Abstract
Three states of the magnetosphere may be identified from polar auroral and magnetic activity: the active auroral oval state, the active polar cap state, and the quiet state. The quiet time magnetosphere occurs when the energy transfer from the solar wind has been maintained at a minimum level for several hours. The conditions for minimal energy transfer occur for low solar wind velocity and for small IMF B and Bz. The quiet magnetosphere is characterized by minimal, but nonzero, cross‐cap electric field potentials (6–20 kV) and, therefore, low earthward convection velocities in the plasma sheet. The auroral oval shrinks to a circle of radius 20° geomagnetic latitude (MLAT) offset, as in active times, toward midnight. Diffuse auroral precipitation is continuous throughout these periods but has low average energy and number flux. Discrete arcs are weak (subvisual) and multiple, often extending to extremely high latitudes, particularly on the dayside of the cap. The cusp lies between 77° and 84° MLAT at noon. The total particle energy flux into both hemispheres is ∼ 10 GW. Region 1 and region 2 field‐aligned current systems frequently are not observed during extended periods of magnetic quiet. However, small‐scale current systems, which are associated with the weak discrete arcs, are still seen. Cusp currents in the sunlit hemisphere remain strong. Of particular interest in modeling the quiet magnetosphere are the following topics: the entry and transport of cusp particle populations, stable convection in the plasma sheet, and the closure of tail lobe magnetic field lines.
Published Version
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