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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 B_Z 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 B_Y, 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.

An additional feature is associated with the expansion phase. If reconnection only occurs across a limited portion of the magnetotail, then sunward‐moving, newly closed field lines will acquire a more dipolar configuration (known as a “dipolarization”) than the stretched field lines to either side as they contract under the action of tension forces. The magnetic shear at these boundaries is associated with upward and downward FACs premidnight and postmidnight, and a westward electrojet in the ionosphere across the highly conducting auroral bulge, known as the “substorm current wedge” (Atkinson, 1967a, 1967b; McPherron et al., 1973), indicated in Figure 2.3b and Figure 2.6b. This current is fed by the cross‐tail current in the nondipolarized regions adjacent to the wedge. The nightside “substorm electrojet” produces a distinct ground magnetic perturbation, known as a “substorm bay,” which is also seen as a negative excursion in AL.

When nightside reconnection first starts, the reconnection site forms within the closed field line region, known as a “near‐Earth X‐line” or “neutral line” (NEXL or NENL) (Hones, 1979), shown approximately by the green X in Figure 2.2. This X‐line maps magnetically to a location near the equatorward edge of the auroral zone (Fig. 2.11a). As reconnection proceeds, closed field lines earthward of the X‐line contract earthward under the action of tension forces, causing the dipolarization and formation of the substorm current wedge, whereas disconnected flux accumulates on the tailward side. The auroral bulge begins to develop as the X‐line location in the ionosphere proceeds poleward (Fig. 2.11b). The disconnected flux is released downtail as a “plasmoid” once the reconnection eats entirely through the closed field line region and open flux begins to be closed (Fig. 2.11c). It is only at this point that Φ_N becomes nonzero: Φ_N is the rate of closure of open flux.

Figure 2.11 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 (dashed line), such that Φ_N > 0.

Observations of the expanding/contracting polar cap, for instance using the convection reversal boundary (Taylor et al., 1996), global auroral imagery (e.g., Milan et al., 2003, 2007; Milan, 2004; Huang et al., 2009), or measurements of FAC distribution (Clausen et al., 2012), can be used to diagnose the dynamics of the magnetosphere, and to infer dayside and nightside reconnection rates and the convection strength. Figure 2.12 presents three periods during which the poleward edge of the auroral oval was used as a proxy for the OCB and changes in open flux were quantified over several hours. The polar cap tended to expand during periods of southward IMF and contract following substorms. For instance, substorm 8 in Figure 2.12 shows an increase in F_PC following a southward turning of the IMF at 10 UT, the growth phase, and a decrease in F_PC following substorm expansion phase onset at 10:45 UT, accompanied by enhanced auroral brightness. The accompanying electrojet indices show clear growth phase and expansion phase signatures: first symmetric positive and negative excursions of AU and AL as convection is driven by dayside reconnection, followed by a sharp excursion in AL associated with the formation of the current wedge. Variations in F_PC allow estimation of Φ_D and Φ_N using equation (2.15), from which Φ_PC can be inferred using equation (2.16). Such observations show that the substorm expansion phase typically lasts 1 hour, with Φ_N ≈ 75 kV, closing approximately 0.3 GWb of flux, equivalent to approximately half the preexisting polar cap flux (Milan et al., 2007). There is ambiguity in such measurements as changes in F_PC are related to the difference between Φ_D and Φ_N, not their absolute values. In this case, estimating Φ_D from solar wind observations (e.g., Milan et al., 2012) removes this ambiguity. If convection measurements are available as well as observations of the polar cap boundary, then Φ_D and Φ_N can be measured independently from the rate of plasma flow across the dayside and nightside OCB (e.g., Baker et al., 1997; Grocott et al., 2002; Hubert et al., 2006, 2017; Chisham et al., 2008). Alternatively, observations of F_PC, Φ_D, and Φ_N can be used to drive a model of convection to compare with measured flow velocities (e.g., Walach et al., 2017).

Figure 2.12 Three examples of observations of the expanding/contracting polar cap from global auroral imaging. Panels from top: Polar cap flux, F_PC; maximum nightside auroral intensity; auroral electrojet indices, AU and AL; IMF B_Z; inferred dayside and nightside reconnection rates, Φ_D (black lines) and Φ_N (grey rectangles); inferred cross‐polar cap potential, Φ_PC

(from Milan et al., 2007; Reproduced with permission of John Wiley and Sons).

The detailed development of the convection pattern following substorm onset has been shown to be dependent on the latitude of substorm onset, that is the amount of open flux that has accumulated in the magnetosphere prior to onset occurring. High‐latitude and low‐latitude onsets are associated with weak and intense auroral responses (Kamide et al., 1999; Milan et al., 2009), which in turn influence the conductance of the auroral bulge. Grocott et al. (2009) demonstrated that the convection speed in the nightside onset region increases at the time of high‐latitude onsets, as is expected due to the contribution to the cross‐polar cap potential by nightside reconnection. However, they also showed that for low‐latitude substorms, the nightside convection counterintuitively slows at onset, which is interpreted as frictional coupling between the ionosphere and atmosphere owing to the high conductance in the auroral bulge (e.g., Morelli et al., 1995); convection can only redistribute flux to return the polar cap to a circular configuration once the conductance decays. In this scenario, we expect that the protrusion of the auroral bulge into the polar cap shown in Figure 2.11 is appropriate for low‐latitude onsets. For high‐latitude onsets, Milan et al. (2018b) have suggested that once reconnection of open flux begins, convection maintains a circular polar cap.

If the IMF remains southward for a prolonged period, the magnetosphere sometimes undergoes steady magnetospheric convection (e.g., Sergeev, 1977; Sergeev et al., 1996; McWilliams et al., 2008). During such events, Φ_N ≈ Φ_D so these are also known as “balanced reconnection intervals” (DeJong et al., 2008), and the polar cap remains of uniform size. Kissinger et al. (2012) and Walach and Milan (2015) showed that many such events begin as a substorm, but segue into SMC if the IMF does not shortly thereafter turn northward. Milan et al. (2018b) have suggested that during prolonged B_Z < 0, SMC can be achieved if the initiating substorm is a high‐latitude onset and convection is unimpeded, but that a sequence of substorms is initiated if the onsets are low latitude and conductance arrests the flow such that a laminar convection state cannot be established.

Statistical studies suggest that during nonsubstorm periods, IMF B_Y plays an important and well‐defined role in determining east‐west asymmetries in the nightside convection pattern (see below), but that after substorm onset, the asymmetries are less straightforward to predict (e.g., Grocott et al., 2010). In general, the development of the Harang reversal, which results in westward low‐latitude return flows in the midnight sector, masks the B_Y effect (e.g., Grocott et al., 2010, 2017). There is evidence that the local time of substorm onset can itself influence the morphology of the nightside flows (e.g., Bristow et al., 2001, 2003), with substorms occurring at atypically early or late local times being associated with asymmetric eastward and westward midnight‐sector return flows, respectively (Grocott et al., 2017), but this appears to be irrespective of the sense of IMF B_Y.

Although it is thought that most nightside reconnection occurs during substorms or the subsequent SMC, reconnection can also occur when the driving of the Dungey cycle is weak and substorm activity is not expected (for instance during periods of northward IMF), albeit at a low rate and in occasional bursts (Senior et al., 2002; Grocott et al., 2003, 2004, 2005, 2008). Such events have become known as “tail reconnection during IMF‐northward, non‐substorm intervals” or TRINNIs (Milan et al., 2005). Fast eastward or westward convection flows are associated with TRINNIs, the direction being determined by the prevailing sense of IMF B_Y, associated with the untwisting of newly closed field lines (Grocott et al., 2005, 2007, 2008; Pitkänen, 2015, 2016; Reistad et al., 2016, 2018); the untwisting of these field lines has also been implicated in the formation of transpolar arcs, auroral features that bisect the dark polar cap (Milan et al., 2005; Goudarzi et al., 2008; Fear et al., 2012a, 2012b).

The question of how long it takes to develop nightside east‐west asymmetries in the nightside convection pattern, and the mechanism by which this occurs, is currently under debate. Mechanisms that have been suggested include pressure asymmetries in the lobe due to asymmetric loading of new open flux (Khurana et al., 1996; Tenfjord et al., 2015; Milan, 2015), and the reconnection of lobe field lines with a significant B_Y component introduced by asymmetric loading and magnetotail twisting (e.g., Cowley 1981b, Taguchi & Hoffman, 1996; Taguchi et al., 1994; Nishida et al., 1994, 1995, 1998; Tanaka, 2001; Grocott et al., 2005, 2007; Browett et al., 2017). It is possible that all these mechanisms are active, but are governed by different timescales, and manifest themselves in different parts of the convection pattern, that is, open and closed field lines, and at different phases of the substorm cycle (e.g., Grocott, 2017; Milan et al., 2018a).