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The cusp processes are important to create localized structures that move into and across the polar cap. Enhanced F‐region ionosphere density is often seen in the polar cap, and density enhancements by a factor of 2 from background of > ~100 km size are called polar cap patches (Crowley, 1996). Among many processes proposed (Carlson, 2012), major ideas are (1) FTEs transporting photoionized plasma from dayside closed field lines into the polar cap across the dayside open‐closed field‐line boundary (Lockwood & Carlson, 1992); (2) time‐varying convection forming storm‐enhanced density (SED) and TOIs (Sojka et al., 1994); and (3) ionization by cusp precipitation and transport by flow channels (Rodger et al., 1994). These are competing processes and the dominant mechanism depends on density distributions on closed field lines, flow strength and duration, and precipitation flux. In fact, both elevated and reduced temperatures from surrounding plasma have been identified to be associated with patches (Yin et al., 2008; Zou et al., 2016; Zhang et al., 2017). Elevated and reduced temperatures are considered as indicators of plasma heated by cusp precipitation and of transport of photoionized plasma from closed field lines.

During enhanced geomagnetic activity such as storms, SEDs are often seen to propagate across the cusp and become patches and TOIs (Foster et al., 2005; Carlson et al., 2006), and these high‐density plasma features have lower temperature than in surrounding plasma (Lockwood et al., 2005). Convection of photoionized plasma can also explain the UT and seasonal dependence of patches (Sojka et al., 1994). During nonstorm times, FTEs or cusp ionization becomes more important, as has been seen from patches that move along flow streamlines that are fully in darkness without access of sunlit plasma (Oksavik et al., 2006; MacDougall & Jayachandran, 2007). A density trough can exist between the cusp and photoionized plasma, indicating that photoionized plasma from lower latitudes does not reach the cusp and polar cap during these times. Instead, cusp precipitation creates patches (Rodger et al., 1994; Walker et al., 1999; Smith et al., 2000; Goodwin et al., 2015). Patch formation in association with PMAFs (Lorentzen et al., 2010; Nishimura et al., 2014a; Hosokawa et al., 2016) is another indication that sudden energy input into the cusp contributes to patches.

Patches are not necessarily a structure passively embedded in large‐scale convection but can be associated with flow channels (Kivanc & Heelis, 1997; Balmforth, 1999; Lockwood et al., 2005; Maynard et al., 2006; Thomas et al., 2013; Zou et al., 2015a; Wang et al., 2016b). Flow channels are seen even deep in the polar cap, and they can exist as quasi‐stable structures because flow channels are connected to their high‐altitude driver via FACs (Zou et al., 2016). Weak soft electron precipitation is also associated with patches (Zou et al., 2017), and the presence of precipitation on open field lines indicates that the flux tube of the patch has different properties from those of surrounding plasma and is possibly connected to a solar‐wind driver. The flow channel system in the polar cap resembles ones at the cusp, nightside auroral oval, and polar cap arc (Fig. 3.2), except that precipitation and FAC magnitudes are much smaller due to low conductance and smaller energy content on the open field lines.

Structures in the polar cap also exist beyond polar cap patches. Precipitation in the polar cap can also be structured (Newell et al., 1997; Huang et al., 2014), and even smaller‐scale flow and density structures of less than 100 km can occur (Gondarenko & Guzdar, 2004; Golovchanskaya & Kozelov, 2010). Those are attributed to gradient drift instability (GDI) and could connect to density irregularities that create radio signal scintillation. Polar cap convection involves mesoscale flow channels of some tens to hundreds km horizontal size particularly during quiet and weakly disturbed conditions (Sojka & Schunk, 1988; Taguchi et al., 1995; Matsuoka et al., 1996). Under northward IMF with large |B_y|, those are typically associated with polar cap arcs (Carlson et al., 1988; Koustov et al., 2008). Polar cap arcs are approximately Sun‐aligned auroral arcs in the polar cap, associated with a few keV or less of electron precipitation (Robinson & Mende, 1990). Similar to arcs in the auroral oval, polar cap arcs are associated with a wedge‐type current system where flow channels with a few kV potential drop are located between upward and downward FACs (Robinson et al., 1987; Maggiolo et al., 2012). While dawn‐dusk motion of polar cap arcs is typically controlled by IMF B_y (Valladares et al., 1994; Hosokawa et al., 2011), dawnside polar cap arcs can show quasi‐periodic poleward motion regardless of the IMF (Shiokawa et al., 1996). While polar cap arcs are typically a quasi‐static structure with slow dawn‐dusk motion, those can also involve substantial antisunward motion similarly to polar cap patches. Such motion particularly occurs for arcs that are associated with lower energy (hundreds of eV) precipitation, and those arcs can actively interact with the nightside auroral oval (Nishimura et al., 2013a; Zou et al., 2015b). Such soft electron precipitation and large electric field also effectively heat plasma and create upflows (Perry et al., 2015).