Space Physics and Aeronomy, Ionosphere Dynamics and Applications. Группа авторов

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Figure 2.5 Plasma flow in the equatorial plane of the magnetosphere. (a) The Dungey cycle contribution to cold plasma flow, associated with a dawn‐to‐dusk convection electric field E₀. Flow streamlines are also contours of the convection electrostatic potential Φ, where Φ₁ < Φ₂ < Φ₃, etc. (b) The corotation contribution to cold plasma flow. (c) The resultant flow, giving rise to Dungey cycle flow in the outer magnetosphere and corotation in the inner magnetosphere, where the plasmasphere forms (shaded). A flow stagnation point exists along the dusk meridian. (d) Convection and gradient‐curvature drift of hot electrons. (e) Convection and gradient‐curvature drift of hot protons. (f) The effect of displacing the plasma sheet (shaded) sunward by convection: where gradient‐curvature drift paths (dotted circles) intersect the inner edge of the plasma sheet, divergence of the partial ring current leads to the formation of field‐aligned currents that form the region 2 FAC system.

      This picture is appropriate for cold plasma, that is the bulk of the plasma sheet particles with gyroradii that are small with respect to the radial magnetic field gradient in the inner dipole. Hot plasma convects earthward from the magnetotail as described above, but experiences gradient‐curvature drift in the inner magnetosphere, with ions and electrons encircling the Earth to the west and east, respectively (Fig. 2.5d and e). This differential ion and electron flow constitutes a westward “ring current.” In addition, divergence of magnetization current in pressure gradients at the inner edge of the earthward‐convecting plasma sheet forms a “partial ring current,” with associated currents flowing along magnetic field lines between the equatorial plane and the polar ionosphere, as shown in Figure 2.5f (Cowley, 2000; Ganushkina et al., 2015). The ramifications of this are discussed in section 2.3.3.

Within the polar cap, the flow is antisunward, associated with flux sinking through the lobes toward the neutral sheet as new open flux is created at the magnetopause and flux is removed from the central plane of the tail as it is reclosed. An additional force on the polar cap field lines needs to be considered when the east‐west or B_Y component of the IMF is nonzero. If we assume that B_Y > 0 in Figure 2.2a, then the northern and southern ends of the newly reconnected field lines are tilted into and out of the page. This exerts a magnetic tension force on the footprints of the field‐lines causing westward and eastward flow in the dayside northern and southern polar caps, respectively (e.g., Heelis, 1984; Reiff & Burch, 1985; Cowley et al., 1991), with the situation reversed for B_Y < 0. The flows crossing the dayside polar cap boundary are directed westward in Figure 2.3b, appropriate for B_Y > 0 in the Northern Hemisphere. Under strong IMF B_Y conditions, the torque exerted by the magnetic tension on the northern and southern lobes can lead to a significant twist on the tail and an induced B_Y component in the lobe field lines that can then introduce east‐west asymmetries into the convection flow on the nightside.

      Combined, the antisunward and sunward flows associated with the Dungey cycle and the east‐west sense of flows in the dayside polar cap produced by tension forces are clearly seen in the empirical convection patterns presented in Figure 2.1. The cross‐polar cap potential, Φ_PC, is largest for southward IMF, when the dayside reconnection rate is largest (e.g., Reiff et al., 1981; Milan et al., 2012, and references therein), that is when the magnetic shear at the subsolar magnetopause is greatest. There is evidence that Φ_PC saturates near 250 kV when driving of the magnetosphere is particularly strong (e.g., Siscoe et al., 2002, 2004; Hairston et al., 2003, 2005); although several models have been proposed to explain this saturation, it has not yet been possible to clearly discriminate between them (e.g., Shepherd, 2006; Borovsky et al., 2009).

      2.3.3 Magnetosphere/Ionosphere Current Systems

Electric currents are an integral consequence of the electrodynamic coupling between the solar wind and the magnetosphere, and the magnetosphere and the ionosphere. In the outer magnetosphere, the field lines deviate from a dipolar configuration due to the confinement of the Earth's field by the solar wind (e.g., Fig. 2.2a), and Ampère's law, equation (2.7), indicates that electric currents must flow; indeed, these currents are associated with the j × B forces (magnetic pressure and tension) discussed earlier. Figure 2.6b shows the Chapman‐Ferraro current on the magnetopause (Chapman & Ferraro, 1931), which provides magnetic pressure to balance the solar wind ram pressure. The magnetic reversal in the magnetotail neutral sheet is also associated with a dusk‐to‐dawn cross‐tail current, which forms current loops with the magnetopause currents.