is driven by dayside reconnection, followed by a sharp excursion in AL associated with the formation of the current wedge. Variations in FPC allow estimation of ΦD and ΦN using equation (2.15), from which ΦPC can be inferred using equation (2.16). Such observations show that the substorm expansion phase typically lasts 1 hour, with ΦN ≈ 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 FPC are related to the difference between ΦD and ΦN, not their absolute values. In this case, estimating ΦD 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 ΦD and ΦN 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 FPC, ΦD, and ΦN 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, FPC; maximum nightside auroral intensity; auroral electrojet indices, AU and AL; IMF BZ; inferred dayside and nightside reconnection rates, ΦD (black lines) and ΦN (grey rectangles); inferred cross‐polar cap potential, ΦPC
(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, ΦN ≈ ΦD 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 BZ < 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 BY 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 BY 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 BY.
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 BY, 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 BY 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).
2.4.3 Lobe Reconnection
The discussion so far has concentrated on periods with IMF BZ < 0 when reconnection occurs at the subsolar magnetopause. When the IMF turns northward, the region of high magnetic shear on the magnetopause moves to high latitudes, where both closed and open field lines are available to reconnect. Figure 2.13 shows the reconnection topologies that can ensue (Cowley, 1981a). Panels (d) and (e) show reconnection in only one hemisphere with a closed or open field line. Panels (a) and (b) show reconnection in both hemispheres with closed field lines. Finally, panel (c) shows reconnection occurring with open field lines in both hemispheres. Panels on the right show the magnetosphere looking from the Sun, with impinging IMF field lines with BZ > 0 and BY < 0 (Imber et al., 2006). In (A), the BY component is such that separate IMF field lines reconnect at the northern and southern reconnection sites, corresponding to panels (d) and (e). Panel (B) shows the limiting clock angle for which a single IMF field line can reconnect at both north and south, corresponding to panels (a) to (c).