1TheMagnetosphere‐IonosphereSystemfromthePerspectiveofPlasmaCirculation†W.Lotko Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000 USA Abstract This tutorial review examines the role of O+ in the dynamics of magnetosphere-ionosphere coupling. The life cycle of an O+ plasma element is considered as it circulates from the mid- to high-latitude ionosphere. Energization and diversion of the convecting plasma element into outflows involves Alfvénic turbulence at the low-altitude base of the cusp and plasmasheet boundary layer and in downward-current “pressure cookers.” Observational evidence indicating that O+ dominates the plasmasheet and ring current during extreme storm intervals is reviewed. The impacts of an O+-enriched plasma on solar wind – magnetosphere – ionosphere coupling are considered at both the micro and global scales. A synthesis of results from observation, theory and simulations shows that the presence of O+ in the magnetosphere is both a disruptive and a moderating agent in maintaining the balance between dayside and nightside magnetic merging. 1. Introduction No pathway to convection could have ever led to greater complexity or richer plasma dynamics of the coupled magnetosphere-ionosphere system than the phenomenon of magnetic reconnection. For four decades, this paradigm has generated theories and controversies for where and when reconnection should occur, why the episodic signatures and processes of reconnection in the magnetotail are so different from those at the dayside, and how the ionosphere regulates reconnection and convection. The scale of reconnection and its impacts on magnetospheric-ionospheric plasma circulation have also challenged observations at both extremes – global and micro. This tutorial will examine the interplay between magnetospheric and ionospheric plasma electrodynamics at these physical extremes. To be sure, observational progress has been steady (cf. Johnson, 1983; Shelley and Collin, 1991; Yau and André, 1997; Hultqvist et al., 1999; Huddleston et al., 2005), and the role of theory and computer simulation in advancing our understanding of magnetosphere-ionosphere system dynamics and plasma circulation has been indispensable. We know that the motional electric field of the solar wind imposes a voltage across solar-wind connected, polar field lines (Reiff and Luhmann, 1986), and simple electrical considerations tell us that the conductivity of the high-latitude ionosphere should regulate the dayside region 1 currents generated by this solar wind dynamo action. However, only recently have we recognized that these currents can significantly modify the geometry of the magnetopause, the magnetosheath flow around it and, consequently, the rate at which solar wind magnetic flux is † Submitted to the special issue on “Global Aspects of Magnetosphere-Ionosphere Coupling” to be published in Journal of Atmospheric and Solar-Terrestrial Physics (April, 2006)2delivered to the magnetosphere (Merkine et al., 2003; Siscoe et al., 2004). This electrodynamic feedback limits the dynamo efficiency, particularly for the strongly driven stormtime system, and it explains in part why the magnetosphere-ionosphere system operates at only a fraction of the power available to it from the solar wind dynamo (Fedder and Lyon, 1987). What exactly determines the fractional rate of power transfer from the solar wind to the magnetosphere, for given ambient conditions, is an important global problem in solar wind - magnetosphere coupling that cannot be understood without consideration of the influence of the ionosphere in this coupling. Our understanding of inertial feedback between the magnetosphere and ionosphere is presently in a far murkier state. It is well known that the high-latitude ionosphere is a persistent source of outflowing plasma and essentially the only source of singly ionized oxygen in the magnetosphere (Yau and André, 1987; Chappell et al., 2000). But are there implications of populating the magnetosphere with ionospheric O+ that result in fundamentally different system dynamics? Is magnetospheric structure sensitive to the relative rates of supply and distribution of plasma from the solar wind and the ionosphere, or to the mass composition of the magnetosphere? Observation has revealed that the ionosphere does release enormous fluxes of O+ during active periods, and it is known that ionospheric O+ can dominate the plasma sheet (Peterson et al., 1981) and ring current (Lennartsson and Sharp, 1982) during such periods. The more recent observations to be considered here focus on the relationship between the mass composition of these regions and storm and substorm dynamics (Kistler et al., 2005; Nosé et al., 2005), especially for extreme events. New insights into the electrodynamic coupling between the solar wind, magnetosphere and ionosphere have emerged from investigations of the integrated system during storm intervals (Ober et al., 2003), and we might therefore expect to improve our understanding of inertial coupling and feedback in the solar wind – magnetosphere – ionosphere system by also focusing on such periods. This descriptive overview examines some basic aspects of ionospheric plasma transport and energization, their implications for an upward exodus of ionospheric plasma, and the fate of the outflowing ionospheric plasma in the magnetosphere. The organizing perspective is the life cycle of an ionospheric O+ plasma element as it convects from the midlatitude, dayside ionosphere through the dayside “convection throat”, across the polar cap, into the nightside auroral zone, eventually merging into the sunward return flow. Along the ionospheric convection path, the plasma content of the element is progressively diminished through diversion of the convective transport into upward, field-aligned flows. Upon escaping gravity through processes that remain poorly understood, the liberated and energized O+ ions join the magnetosphere circulation, but their presence seemingly has the capacity to undermine the dayside-nightside balance of magnetic flux transport. The impact of O+ ionospheric outflow on solar wind –magnetosphere – ionosphere system dynamics is considered, although, as suggested above, the discussion here is somewhat tentative. 2. Surges in ionospheric plasma transport Storm-enhanced transport of ionospheric plasma to high-latitudes is a