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 BY component of the IMF is nonzero. If we assume that BY > 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 BY < 0. The flows crossing the dayside polar cap boundary are directed westward in Figure 2.3b, appropriate for BY > 0 in the Northern Hemisphere. Under strong IMF BY 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 BY 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, ΦPC, 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 ΦPC 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.
Figure 2.6 (a) A 3‐D representation of the Earth's magnetic field (southern lobe field lines suppressed for clarity); (b) current systems formed by the deformation of the magnetic field by the flow of the solar wind; (c) current systems formed by convection within the magnetosphere (after Milan et al., 2017; Licensed under CCBY).
Currents also form due to the deformation of the field by the Dungey cycle flow in the magnetosphere and its coupling to the ionosphere (Fig. 2.6c). As described above, ion‐neutral collisions produce a drag on the ionospheric flow. Figure 2.3a sketches the pattern of electric field E, and Figure 2.3b shows the Pedersen and Hall currents that flow in the ionospheric E region in E and −E × B directions as green and orange arrows, respectively. In the polar cap region, where the flow is antisunward, a dawn‐to‐dusk ionospheric Pedersen current is generated. At the magnetopause, a current flows from dusk‐to‐dawn (in the same sense as the Chapman‐Ferraro current), associated with tension forces between the IMF and the internal field (at the kinks in the newly reconnected field lines in Fig. 2.2a). The combined action of these currents and the magnetic perturbations they produce is to cause a bend‐back of the open field lines above the ionosphere, as shown in Figure 2.7b. The kink in the field at the ionosphere is associated with a j × B tension force that acts to balance the frictional drag; the kink at the magnetopause is associated with a j × B tension force that acts as a drag on the flow of the solar wind. These two currents hence transfer momentum from the solar wind to the ionosphere. A similar process takes place in the closed flux, return flow region: sunward‐moving field lines are bent sunward by the drag of the ionosphere; here, the bend‐back is in the opposite sense to that in the polar cap and the ionospheric Pedersen current is similarly reversed to be directed dusk‐to‐dawn (Fig. 2.3b). In other words, at shears or vorticity in the ionospheric convective flow, there is also a magnetic shear above, between the opposed directions of field bend‐back; again, Ampère's law, equation (2.7), requires that currents flow along the magnetic field lines threading the shear (Fig. 2.7a and b), fed by the divergence of horizontal currents in the ionosphere. Magnetic perturbations above the auroral zones were first discovered experimentally by Zmuda et al. (1966, 1967), and Cummings and Dessler (1967) identified these as being produced by the field‐aligned currents first predicted by Birkeland (1908). The overall morphology of these FACs was subsequently described more fully by Iijima and Potemra (1976a, b, 1978) (see left panel of Fig. 2.8). They form two concentric rings of FAC, termed region 1 (R1), which maps to the magnetopause, and region 2 (R2), which maps to the inner magnetosphere. FACs also flow in the dayside polar cap associated with east‐west flow asymmetries produced by tension forces when IMF BY ≠ 0. These FACs are shown by circled dots and crosses in Figure 2.3b. On open field lines, the R1 FAC closes across the magnetopause as described above. At the equatorward edge of the return flow