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

Автор: Группа авторов

Издательство: John Wiley & Sons Limited

Жанр: Физика

Серия:

isbn: 9781119815532

isbn:

СКАЧАТЬ 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 region, the R2 FAC connects to the partial ring current associated with the sunward convection of hot plasma sheet plasma described in section 2.3.2.

Schematic illustrations of (a) a cut through the magnetosphere in the X = 0 plane, showing the sense of plasma flow (black arrows into and out of the page) in the open, closed, and plasmasphere regions, along with the sense of current flows (green) at the magnetopause, ionosphere (polar cap and return flow regions), R1 and R2 FACs, and partial ring current. (b) The deformation of open (blue) and closed (red) field lines by flows in the solar wind and inner magnetosphere and the frictional forces of the ionosphere, and the sense of field-aligned current produced by the magnetic shear. Schematic illustrations of (Left) Field-aligned current pattern associated with vorticity in the convection flow, with black as downward current and grey as upward current. The two concentric rings of FAC are region 1 or R1 (high latitude) and region 2 or R2 (low latitude).

      (from Iijima & Potemra, 1976).

      (Right) The Harang discontinuity as seen in FACs and convection

      (adapted from Iijima & Potemra, 1976; Koskinen & Pulkkinen, 1995).

      in which the three terms on the RHS are the divergence of JP due to shears in the convection flow, the divergence of JP due to gradients in the Pedersen conductance, and the divergence of JH due to gradients in Hall conductance. In the limit of uniform conductance, this reduces to Poisson's equation. The distribution of FACs can then be compared with or inferred from the vorticity in the convective flow (e.g., Sofko et al., 1995; McWilliams et al., 1997; Green et al., 2006; Chisham et al., 2009). In the auroral zone, with a Pedersen conductance of 10 S, a flow speed of 500 m s−1 produces an ionospheric current across the flow of 0.25 A m−1. If the convection reversal associated with the low‐latitude duskside flow shear is 2,000 km in length, then 0.5 MA of FAC flows in the R2 current there. The R1 current poleward of this will be of a similar magnitude, possibly enhanced somewhat by a contribution from Pedersen current flowing across the polar cap, as shown in Figure 2.3b (though see Laundal et al., 2018, for a discussion of horizontal current closure). By this estimate, it is expected that when convection is ongoing, approximately 2 MA flows into and out of the ionosphere in each hemisphere in the whole R1/R2 system, increasing during periods of strong driving, in agreement with the observations of Iijima and Potemra (1978).