Читать книгу Space Physics and Aeronomy, Ionosphere Dynamics and Applications - Группа авторов - Страница 51

While many dayside flow channels decay in the vicinity of the cusp, a portion of them propagates over much longer distances and have a major impact on nightside processes (day‐night interaction) (Nishimura et al., 2014a; Lyons et al., 2016b). A flow channel initiated in the cusp (cusp auroral brightening and PMAF) can propagate into the polar cap (seen as polar cap patches and arcs) (Lockwood, 1991), and even reach the nightside auroral oval. When reaching the nightside auroral poleward boundary, the flow channel drives a PBI and then streamer (de la Beaujardière et al., 1994; Lorentzen et al., 2004; Moen et al., 2007; Zou et al., 2014; Ohtani & Yoshikawa, 2016), indicating triggering of nightside reconnection. If it occurs during the substorm growth phase, the streamer may trigger a substorm by making the near‐Earth plasma sheet unstable (Oguti, 1973; Kepko et al., 2009; Nishimura et al., 2010b; Lyons et al., 2011; Kornilova & Kornilov, 2012). Streamers/auroral flow channels further propagate into the subauroral ionosphere and drive subauroral processes (subauroral polarization streams (SAPS) (Gallardo‐Lacourt et al., 2017; Mishin et al., 2017) and proton aurora (Nishimura et al., 2014b). The day‐night interaction process can also be seen as enhanced plasma density drifting from the dayside to nightside and into the auroral oval, and then further propagating back to the dayside (Zhang et al., 2013b).

Localized ionosphere structures can affect large‐scale magnetosphere‐ionosphere dynamics. As mentioned in section 3.1, global ionosphere conductance can increase due to small‐scale density structures (Oppenheim & Dimant, 2013), and this provides a net positive increase in large‐scale conductance. Global MHD simulations have shown that such small‐scale conductance enhancements can have a global impact by affecting FACs and storm development (Wiltberger et al., 2017). Such studies indicate the importance of mesoscale/small‐scale structures for understanding global processes, and call for studies of characterizing properties of mesoscale/small‐scale structures. Figure 3.5 shows representative global distributions of mesoscale/small‐scale structures of selected quantities. FACs have localized variabilities predominantly in the cusp, and smaller amplitudes are found in the rest of the auroral oval (Neubert & Christiansen, 2003; Rother et al., 2007; McGranaghan et al., 2017). Electric field, Poynting flux, and density irregularities follow a similar spatial distribution (Cousins & Shepherd, 2012; Prikryl et al., 2015; Hatch et al., 2018), while the distribution of the electric field variability is highly dependent on IMF conditions, and the energy flux in the nightside can be larger on the nightside, likely due to substorms. Also, the probability distribution of mesoscale/small‐scale structures does not have a discrete peak but has a broad spectrum that varies in regions (Golovchanskaya, 2008; Lühr et al., 2015). Consideration of mesoscale/small‐scale electric fields and their variability can substantially increase the Joule heating (Codrescu et al., 1995; Deng et al., 2009; Yigit & Ridley, 2011; Zhu et al., 2018). These studies show that statistical parameterization is a possible pathway for specifying distribution of mesoscale/small‐scale quantities that are difficult to resolve over a large spatial scale at each instance. However, currently parameterizations have been conducted only during limited geomagnetic and geographic conditions, and statistical averaging tends to underestimate actual amplitude of multiscale parameters due to smoothing over their localized and transient structures. As studies above have shown, mesoscale/small‐scale features (such as flows) are not necessarily random fluctuations around a large‐scale mean, but are often oriented in a certain direction, and thus do not average out but have net effects on the large‐scale. Moreover, coherent structures and physical connections among the quantities are missed by statistical averaging, because information of size, orientation, and duration is lost, and individual parameters are processed without considering behavior of other parameters. Although parameterization is a useful and promising approach, those cautions should be noted when using such parameterization for model input, and a challenge to data analysis is to create an approach of comprehensive parameterization without underestimating mesoscale and small‐scale features.

Figure 3.5 Representative statistical distributions of mesoscale/small‐scale quantities mapped to the high‐latitude ionosphere: (a) Electric field from SuperDARN; (b) FAC (Swarm); (c) GPS phase scintillation as a proxy of small‐scale density irregularities; (d,e) electron energy flux and Poynting flux by the FAST satellite; and (f) Joule heating (GITM simulation). The Sun is to the top of each plot

(a: Cousins & Shepherd, 2012. Reproduced with permission of John Wiley & Sons; b: McGranaghan et al., 2017. Licensed under CC‐BY 3.0; c: Prikryl et al., 2015. Licensed under CC‐BY 3.0; d,e: Hatch et al. (2018). Reproduced with permission of Elsevier; f: Yigit & Ridley, 2011. Reproduced with permission of Elsevier).