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The dawn‐dusk asymmetry of magnetic depression is a characteristic feature of the storm main phase. Recently Ohtani (2021,https://doi.org/10.1029/2021JA029643) reported that its magnitude is correlated with the dawnside westward auroral electrojet (AEJ) intensity, and suggested that the dawnside AEJ intensification is a fundamental process of the stormtime magnetosphere‐ionosphere coupling. In this study we observationally address the cause of the dawnside AEJ intensification in terms of four scenarios. That is, the dawnside AEJ intensifies because (a) the external driving of global convection strengthens, (b) solar wind compression enhances energetic electron precipitation, and therefore, ionospheric conductance, through wave‐particle interaction, (c) the substorm current wedge forms in the dawn sector, and (d) energetic electrons injected by nightside substorms drift dawnward, and the subsequent precipitation enhances ionospheric conductance. We find an event that fits each scenario, and therefore, none of these scenarios can be precluded. However, the result of a superposed epoch analysis shows that some causes are more prevalent than others. More specifically, (a) although the enhancement of external driving may precondition the dawnside AEJ intensification, it is rarely the direct cause; (b) external compression probably explains only a small fraction of the events; (c) prior to the dawnside AEJ intensification, the westward AEJ tends to intensify in the midnight sector along with mid‐latitude positive bays, which suggests that the substorm injection of energetic electrons is the most prevalent cause. This last result may also be explained by the dawnside expansion of the substorm current wedge, which, however, is arguably far less common.

In the present study we examine three substorm events, Events 1–3, focusing on the spatio‐temporal development of auroral electrojets (AEJs) before auroral breakup. In Events 1 and 2, auroral breakup was preceded by the equatorward motion of an auroral form, and the ground magnetic field changed northward and southward in the west and east of the expected equatorward flow, respectively. Provided that these magnetic disturbances were caused by local ionospheric Hall currents, this feature suggests that the equatorward flow turned both eastward and westward as it reached the equatorward part of the auroral oval. The auroral breakup took place at the eastward‐turning and westward‐turning branches in Events 1 and 2, respectively, and after the auroral breakup, the westward AEJ enhanced only on the same side of the flow demarcation meridian. The zonal flow divergence is considered as an ionospheric manifestation of the braking of an earthward flow burst in the near‐Earth plasma sheet and subsequent dawnward and duskward turning. Therefore, in Events 1 and 2, the auroral breakup presumably mapped to the dawnward and duskward flow branches, respectively. Moreover, for Event 3, we do not find any pre‐onset auroral or magnetic features that can be associated with an equatorward flow. These findings suggest that the braking of a pre‐onset earthward flow burst itself is not the direct cause of substorm onset, and therefore, the wedge current system that forms at substorm onset is distinct from the one that is considered to form as a consequence of the flow braking.

Embedded Region 1 and 2 field‐aligned currents (FACs), intense FAC layers of mesoscale latitudinal width near the interface between large‐scale Region 1 and Region 2 FACs, are related to dramatic phenomena in the ionosphere such as discrete arcs, inverted‐V precipitation, and dawnside auroral polarization streams. These relationships suggest that the embedded FACs are potentially important for understanding ionospheric heating and magnetosphere‐ionosphere (M‐I) coupling and instabilities. Previous case studies of embedded FACs have led to the speculation that they may result from enhanced M‐I convection during active times. To explore this idea further, we investigate statistically their occurrence rates under a variety of geomagnetic conditions with a large event list constructed from 17 years of Defense Meteorological Satellite Program observations. The identification procedure is fully automated and explicit. The statistical results indicate that embedded Region 1 and 2 FACs are common, and that they have a higher chance to occur when the level of geomagnetic activity is higher (given by various indices), supporting the idea that they result from enhanced M‐I convection.

The present study investigates dipolarization signatures in the inner magnetosphere using sharp geosynchronous dipolarizations as a reference. The results are summarized as follows: (1) The region of sharp and structured dipolarizations expands earthward while dipolarizations are sustained at geosynchronous orbit; (2) within 5R_Efrom Earth, dipolarization signatures are often smooth and gradual, resembling midlatitude positive bays, and they start simultaneously with substorm onsets; (3) off the equator (>0.5R_E), sharp dipolarizations often take place before geosynchronous dipolarizations. These results can be explained by a model current system with R1‐sense and R2‐sense current wedges (R1CW and R2CW) if (a) the R1CW, which is located outside, is more intense than the R2CW in total current, (b) the R1CW stays outside of geosynchronous orbit, and (c) the R2CW moves earthward. The model suggests that the region of sharp dipolarizations is confined between the two current wedges, and it expands earthward as the R2CW moves earthward (Result 1). Sufficiently earthward of the R2CW, the remote effect of the R1CW dominates that of the R2CW, and accordingly, magnetic disturbances resemble midlatitude positive bays (Result 2). Since the timing of sharp dipolarizations is determined by the passage of the R2CW, they take place earlier for outer flux tubes. Away from the magnetic equator, sharp dipolarizations can precede geosynchronous dipolarizations especially if the magnetic configuration is stretched (Result 3). Thus, this double‐current wedge model explains the variability of dipolarization signatures at different distances, and it may be regarded as a generalized substorm current wedge model.

Dispersionless injections, involving sudden, simultaneous flux enhancements of energetic particles over some broad range of energy, are a characteristic signature of the particles that are experiencing a significant acceleration and/or rapid inward transport at the leading edge of injections. We have statistically analyzed data from Van Allen Probes (also known as Radiation Belt Storm Probes [RBSP]) to reveal where the proton (H⁺) and electron (e^–) dispersionless injections occur preferentially inside geosynchronous orbit and how they develop depending on local magnetic field changes. By surveying measurements of RBSP during four tail seasons in 2012–2019, we have identified 171 dispersionless injection events. Most of the events, which are accompanied by local magnetic dipolarizations, occur in the dusk‐to‐midnight sector, regardless of particle species. Out of the selected 171 events, 75 events exhibit dispersionless injections of both H⁺and e^–, which occur within 2 min of each other. With only three exceptions, the both‐species injection events are further divided into two main subgroups: One is the H⁺preceding e^–events with a time offset of tens of seconds between H⁺and e^–, and the other the concurrent H⁺and e^–events without any time offset. Our superposed epoch results raise the intriguing possibility that the presence or absence of a pronounced negative dip in the local magnetic field ahead of the concurrent sharp dipolarization determines which of the two subgroups will occur. The difference between the two subgroups may be explained in terms of the dawn‐dusk asymmetry of localized diamagnetic perturbations ahead of a deeply penetrating dipolarization front.

The expansion phase of auroral substorms is characterized by the formation of an auroral bulge, and it is generally considered that a single bulge forms following each substorm onset. However, we find that occasionally two auroral intensifications takes place close in time but apart in space leading to the formation of double auroral bulges, which later merge into one large bulge. We report three such events. In those events the westward auroral electrojet intensified in each auroral bulge, and geosynchronous magnetic field dipolarized in the same sector. It appears that two substorms took place simultaneously, and each substorm was accompanied by the formation of its own substorm current wedge system. This finding strongly suggests that the initiation of auroral substorms is a local process, and there is no global reference frame for their development. For example, ideas such as (i) the auroralbreakup takes place in the vicinity of the Harang reversal and (ii) the westward traveling surge maps to the interface between the plasma sheet and low‐latitude boundary layer, do not necessarily hold for every substorm. Even if those ideas may be suggestive of causal magnetospheric processes, the reference structures themselves are probably not essential. It is also found that despite the formation of two distinct auroral bulges, the overall magnetosphere‐ionosphere current system is represented by one globally coherent system, and we suggest that its structure is determined by the relative intensities and locations of the two substorm current wedges that correspond to the individual auroral bulges.

Four closely located satellites at and inside geosynchronous orbit (GEO) provided a great opportunity to study the dynamical evolution and spatial scale of premidnight energetic particle injections inside GEO during a moderate substorm on 23 December 2016. Just following the substorm onset, the four spacecraft, a LANL satellite at GEO, the two Van Allen Probes (also called “RBSP”) at ~5.8R_E, and a THEMIS satellite at ~5.3R_E, observed substorm‐related particle injections and local dipolarizations near the central meridian (~22 MLT) of a wedge‐like current system. The large‐scale evolution of the electron and ion (H, He, and O) injections was almost identical at the two RBSP spacecraft with ~0.5R_Eapart. However, the initial short‐timescale particle injections exhibited a striking difference between RBSP‐A and ‐B: RBSP‐B observed an energy dispersionless injection which occurred concurrently with a transient, strong dipolarization front (DF) with a peak‐to‐peak amplitude of ~25 nT over ~25 s; RBSP‐A measured a dispersed/weaker injection with no corresponding DF. The spatiotemporally localized DF was accompanied by an impulsive, westward electric field (~20 mV m⁻¹). The fast, impulsiveE × Bdrift caused the radial transport of the electron and ion injection regions from GEO to ~5.8R_E. The penetrating DF fields significantly altered the rapid energy‐ and pitch angle‐dependent flux changes of the electrons and the H and He ions inside GEO. Such flux distributions could reflect the transient DF‐related particle acceleration and/or transport processes occurring inside GEO. In contrast, O ions were little affected by the DF fields.