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

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СКАЧАТЬ in the magnetosphere, such as pitch angle scattering.

      Pedersen currents do not in general produce significant magnetic perturbations on the ground, as these are cancelled by the perturbations produced by the FACs themselves (Fukushima, 1976), except in specific situations in which there are significant horizontal gradients in the conductance (Laundal et al., 2015, 2018). However, the Hall currents in the high conductance auroral zones, known as the eastward and westward “auroral electrojets” at dusk and dawn (Fig. 2.3b), produce northward and southward directed perturbations on the ground, which can be used as a measure of convection strength. These perturbations are measured by the upper and lower auroral electrojet indices (AU and AL) (Davis & Sugiura, 1966), which quantify the maximum northward and southward directed perturbations associated with the eastward and westward electrojets, respectively.

      An additional upward FAC forms in the midnight sector, which tends to bridge the upward FAC of the dusk R1 current with the upward FAC of the dawn R2 current, as shown in the right‐hand panels of Figure 2.8 (Kamide & Vickrey, 1983; Kunkel et al., 1986). This is thought to be due to a pressure buildup in the premidnight sector (Erikson et al., 1991) as hot protons convect earthward and gradient‐curvature drifts toward dawn (section 2.3.2). This produces the “Harang discontinuity” or reversal, a deformation of the dusk convection cell in the nightside convection pattern (e.g., Koskinen & Pulkkinen, 1995).

2.4 TIME‐DEPENDENT CONVECTION

      The discussion so far has concentrated on steady‐state convection when the IMF is directed southward and the dayside and nightside reconnection rates are equal, a phenomenon known as “steady magnetospheric convection” or SMC. However, the IMF orientation and solar wind speed continually change, varying the dayside reconnection rate. The nightside reconnection process is largely decoupled from the dayside and cannot instantaneously adjust itself to changes in the solar wind. This means that magnetospheric dynamics and ionospheric convection are inherently time dependent. Much research in recent years has focused on the time dependence of convection associated with the substorm cycle, or the timescales of response to changes in IMF orientation.

      2.4.1 The Expanding/Contracting Polar Cap Model

      The consequences of nonsteady and unequal dayside and nightside reconnection for magnetospheric and ionospheric convection were first explored by Russell (1972), Siscoe and Huang (1985), and Holzer et al. (1986), and later developed by Freeman and Southwood (1988), Cowley and Lockwood (1992), and Lockwood and Cowley (1992), resulting in what became known as the expanding/contracting polar cap (ECPC) model of the Dungey cycle. If the dayside and nightside reconnection rates are unequal, the proportion of the magnetic flux associated with the Earth's dipole that is open, the open magnetic flux content of the magnetosphere, also known as the “polar cap flux,” F_PC, will change with time. Global auroral imagery, used to estimate the size of the polar cap, shows that typically 0.5 GWb of the 8 GWb associated with the Earth's dipole is open, though this can vary between 0.2 and 1.2 GWb (e.g., Milan et al., 2007; Huang et al., 2009). Assuming a polar magnetic field strength of 50,000 nT, with 0.5 GWb of open flux the polar cap is approximately 1,800 km in radius.

      The reconnection rate at the magnetopause is measured by a voltage, the amount of flux that is opened by reconnection in unit time, or equally the rate at which magnetic flux is transported into a region where its topology changes from closed to open. If the dayside and nightside reconnection rates, Φ_D and Φ_N, are equal, flux is closed as rapidly as it is opened, and F_PC is constant. In such a situation, the rate of open and closing flux is also the rate at which flux is transported through the magnetosphere by the Dungey cycle, so Φ_D = Φ_N = Φ_PC. On the other hand, if the rates are unequal, the rate of change of F_PC is

(2.15)

Consider the situation in which Φ_D > 0, Φ_N = 0. Magnetic flux is opened at the dayside magnetopause and carried into the tail by the flow of the solar wind. The dayside magnetosphere is eroded, and the magnetotail flares outward, as indicated in Figure 2.9a, in which a limited portion of the subsolar magnetosphere has reconnected. The magnetopause is no longer everywhere in stress balance with the flow of the solar wind; there is a mismatch between the angle of attack of the solar wind on the magnetopause and the magnetic pressure within. As a consequence, the internal magnetic flux and plasma are redistributed by the pressure imbalance to re‐attain equilibrium: the eroded magnetopause moves outward, the magnetotail magnetopause moves inward, and there is a general sunward movement of flux in the closed magnetosphere. Once equilibrium is reached, the magnetosphere as a whole is blunter and the magnetotail larger in cross‐section, as flux has been removed from the dayside and added to the tail. Figure 2.9b (i–iv) shows the situation in the ionosphere: equatorward of the original dayside polar cap is a region of newly opened magnetic flux. The flows excited within the magnetosphere are communicated to the ionosphere, and ionospheric convection redistributes the flux to return the polar cap to a circular, but larger configuration, with a general antisunward motion in the polar cap and sunward motion at lower latitudes. A burst of nightside reconnection has a similar effect, closing open flux at the nightside of the polar cap, again leading to antisunward and sunward flow at higher and lower latitudes, respectively (Fig. 2.9c (i–iv)), reducing the width of the tail and returning closed flux to the dayside.

Figure 2.9 (a) Deformation of the magnetopause and resulting plasma flows following a burst of dayside reconnection. (b) (i)–(iv) The flows in the ionosphere excited by a burst of dayside reconnection. (c) Ionospheric flows excited by nightside reconnection. (d) Ionospheric flows excited due to dayside and nightside reconnection, in the limit that the redistribution of flux maintains a circular polar cap at all times

      (from Cowley & Lockwood, 1992).

Figure 2.9d shows the convection pattern expected for dayside and nightside reconnection in the limit that the redistribution of flux in the magnetosphere is sufficiently rapid that the magnetopause and polar cap boundary are kept close to equilibrium at all times. The reconnecting portion of the OCB is shown dotted. Dayside and nightside reconnection frequently occur at the same time, and in this situation the convection is a superposition of these patterns, with the polar cap expanding or contracting at a rate given by equation (2.15). Figure 2.10 presents a sequence of simulated convection patterns in response to dayside and nightside reconnection (Milan, 2013). In panels (a) and (b) Φ_D > 0, Φ_N = 0, in (c) Φ_D < Φ_N, and in (d) Φ_D = СКАЧАТЬ