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The cusps are regions with enhanced ion (~keV) and electron (tens to hundreds eV) precipitation in the dayside auroral oval near noon in the northern and southern high latitude ionosphere. Number of fluxes of precipitation into the ionosphere are generally the largest in the cusps, which are the regions most directly affected by solar‐wind condition changes. The cusp is typically a single spot extending over a few hours of MLT (~500–1,000 km azimuthally), and its location overall follows the interplanetary magnetic field (IMF) B_y and B_z (Frey et al., 2003; Newell et al., 2006; Zhang et al., 2013a). However, the cusp is not always a single structure but can bifurcate to two or more, particularly at large IMF |B_y| or during IMF orientation changes. Latitudinal bifurcation is considered to reflect low‐ and high‐latitude reconnection at the magnetopause (Sandholt et al., 1998; Pitout et al., 2002), and azimuthal bifurcation has been attributed to component and antiparallel reconnections (Sandholt & Farrugia, 2003; Massetti et al., 2006).

The cusp also exhibits localized and transient structures (Moen et al., 2013). One of the most common types is poleward‐moving auroral forms (PMAFs), which initiate as brightening near the equatorward boundary of the discrete aurora followed by poleward propagation (Sandholt et al., 1998; Sandholt & Farrugia, 2007). PMAFs are associated with soft electron precipitation and dominantly occur under a southward IMF (Drury et al., 2003; Xing et al, 2012). Some of them are triggered by IMF orientation changes (Moen et al., 1999; Milan et al., 2002; Sandholt & Farrugia, 2008; Wang et al., 2016a), while other events do not have any clear interplanetary trigger (Sandholt et al., 2003).

PMAFs are an ionospheric manifestation of bursty reconnection at the dayside magnetopause and subsequent flux transfer events (FTEs) (Lockwood et al., 2001; Neudegg et al., 2001). They are associated with localized flow channels and a wedge‐type FAC loop, where PMAFs correspond to the footprint of the upward FAC (McWilliams et al., 2000; Oksavik et al., 2004). Each localized flow channel has a substantial level of potential drop (~1–10 kV) (Greenwald et al., 1996; Moen et al., 2008) or ~1%–10% of the open flux in the magnetosphere (Fear et al., 2017).

The cusp also involves smaller‐scale variabilities of < ~10 km size, including precipitation (Tanaka et al., 2005; Ebihara et al., 2008), FAC (Neubert & Christiansen, 2003), flow (Keady & Heelis, 1999), and density (Kivanc & Heelis, 1999). Small‐scale density irregularities contribute to radio signal scintillation (Oksavik et al., 2015; Spicher et al., 2016; Jin et al., 2017).

Other types of dayside auroral brightenings are discussed in a separate review article by Nishimura et al. (2018a). These include discrete aurora including throat aurora, and diffuse aurora (Fig. 3.2). They are sensitive to solar‐wind‐condition changes and magnetosheath transients and can occur even under small IMF B_z.