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Localized and transient structures in the nightside auroral oval (Fig. 3.2) are primarily driven by electromagnetic and precipitation energy from the magnetotail, and ionosphere observations are useful for highlighting magnetosphere‐ionosphere coupling processes. Here we focus on ionosphere processes, and related magnetosphere processes are described in the companion book (Nishimura et al., in preparation).

Auroral arcs are one of the most common auroral features. Arcs typically have widths between several hundred m and tens of km (Knudsen et al., 2001; Partamies et al., 2010). Quasi‐static arcs are associated with inverted‐V electron precipitation (Stenbaek‐Nielsen et al., 1998), while arcs with vortical forms such as curls (~5 km wavelength along the arc; Hallinan & Davis, 1970) involve Alfvenic precipitation (Chaston et al., 2010). Quiet arcs at evening to premidnight hours are located within the large‐scale upward Region‐1 FAC (Wu et al., 2017). The most intense arc is typically located close to the Region‐1/Region‐2 FAC boundary (Ohtani et al., 2010; Coroniti & Pritchett, 2014), and fast azimuthal flow channels are also seen at this boundary (Archer et al., 2017). While the substorm growth phase arc can also be found near the same boundary (Marghitu et al., 2009; Jiang et al., 2012), the growth phase arc migrates into the downward Region‐2 FAC during the late growth phase, forming two or more upward FACs in the auroral oval (Nishimura et al., 2012; Motoba et al., 2015). Such multipolar FACs tend to occur near midnight associated with substorms (Wu et al., 2017).

Arcs are associated with enhanced precipitation, FAC, and electric field, and those drive active ionosphere feedback. Heating by precipitation and electric field drives plasma upflows, and fast flows enhance recombination. Those processes result in dynamic density structures (Zettergren et al., 2014; Lynch et al., 2015). The Alfvén resonator associated with FACs also contributes to create smaller‐scale density structures (Cohen et al., 2013).

Poleward boundary intensifications (PBIs) are auroral brightenings along the poleward boundary of the auroral oval (Lyons et al., 1999). PBIs are more intense during geomagnetically active times but can occur at all levels of geomagnetic activity. PBIs involve intense broad‐band Alfvénic electron precipitation (Mella et al., 2011). Cluster spacecraft observations showed that a growth of PBI corresponds to enhanced Alfvénic precipitation and then quasi‐static inverted‐V precipitation (Hull et al., 2016). PBIs are often followed by equatorward‐moving auroral arcs emanating from them. Those are called auroral streamers to distinguish from more stable east‐west oriented arcs. PBIs and streamers are the ionospheric signature of flow bursts in the nightside plasma sheet (Lyons et al., 1999) and a wedge‐type current system, where streamers mark upward FACs on the duskward edge of flow bursts (Sergeev et al., 2004). Radar observations show that streamers are located along the duskward edge of flow channels, where flows are generally substantially faster than large‐scale background convection speed (Shi et al., 2012; Gallardo‐Lacourt et al., 2014a; Pitkanen et al., 2011; Gabrielse et al., 2018). Even during storm times, fast flow channels and structured FACs are found to be the dominant mode of nightside auroral convection during the storm main phase (Lyons et al., 2016a).

The most prominent auroral structure is the auroral substorm. Details of a typical substorm auroral sequence are described by Akasofu (1964) and Voronkov et al. (2003). Substorm auroral onset is characterized by sudden brightening (initial brightening) of one of the growth‐phase auroral arcs within the equatorward portion of the auroral oval. Initial brightening involves wavy modulation of auroral intensity along the arc, called beads or rays depending on looking angles (Donovan et al., 2006). Beads are associated with intense flow oscillation (Hosokawa et al., 2013; Gallardo‐Lacourt et al., 2014b).

During poleward expansion of the arc, numerous auroral streamers originate in the vicinity of the poleward expanding arc and propagate equatorward. Individual streamers are associated with an intense wedge‐type current system and are a major contributor to local magnetic bays in ground magnetic field (Lyons et al. 2013). Although individual streamers are localized, a number of streamers arise during the expansion phase, and create modulation of the large‐scale substorm current wedge (SCW) (Forsyth et al., 2014). Streamers are associated with intense flows and particle injections into the inner magnetosphere and thus intensify various types of auroral activity in the equatorward portion of the auroral oval and subauroral ionosphere (Henderson, 2013), including pulsating aurora (Nishimura et al., 2018b) and omega bands (Henderson et al., 2006; Nishimura et al., 2013b). Diffuse auroral patch formation during substorms (Shiokawa et al., 2014) could also be a process related to substorm injection. Such localized aurora corresponds to localized precipitation (Hargreaves et al., 2010), which in many cases involves wave‐particle interaction in the magnetosphere (Nishimura et al., 2010a). Most intense electrodynamics during substorms can be seen along the poleward expanding arc and in the head of the westward traveling surge (Opgenoorth et al., 1983; Fujii et al., 1994; Gjerloev & Hoffman, 2002). The surge transitions to a quasi‐steady hook‐shaped auroral arc in the premidnight sector. Its clockwise open‐loop structure is an optical manifestation of the flow shear around the Harang reversal (thus called Harang aurora; Nishimura et al., 2010b).

There are also auroral substructures embedded in the phenomena mentioned above. Electron diffuse aurora involves small‐scale (< ~km) curls, filaments, and black aurora (Maggs & Davis, 1968; Stenbaek‐Nielsen et al., 1999). Pulsating auroral patches include substructures of some tens of km (Nishiyama et al., 2016). Those are considered to include kinetic plasma dynamics in the magnetosphere. Measurements of radio signal scintillation indicate that small‐scale density irregularities occur associated with aurora and high‐density plasma (van der Meeren et al., 2014, 2015; Jin et al., 2014), and irregularities are tightly coupled to auroral arc motion but with a slight time lag, suggesting instability processes associated with density gradient and flow shear (Mrak et al., 2018).

The studies mentioned above have documented properties of mesoscale aurora and flows structures, but mesoscale precipitation has been rather poorly characterized. Global imaging can detect only bright structures of > ~100 km size but attempts have been made to utilize ground‐based imaging to deduce precipitating electron fluxes by resolving individual auroral structures (Partamies et al., 2004; Grubbs et al., 2018). While their studies are limited to a single all‐sky imager, here we present an approach to use the THEMIS ASI array to cover a semicontinental scale, which is critical for account for the entire substorm region. Figure 3.3 shows 2‐D energy flux during a substorm at 7 UT on 26 March 2014. Technical details are beyond the scope of this chapter and will be reported more in detail elsewhere (Nishimura et al., 2018c) but, in brief, the THEMIS ASI data were used to convert white light images to red‐green‐blue colors by comparing to the nearest NORSTAR meridian scanning photometers, and then the color ratios were converted to energy fluxes and characteristic energies using the Rees and Luckey (1974) formulas. While the data were mapped to 110 km altitude, mapping and viewing angle effects provide uncertainties in interpretation of derived parameters.

Figure 3.3 Energy flux distribution and its temporal variation during the 7 UT 26 March 2014 substorm detected by THEMIS ASIs. (a) H‐component of ground magnetic field at Fort Smith (FSMI); (b) THEMIS ASI keogram at FSMI; (c) energy flux in the same format as Panel (b); (d) total energy flux integrated over the five imagers (black: total; colored: mesoscale component detrended over 500 (red), 300 (green), and 100 (blue) km size); (e) the ratio between the mesoscale and total energy fluxes; (f) averaged energy over the five imagers (energy flux‐weighted average); and (g–l) selected snapshots of energy flux maps.

This is a classical isolated substorm, whose onset was right after 7 UT followed by a ~ ‐300 nT magnetic bay, auroral breakup, and poleward expansion (Fig. 3.3a,b). During the growth phase, a quiet arc was located at ~67 deg MLAT with 1–2 erg/cm²/s energy flux (Fig. 3.3c,g). The total energy flux and averaged characteristic energy integrated over the available imager FOVs were ~30 GW and ~700 eV (Fig. 3.3d,f). Then the auroral intensity and energy flux showed a rapid two‐step increase during the substorm expansion phase, reaching ~50 erg/cm²/s along the poleward expanding arc and streamers. The total energy flux and averaged energy reached 120 GW and 2.5 keV. It is interesting that the auroral structures during the expansion phase involve numerous mesoscale auroras of the order of 100 km size (Fig. 3.3i–k). Those correspond to repetitive auroral intensifications (PBIs and streamers). To quantify the contribution of such mesoscale precipitation, 2‐D energy flux distributions were smoothed over 100, 300, and 500 km at each time, and the difference from the original data was defined as the mesoscale precipitation below the specified scale (shown in colors in Fig. 3.3d,e). Mesoscale precipitation repetitively increased the total energy flux by ~20–40 GW for a duration of ~10–20 min each. Their contribution relative to the total energy flux reached ~25 (<100 km), 40 (<300 km), and 50% (<500 km) during the expansion phase. This indicates that mesoscale precipitation is critically important to describe the total precipitation energy input. The average energy stayed almost constant after the initial rise.

While the imager array can be used as in Figure 3.3 to potentially improve specification of instantaneous mesoscale precipitation over a regional scale, it is currently difficult to specify convection and currents with a similar level of resolution and coverage. Figure 3.4 shows a comparison of the energy flux, SuperDARN convection map, and vertical and horizontal currents from ground magnetometers at the same region and time of Figure 3.3j. The currents were obtained by the spherical elementary current systems technique (Weygand et al., 2011), and the vertical currents are a proxy of FACs. Generally coherent radar echoes in the nightside auroral oval are sparse. In this example, a good amount of radar echoes exists, but the flow pattern does not reproduce mesoscale structures corresponding to the mesoscale energy flux structures. Instead, the flows are much smoother, only showing the large‐scale flow pattern. This is likely due to the spherical harmonic fitting of the radar data and to the limited spatial and temporal resolution of the data. Line‐of‐sight velocity measurements (not shown) show more structured flows, and techniques, such as divergence free fitting (Amm et al., 2010; Bristow et al., 2016), may be able to provide 2‐D flow structures near the echo areas, although it is not possible to reproduce flows where radar echoes are sparse. The vertical currents (FACs) show enhanced upward currents at the auroral structures extending over > ~500 km (westward traveling surge at ~22 MLT and a group of bright streamers at ~0–1 MLT). However, many of the mesoscale auroral structures are missed. Similarly, the horizontal currents highlight an intense electrojet along the poleward‐expanding arc, while the resolution is not sufficiently high to resolve currents associated with the surge and streamers. This comparison signifies limitation of our current capability of capturing mesoscale structures, and further advances are necessary to properly specify mesoscale structures.

Figure 3.4 (a) Energy flux; (b) SuperDARN fitted convection map; (c) vertical current (red = upward, blue = downward) technique; and (d) horizontal current at 7:15 UT on 26 March 2014. Panels (b–d) use THEMIS ASI counts as the black‐white background.