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

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


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al. (1988), Todd et al. (1988), Lockwood et al. (1993), and Khan and Cowley (1999) did suggest a propagation of the convection response to an IMF change away from its initial observation in the dayside ionosphere. Ridley et al. (1997, 1998), Ruohoniemi and Greenwald (1998), and Ruohoniemi et al. (2002), on the other hand, argued on the basis of globally derived patterns that an instantaneous response of the ionospheric convection is evident near‐simultaneously over the whole polar cap. Lockwood and Cowley (1999) countered that a near‐instantaneous response does not preclude an expansion of the pattern. Indeed, a two‐stage ionospheric response, with fast global onset communicated in the form of a fast magnetosonic wave and slower reconfiguration that spreads from day to night over ~25 minutes, was suggested based on observations by Lu et al. (2002). A number of efforts to model a convection response that can account for both types of observation have been described (e.g., Freeman, 2003; Lockwood & Morley, 2004; Morley & Lockwood, 2005). One particular factor that has been considered in controlling the variable timing of the reconfiguration is preconditioning of the magnetosphere by prior activity (e.g., Watanabe et al., 2000; Morley & Lockwood, 2006). This idea has further implications when considering the combined effect on the convection evolution of dayside and nightside reconnection (as discussed, for example, in section 2.4.3).

      2.4.2 The Substorm Cycle and the ECPC

      Expansions and contractions of the polar cap are necessarily associated with shears in ionospheric convection, and the generation of FACs and auroras; the locations of these FACs and hence the auroral zone are associated with the size of the convection pattern, in turn related to the size of the polar cap. As described by Lockwood and Cowley (1992), the time‐dependent convection just described can be put in the context of the auroral changes observed in the magnetospheric substorm cycle (Akasofu, 1964; Atkinson, 1967a, 1967b; Russell & McPherron, 1973; Hones, 1979; Rostoker et al., 1980).

      The substorm growth phase (McPherron, 1970) occurs during periods of southward IMF, when dayside reconnection causes the polar cap to expand and is observed as poleward flows into the dayside polar cap and an equatorward progression of the auroral zone. The strengthening of the convection during the growth phase leads to enhanced magnetic perturbations produced by the eastward and westward electrojets, measured by the AU and AL indices. Dayside reconnection can occur in bursts on timescales of a few minutes, even when IMF BZ is steady, leading to the formation of “flux transfer events” (FTEs). Each FTE is associated with a small addition of new open flux to the dayside polar cap (Fig. 2.9b), leading to a sequence of poleward flow bursts in the dayside convection throat, which may also have a significant east‐west component dependent on the polarity of IMF BY, and auroral counterparts known as poleward‐moving auroral forms (PMAFs) (e.g., Lockwood et al., 1989; Milan et al., 2000a, 2016; Wild et al, 2001; Fear et al., 2017).

      As the growth phase progresses, the magnetotail inflates and pressure in the lobes and plasma sheet intensifies. This eventually leads to the onset of nightside reconnection, though the exact conditions under which this occurs are still poorly understood, and could involve plasma instabilities in the inner magnetosphere. The expansion phase is associated with the formation of a poleward‐expanding “substorm auroral bulge” in the nightside auroral zone (McPherron et al., 1973), which we identify with the protrusion of the nightside OCB into the polar cap in Figure 2.9c (ii). The enhanced conductance and convection strength also enhances the electrojets. The substorm recovery phase marks the return of the system to quieter conditions after the IMF has turned northward.

Schematic illustrations of schematic of the formation of the substorm auroral bulge. (a) A near-Earth X-line (dotted line) forms on closed field lines, mapping to near the equatorward portion of the nightside auroral zone. The black circle represents the open/closed field line boundary. (b) Reconnection of closed magnetic flux proceeds, with bright auroras (dark grey) where flux has reconnected. (c) A portion of the X-line eats through the closed field line region and proceeds to reconnect open field lines.
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