 UT on January 10, 1997. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mercury’s Atmosphere: A Surface-Bounded Exosphere

The existence of a surface-bounded exosphere about Mercury was discovered through the Mariner 10 airglow and occultation experiments. Most of what is currently known or understood about this very tenuous atmosphere, however, comes from ground-based telescopic observations. It is likely that only a subset of the exospheric constituents have been identified, but their variable abundance with location, time, and space weather events demonstrate that Mercury’s exosphere is part of a complex system involving the planet’s surface, magnetosphere, and the surrounding space environment (the solar wind and interplanetary magnetic field). This paper reviews the current hypotheses and supporting observations concerning the processes that form and support the exosphere. The outstanding questions and issues regarding Mercury’s exosphere stem from our current lack of knowledge concerning the surface composition, the magnetic field behavior within the local space environment, and the character of the local space environment.     

Laboratory and Numerical Simulations of the Impulsive Penetration Mechanism

Plasma interaction at the interface between the magnetosheath and magnetosphere has been extensively studied during recent years. As a consequence various theoretical models have emerged. The impulsive penetration mechanism initially proposed by Lemaire and Roth as an alternative approach to the steady state reconnection, is a non-stationary model describing the processes which take place when a 3-D solar wind plasma irregularity interacts with the outer regions of the Earth's magnetosphere. In this paper we are reviewing the main features of the impulsive penetration mechanism and the role of the electric field in driving impulsive events. An alternative point of view and the controversy it has raised are discussed. We also review the numerical codes developed to simulate the impulsive transport of plasma across the magnetopause. They have illustrated the relationship between the magnetic field distribution and the convection of solar-wind plasma inside the magnetosphere and brought into perspective non-stationary phenomena (like instabilities and waves) which were not explicitly integrated in the early models of impulsive penetration. Numerical simulations devoted to these processes cover a broad range of approximations, from ideal MHD to hybrid and kinetic codes. The results show the limitation of these theories in describing the full range of phenomena observed at the magnetopause and magnetospheric boundary layers.     

Solar System Magnetospheres

This article proposes a short review of our present knowledge of solar system magnetospheres, with the purpose of placing the study of Saturn’s magnetosphere in the context of a comparative approach. We describe the diversity of solar system magnetospheres and the underlying causes of this diversity: nature and magnetization state of the planetary obstacle, presence or not of a dense atmosphere, rotation state of the planet, existence of a system of satellites, rings and neutral gas populations in orbit around the planet. We follow the “russian doll” hierarchy of solar system magnetospheres to briefly describe the different objects of this family: the heliosphere, which is the Sun’s magnetosphere; the “elementary” magnetospheres of the inner planets, Earth and Mercury; the “complex” magnetospheres of the giant planets, dominated by planetary rotation and the presence of interacting objects within their magnetospheric cavities, some of which, like Ganymede, Io or Titan, produce small intrinsic or induced magnetospheres inside the large one.We finally describe the main original features of Saturn’s magnetosphere as we see them after the Voyager fly-bys and before the arrival of Cassini at Saturn, and list some of the key questions which Cassini will have to address during its four-year orbital tour.     

A Class of TVD Type Combined Numerical Scheme for MHD Equations With a Survey About Numerical Methods in Solar Wind Simulations

It has been believed that three-dimensional, numerical, magnetohydrodynamic (MHD) modelling must play a crucial role in a seamless forecasting system. This system refers to space weather originating on the sun; propagation of disturbances through the solar wind and interplanetary magnetic field (IMF), and thence, transmission into the magnetosphere, ionosphere, and thermosphere. This role comes as no surprise to numerical modelers that participate in the numerical modelling of atmospheric environments as well as the meteorological conditions at Earth. Space scientists have paid great attention to operational numerical space weather prediction models. To this purpose practical progress has been made in the past years. Here first is reviewed the progress of the numerical methods in solar wind modelling. Then, based on our discussion, a new numerical scheme of total variation diminishing (TVD) type for magnetohydrodynamic equations in spherical coordinates is proposed by taking into account convergence, stability and resolution. This new MHD model is established by solving the fluid equations of MHD system with a modified Lax-Friedrichs scheme and the magnetic induction equations with MacCormack II scheme for the purpose of developing a combined scheme of quick convergence as well as of TVD property. To verify the validation of the scheme, the propagation of one-dimensional MHD fast and slow shock problem is discussed with the numerical results conforming to the existing results obtained by the piece-wise parabolic method (PPM). Finally, some conclusions are made. This revised version was published online in August 2006 with corrections to the Cover Date.     

太阳能无人机能源系统的多维耦合建模

针对太阳能无人机能源系统建模的耦合性考虑不足、能量流动过程描述不全面等问题,将外部大气环境、无人机的姿态角度等不同维度下影响能源系统运行的多个因素纳入到建模过程中,先后给出了非均质大气环境模型、无人机运动模型、电机模型、减速器模型、螺旋桨模型、光伏电池模型和蓄电池模型的建立过程,并建立了各模型间的耦合关系,得到反映非均质大气环境下、动态飞行行为的太阳能无人机中能量流动的耦合模型。仿真结果表明：该模型可以完整的描述外部大气环境和无人机飞行姿态对无人机发电功率以及动力负荷功率的影响等耦合作用,适合用于仿真分析和地面半实物仿真平台的搭建。     

Space weather: its effects and predictability

Terrestrial technology is now, and increasingly, sensitive to space weather. Most space weather is caused by solar storms and the resulting changes to the Earth's radiation environment and the magnetosphere. The Sun as the driver of space weather is under intense observation but remains to be adequately modelled. Recent spacecraft measurements are greatly improving models of solar activity, the interaction of the solar wind with the magnetosphere, and models of the radiation belts. In-situ data updates the basic magnetospheric model to provide specific details of high-energy electron flux at satellite orbits. Shock wave effects at the magnetopause can also be coarsely predicted. However, the specific geomagnetic effects at ground level depend on the calculation of magnetic and electric fields and further improvements are needed. New work on physical models is showing promise of raising geomagnetic and ionospheric predictability above the synoptic climatological level. This revised version was published online in August 2006 with corrections to the Cover Date.     

Primary sources of large-scale Birkeland currents

We review generation mechanisms of Birkeland currents (field-aligned currents) in the magnetosphere and the ionosphere. Comparing Birkeland currents predicted theoretically with those studied observationally by spacecraft experiments, we present a model for driving mechanism, which is unified by the solar wind-magnetosphere interaction that allows the coexistence of steady viscous interaction and unsteady magnetic reconnection. The model predicts the following: (1) the Region 1 Birkeland currents (which are located at poleward part of the auroral Birkeland-current belt, and constitute quasi-permanently and stably a primary part of the overall system of Birkeland currents) would be fed by vorticity-induced space charges at the core of two-cell magnetospheric convection arisen as a result of viscous interaction between the solar wind and the magnetospheric plasma, (2) the Region 2 Birkeland currents (which are located at equatorward part of the auroral Birkeland-current belt, and exhibit more variable and localized behavior) would orginate from regions of plasma pressure inhomogeneities in the magnetosphere caused by the coupling between two-cell magnetospheric convection and the hot ring current, where the gradient-B current and/or the curvature current (presumably the hot plasma sheet-ring current) are forced to divert to the ionosphere, (3) the Cusp Birkeland currents (which are located poleward of and adjacent to the Region 1 currents and are strongly controlled by the interplanetary magnetic field (IMF)) might be a diversion of the inertia current which is newly and locally produced in the velocity-decelerated region of earthward solar wind where the magnetosphere is eroded by dayside magnetic reconnection, (4) the nightside Birkeland currents which are connected to a part of the westward auroral electrojet in the Harang discontinuity sector might be a diversion of the dusk-to-dawn tail current resulting from localized magnetic reconnection in the magnetotail plasma sheet where plasma density and pressure are reduced.     

Review of Pi2 Models

More than half a century after the discovery of Pi2 pulsations, Pi2 research is still vigorous and evolving. Especially in the last decade, new results have provided supporting evidence for some Pi2 models, challenged earlier interpretations, and led to entirely new models. We have gone beyond the inner magnetosphere and have explored the outer magnetosphere, where Pi2 pulsations have been observed in unexpected places. The new Pi2 models cover virtually all magnetotail regions and their coupling, from the reconnection site via the lobes and plasma sheet to the ionosphere. In addition to understanding the Pi2 phenomenon in itself, it has also been important to study Pi2 pulsations in their role as transient manifestations of the coupling between the magnetosphere and the ionosphere. The transient Pi2 is an integral part of the substorm phenomenon, especially during substorm onset. Key questions about the workings of magnetospheric substorms are still awaiting answers, and research on Pi2 pulsations can help with those answers. Furthermore, the role of Pi2 pulsations in association with other dynamic magnetospheric modes has been explored in the last decade. Thus, the application of Pi2 research has expanded over the years, assuring that Pi2 research will remain active in this decade and beyond. Here we review recent advances, which have given us a new understanding of Pi2 pulsations generated at various places in the magnetosphere during different magnetospheric modes. We review seven Pi2 models found in the literature and show how they are supported by observations from spacecraft and ground observatories as well as numerical simulations. The models have different degrees of maturity; while some enjoy wide acceptance, others are still speculative.     

重力姿轨耦合效应引起的太阳能电站轨道共振

以任意相控阵天线式空间太阳能电站为研究对象,主要研究了其在轨运行过程中受到的重力姿轨耦合效应对其轨道运动的影响。首先,通过Hamilton原理建立起考虑重力姿轨耦合效应时的姿态运动和轨道运动的方程。其中,任意相控阵天线式空间太阳能电站被简化成刚体,它的重力势能以其结构尺寸和其轨道半径的比值为小量进行泰勒展开,并保留至二阶项。之后,采用解析的方法对方程进行分析,并发现当电站的姿态运动满足一定条件时,其轨道运动将会出现共振现象。此外,重力姿轨耦合效应还会引起空间太阳能电站轨道运动长期的漂移,通过选择合适的轨道运动初始条件可以消除漂移;而且,在一定条件下,重力姿轨耦合效应还会引起轨道运动的发散。最后,数值仿真结果验证了分析的正确性。     

Microphysics of Cosmic Plasmas: Background, Motivation and Objectives

With the maturing of space plasma research in the solar system, a more general approach to plasma physics in general, applied to cosmic plasmas, has become appropriate. There are both similarities and important differences in describing the phenomenology of space plasmas on scales from the Earth’s magnetosphere to galactic and inter-galactic scales. However, there are important aspects in common, related to the microphysics of plasma processes. This introduction to a coordinated collection of papers that address the several aspects of the microphysics of cosmic plasmas that have unifying themes sets out the scope and ambition of the broad sweep of topics covered in the volume, together with an enumeration of the detailed objectives of the coverage.     

Processes that Promote and Deplete the Exosphere of Mercury

It has been speculated that the composition of the exosphere is related to the composition of Mercury’s crustal materials. If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been believed that H and He come primarily from the solar wind (Goldstein, B.E., et al. in J. Geophys. Res. 86:5485–5499, 1981), Na and K come from volatilized materials partitioned between Mercury’s crust and meteoritic impactors (Hunten, D.M., et al. in Mercury, pp. 562–612, 1988; Morgan, T.H., et al. in Icarus 74:156–170, 1988; Killen, R.M., et al. in Icarus 171:1–19, 2004b). The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization, photon-stimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal and spatial dependence. The exosphere is strongly influenced by Mercury’s highly elliptical orbit and rapid orbital speed. As a consequence the surface undergoes large fluctuations in temperature and experiences differences of insolation with longitude. Because there is no inclination of the orbital axis, there are regions at extreme northern and southern latitudes that are never exposed to direct sunlight. These cold regions may serve as traps for exospheric constituents or for material that is brought in by exogenic sources such as comets, interplanetary dust, or solar wind, etc. The source rates are dependent not only on temperature and composition of the surface, but also on such factors as porosity, mineralogy, and space weathering. They are not independent of each other. For instance, ion impact may create crystal defects which enhance diffusion of atoms through the grain, and in turn enhance the efficiency of PSD. The impact flux and the size distribution of impactors affects regolith turnover rates (gardening) and the depth dependence of vaporization rates. Gardening serves both as a sink for material and as a source for fresh material. This is extremely important in bounding the rates of the other processes. Space weathering effects, such as the creation of needle-like structures in the regolith, will limit the ejection of atoms by such processes as PSD and ion-sputtering. Therefore, the use of laboratory rates in estimates of exospheric source rates can be helpful but also are often inaccurate if not modified appropriately. Porosity effects may reduce yields by a factor of three (Cassidy, T.A., and Johnson, R.E. in Icarus 176:499–507, 2005). The loss of all atomic species from Mercury’s exosphere other than H and He must be by non-thermal escape. The relative rates of photo-ionization, loss of photo-ions to the solar wind, entrainment of ions in the magnetosphere and direct impact of photo-ions to the surface are an area of active research. These source and loss processes will be discussed in this chapter.     

26-day Analysis of Energetic Ion Observations at High and Low Heliolatitudes: Ulysses and ACE

We present observations of energetic (0.34–8 MeV) ions from the Ulysses spacecraft during its second ascent to southern high latitude regions of the heliosphere. We cover the period from January 1999 until mid-2000 as Ulysses moved from 5.2 AU and 18° S to 3.5 AU and 55° S. In contrast to the long-lived and well-defined ∼26-day recurrences that were observed throughout Ulysses‘ first southern pass, energetic ion fluxes during the first portion of the Ulysses’ second polar orbit are highly irregular. Although corotating interaction regions (CIRs) are clearly present in solar wind and magnetic field data throughout the first half of 1999, their effects on energetic ion intensities are quite different from what they were in 1992–1993. No dominant strictly recurrent ion flux increases are observed in association with the arrival of these CIRs. Correspondingly, there is no stable structure of large polar coronal holes during the same period. Isolated transient solar energetic particle (SEP) events are observed at low and high latitudes. We compare energetic ion observations from the ACE and Ulysses spacecraft during the first half of 1999 to determine the influence of these SEP events in the observed recurrent CIR structure. Such SEP events occurred only occasionally during 1992–1993, but when they occurred, they obscured the recurrences in a manner similar to that observed in 1999–2000. We therefore conclude that the basic differences in the behavior of energetic ion events between the first and second southern passes are due to the short life of the corotating structure and the higher frequency of SEP events occurring in 1999–2000. This revised version was published online in August 2006 with corrections to the Cover Date.