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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.

Mining of substorm magnetic field data reveals the formation of two X‐lines preceded by the flux accumulation at the tailward end of a thin current sheet (TCS). Three‐dimensional particle‐in‐cell simulations guided by these pre‐onset reconnection features are performed, taking also into account weak external driving, negative charging of TCS and domination of electrons as current carriers. Simulations reveal an interesting multiscale picture. On the global scale, they show the formation of two X‐lines, with stronger magnetic field variations and inhomogeneous electric fields found closer to Earth. The X‐line appearance is preceded by the formation of two diverging electron outflow regions embedded into a single diverging ion outflow pattern and transforming into faster electron‐scale reconnection jets after the onset. Distributions of the agyrotropy parameters suggest that reconnection is provided by ion and then electron demagnetization. The bulk flow and agyrotropy distributions are consistent with MMS observations.

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.

South Pole Station, Antarctica (SPA, magnetic latitude = −74.5°, magnetic local time (MLT) = UT–3.5 h), is a unique observatory which can capture daytime auroral forms throughout austral winter season. We have studied the properties and origin of ultralow‐frequency (ULF) range modulation of daytime diffuse aurora, using data acquired on June 23, 2017 by multi‐instrument measurements at SPA and in situ measurements in the dayside outer magnetosphere. At 1500–1600 UT, monochromatic Pc5‐range pulsations (period ∼10 min) emerged in the midday diffuse auroral region. The sequential 2‐D images reveal that the auroral pulsations result from the repetitive formation of faint, diffuse auroral patches, propagating poleward at a speed of ∼1.5 km s⁻¹. Interestingly, no obviously similar magnetic pulsations were found at SPA. The results differ fundamentally from the ground optical and magnetic signatures expected for a standing field line resonance. On the other hand, the co‐located riometer and VLF receiver observed clearly synchronized pulsations, suggesting that tens‐of‐keV electrons interact with modulated chorus waves and then are scattered down to the auroral pulsation region. During the same interval, the THEMIS‐D spacecraft detected corresponding Pc5 oscillations in the dayside outer magnetosphere (9–10R_Eand ∼15 MLT). The compressional component of the magnetospheric Pc5 waves, presumably driven by an external source, exhibited a good correspondence to the daytime Pc5 auroral pulsations. The simultaneous SPA–THEMIS observations highlight the role of compressional Pc5 pulsations in the dayside outer magnetosphere in determining the periodicity of daytime high‐latitude diffuse auroral pulsations.

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.

Substorm‐type evolution of the Earth's magnetosphere is investigated by mining more than two decades (1995–2017) of spaceborne magnetometer data from multiple missions including the first two years (2016‐2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal magnetohydrodynamics (MHD) behavior: X‐lines, thin current sheets, and regions with the tailward gradient of the equatorial magnetic fieldB_z. X‐lines are found to form mainly beyond 20R_E, but for strong driving, with the solar wind electric field exceeding ∼5mV/m, they may come closer. For substorms with weaker driving, X‐lines may be preceded by redistribution of the magnetic flux in the tailwardB_zgradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2R_E, comparable to the thermal ions gyroradius, and at the same time, as long as 15R_E. Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement microscale analysis of nonideal effects.