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Following the auroral substorm onset, the active aurora undergoes expansion, which can vary in spatial and temporal extent. The spatiotemporal development of the expansion phase active aurora is controlled by new auroral intensifications that often follow the initial onset. Using seven examples, we investigate the nature of these new auroral intensifications and address a question: are they new auroral onsets, that is, “successive onsets” or poleward‐boundary intensifications (PBIs) and ensuing auroral streamers? We observed events that included both types of auroral features—successive onsets and PBIs—and their combinations. For multiple‐onset substorms, successive onsets may occur eastward, westward, and poleward of the initial onset, resulting in a diverse range of expansion phase spatial extent and durations. Single‐onset substorms show only one auroral onset, but their spatiotemporal development can resemble that of multiple‐onset substorms. However, the additional activations are mainly PBIs and subsequent streamers. In some cases, PBIs undergo explosion, leading to a rapid poleward and azimuthal expansion of the aurora, resembling the auroral substorm onset. A prolonged sequence of PBIs and its longitudinal extension can contribute significantly to the spatiotemporal development of substorms expansion phase. Results suggest that post‐onset flow channels drive the spatiotemporal development of the substorm expansion phase by (a) triggering successive onsets and (b) inducing bursts of PBIs and their prolonged sequence. We speculate that post‐onset flow channels likely originate from the polar cap, but more evaluation is required. Our findings highlight the significance of examining imager data before solely relying on magnetometers to identify substorm onsets. Successive onsets, often preceded by post‐onset streamers, cause diverse range of expansion phase coverage and durationsA prolonged sequence of poleward‐boundary intensifications and auroral streamers play a crucial role in driving spatiotemporal development of substorm expansion phasePost‐onset flow channels play an important role in spatiotemporal development of substorm expansion phase Successive onsets, often preceded by post‐onset streamers, cause diverse range of expansion phase coverage and durations A prolonged sequence of poleward‐boundary intensifications and auroral streamers play a crucial role in driving spatiotemporal development of substorm expansion phase Post‐onset flow channels play an important role in spatiotemporal development of substorm expansion phase

We report the first observations of the association between equatorward extending streamers and overshielding using the THEMIS all‐sky imagers and ground magnetometers. Because auroral streamers indicate plasma sheet flow bursts, these observations uncover the effect of flow bursts on overshielding. Results show that, in general, bright equatorward extended streamers were associated with an increase in equatorial electrojet (EEJ) on the nightside and a decrease in the dayside EEJ, indicating a striking correspondence between auroral streamers and overshielding conditions. Thus, the driving of overshielding at equatorial latitudes can be identified via bright equatorward extended streamers, indicating that flow bursts are an alternate means to discern the earthward injections that increase the region 2 field aligned currents and associated overshielding electric fields. Repetitive auroral streamers were associated with repetitive overshielding, resulting in a stepwise development of the dayside and nightside EEJ. The stepwise intensifications were also observed in the midlatitude positive bay and Pi2 pulsations. Our results could explain the occurrence of overshielding conditions at equatorial latitudes during substorms and nonsubstorm times without a northward turning of IMF‐Bz. As seen through streamers, the localized current structures (wedgelets) associated with flow bursts giving injection that leads to overshielding is titled northeast‐to‐southwest. Our results add a new element to the understanding of high‐to‐low latitude electrodynamical coupling by demonstrating the association between bright equatorward extended auroral streamers and enhanced shielding electric fields caused by earthward injections associated with flow bursts. The driving of overshielding seen at equatorial latitudes by injections can be identified via equatorward extending auroral streamersRepetitive streamers, thus repetitive injections, are associated with repetitive overshielding conditionsAs seen via streamers, localized current system with flow bursts giving injection driven overshielding is tilted northeast‐to‐southwest The driving of overshielding seen at equatorial latitudes by injections can be identified via equatorward extending auroral streamers Repetitive streamers, thus repetitive injections, are associated with repetitive overshielding conditions As seen via streamers, localized current system with flow bursts giving injection driven overshielding is tilted northeast‐to‐southwest

Auroral streamers have been found to be a particularly important cause of magnetic perturbations within the auroral oval during substorms. We present a detailed investigation on the association of equatorward extending auroral streamers with ground magnetic perturbations at locations initially equatorward of the auroral oval using Time History of Events and Macroscale Interactions imagers and ground magnetometers. Based on the analysis of four events, we show that negative perturbations in the horizontal (H) component at initially subauroral latitudes were not seen prior to streamer onset, and the peak negative H‐bay at subauroral latitudes coincided with the equatorward extending auroral streamers. Although stations close to the poleward boundary of auroral oval showed modest H‐component perturbations in response to the substorm onset and poleward auroral intensifications, the abrupt and large responses within the oval were associated with the onset of streamers. Results confirm that ground magnetic perturbations that are traditionally perceived as signatures of magnetic substorm onset are not always an auroral substorm onset but can also be related to auroral streamers. These results are supported by the remarkable correlation of positive bay onset and sudden Pi2 enhancement with the onset of auroral streamers, irrespective of the classical substorm onset. In addition, the magnitude of positive bay and amplitude of Pi2 pulsations, particularly at subauroral latitudes, are enhanced during the events of equatorward extending streamers. In general, all streamer events were accompanied by dipolarization and electron flux enhancement at geosynchronous orbit that continue to strengthen during the emergence of bright streamers. Large, rapid perturbations in the ground magnetic field at high latitudes have been considered a major space weather hazard as they can harm technological systems. Such magnetic perturbations are common at auroral latitudes during geomagnetic storms and substorms. However, it is a matter of research to comprehend under what conditions such perturbations can extend equatorward of the auroral oval. The ground magnetic perturbations at auroral latitudes are shown to be linked with auroral streamers. Auroral streamers are roughly north‐south oriented narrow bands of auroral luminosity. They originate near the poleward edge of the auroral oval and propagate equatorward. Auroral streamers are related to flow bursts in the plasma sheet. The equatorward motion of auroral streamer is related to the earthward motion of flow bursts in the plasma sheet. This paper shows that the abrupt magnetic perturbations at latitudes equatorward of the auroral oval (i.e., nominal subauroral latitudes) are linked with the equatorward extending auroral streamers. The observations suggest that as the streamers extended to lower latitudes, the auroral oval expands equatorward to reach previously subauroral latitudes. We found that auroral streamers are, in general, followed by abrupt decreases in the H (horizontal)‐component at auroral and subauroral latitudes. The onset of abrupt negative perturbations in H‐component at auroral and part of subauroral latitudes coincided with the onset of auroral streamerThe peak negative bay in H‐component at subauroral latitudes coincided with the equatorward extending auroral streamersBright equatorward extended auroral streamers were associated with the peak enhancement in electron flux at geosynchronous orbit The onset of abrupt negative perturbations in H‐component at auroral and part of subauroral latitudes coincided with the onset of auroral streamer The peak negative bay in H‐component at subauroral latitudes coincided with the equatorward extending auroral streamers Bright equatorward extended auroral streamers were associated with the peak enhancement in electron flux at geosynchronous orbit

Auroral observations were first to identify the substorm, and later used to propose that substorm onset is triggered in the inner plasma sheet (equatorward portion of the auroral oval) by an intrusion of low entropy plasma comprising plasma sheet flow channels. Longitudinal localization makes the intruding flow channels difficult to observe with spacecraft. However, they are detectable in the ionosphere via the broader, two‐dimensional coverage by radars. Line‐of‐sight radar flow measurements have provided considerable support for the onset proposal. Here we use two‐dimensional, ionospheric flow maps for further testing. Since these maps are derived without the smoothing from global fits typically used for global convection maps, their spatial resolution is significantly improved, allowing representation of localized spatial structures. These maps show channels of enhanced ionospheric flow intruding to the time and location of substorm onset. We also see evidence that these intruding flows enter the plasma sheet from the polar cap, and that azimuthal spread of the reduced entropy plasma in the inner plasma sheet contributes to azimuthal onset spreading after initial onset. Identified events with appropriate radar data remain limited, but we have found no exceptions to consistency with flow channel triggering. Thus, these analyses strongly support the proposal that substorm onset is due to the intrusion of new plasma to the onset region. The lower entropy of the new plasma likely changes the entropy distribution of inner plasma sheet, a change possibly important for the substorm onset instability seen via the growing waves that demarcate substorm auroral onset. Two‐dimensional maps based on divergence free ionospheric flow provide further testing of substorm onset triggered by intruding flow channelMaps show channels of enhanced flow intruding to the time and location of substorm onset and evidence that flows enter oval from polar capMaps imply azimuthal spread of reduced entropy plasma in inner plasma sheet contributes to azimuthal onset spreading after initial onset Two‐dimensional maps based on divergence free ionospheric flow provide further testing of substorm onset triggered by intruding flow channel Maps show channels of enhanced flow intruding to the time and location of substorm onset and evidence that flows enter oval from polar cap Maps imply azimuthal spread of reduced entropy plasma in inner plasma sheet contributes to azimuthal onset spreading after initial onset

In order to characterize plasma sheet and nightside auroral disturbances in response to interplanetary shocks, we have examined three interplanetary shock events that occurred when multiple Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft were located in the plasma sheet near midnight while ground‐based aurora data were available near the spacecraft footprints. Large‐scale responses we found are that the magnetotail magnetic pressure started to increase within ~2 min of the SYM‐H jump, and the diffuse aurora near the auroral equatorward boundary intensified over a wide magnetic local time range, due to the shock compressional effect, on average 3 min after the shock arrival. In addition, we also identified plasma sheet and auroral disturbances that are more transient and localized. Earthward or equatorward flow bursts are observed in the near‐Earth plasma sheet on average 5 min after the SYM‐H increase. We find that these fast flows, originating downtail of the near‐Earth spacecraft, form a localized channel, since only some of the spacecraft detected the flow bursts. Poleward boundary intensifications (PBIs) and subsequent north‐south directed auroral streamers are then formed, while no substorm activity was detected. Those auroral forms are also localized in space near midnight and around the footprint of the spacecraft. These results indicate that the fast flows are azimuthally localized channels and are the magnetotail counterpart of the PBIs and streamers and that such localized disturbances are triggered by the interplanetary shocks in addition to the large‐scale compression of the magnetosphere. We performed coordinated observations of shocks with THEMIS spacecraft and ASIs.Transient and localized plasma flow and auroral disturbances are found.PBI and north‐south directed streamers are formed, no substorm was detected.

A merged data set from Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) and Magnetometers‐Ionospheric Radars‐All‐Sky Cameras Large Experiment (MIRACLE) magnetometer stations is used to study variations observed at widely separated locations. Three years of 10‐s data are analyzed using canonical correlation techniques to determine the temporal offset of similar fluctuations observed at pairs of sites. Nearly simultaneous (<2‐min delay) signatures are often found between sites separated by 12 hours magnetic local time, with no evidence of more slowly propagating events (> 10‐min delay). Correlated episodes occur more often (roughly 10% of the time) when sites are in the dawn/dusk sectors and less often (∼5%) for noon/midnight configurations.

Although triggering mechanisms for substorm onsets remain highly controversial, consensus has reached that violation of frozen‐in flux condition in the central plasma sheet is required. In this study, we carry out a numerical gedanken experiment to investigate the effects of the violation by assuming ions slip with respect to the magnetic field lines in the late substorm growth phase while electrons remain magnetized, without specifying the microphysics. The simulation results predict (1) a thin arc and a strong westward electrojet associated with downward‐upward‐downward field‐aligned currents and westward‐eastward‐westward horizontal flows in the ionosphere, which are found to be consistent with a preonset arc observed by the Swarm and the all‐sky imager; (2) a rapid creation of a bubble‐blob pair in the plasma sheet with a tailward hump of Bzthat may lead to tearing instability. Aurora borealis are fascinating phenomena in Earth's high latitude regions. Their various forms, ranging from spirals, rays, streamers, beads, and arcs, change from time to time. One particular structure, as known as the substorm preonset arc, emerges in the sky prior to a very dynamic energy conversion process in near‐Earth space. Several mechanisms for the formation of these east‐west‐aligned arcs have been proposed, and in this study, we propose a different one. Using a numerical simulation, we show that preonset arcs connect with the nightside magnetosphere, where the conservation of entropy is violated. Such a violation creates higher concentrations of charged particles at a larger radial distance, lower concentrations of particles at a smaller radial distance. Significant electric currents arising from that process leads to the precipitation of electrons into the ionosphere, causing aurora light. We also found that a microphysical instability, called tearing, may be triggered in the current sheet, causing further release of magnetic energy. Our simulation results, which are found to be consistent with satellite observations in Earth's ionosphere, shed some light on how substorms progress from the end of growth phase toward onset. Effects of ion slippage that violates frozen‐in flux condition are studied with RCM‐I modelPreonset thin arc and westward electrojet may occur associated with sheets of strong field‐aligned currents and horizontal ionospheric flowsRapid creation of bubble‐blob leads to a bigger tailward Bzhump than growth phase, a feature related to potential tearing instability Effects of ion slippage that violates frozen‐in flux condition are studied with RCM‐I model Preonset thin arc and westward electrojet may occur associated with sheets of strong field‐aligned currents and horizontal ionospheric flows Rapid creation of bubble‐blob leads to a bigger tailward Bzhump than growth phase, a feature related to potential tearing instability

Longitudinal expansion of brightening of the substorm onset arc likely represents longitudinal expansion of the substorm onset instability. We used Poker Flat radar campaign auroral observations to identify seven substorm onsets seen east or west of the radar that expanded into the radar field‐of‐view. The radar observations were used to show flows relative to the expanding onset aurora. Flow enhancements were found equatorward (poleward) of the westward (eastward) longitudinal expansion of the onset arc brightening that agree with plasma sheet bubble modeling. This lends significant additional supporting evidence that an incoming reduced entropy flow channel leads to onset. As a flow channel enters the auroral oval (plasma sheet) from the polar cap (magnetotail lobes) and starts to move equatorward (earthward in the plasma sheet) as a bubble, it simultaneously expands azimuthally due to the combination of electric drift along dawnward tilted equipotentials and westward magnetic drift of more energetic ions. As the azimuthally expanding bubble reaches near the equatorward boundary of the electron auroral oval, it leads to the onset instability indicated by the auroral beading and associated flows if conditions are appropriate. The instability grows in the initial onset region, as it simultaneously extends longitudinally with the continued azimuthal expansion of the bubble within the plasma sheet. Additionally, after onset, the auroral bulge expands poleward and can lead to a westward traveling surge. We observe strong flows heading toward the front of the surge that appear to bring low entropy plasma, perhaps from the polar cap, into the surge. Longitudinal expansion of brightening of the substorm onset arc likely represents longitudinal expansion of the substorm onset instabilityRadar flow enhancements with auroral onset longitudinal expansion agree with instability due to longitudinal expansion of intruding bubbleWestward traveling surge associated with separate flow channel bringing new plasma to field lines head of expansion phase auroral bulge Longitudinal expansion of brightening of the substorm onset arc likely represents longitudinal expansion of the substorm onset instability Radar flow enhancements with auroral onset longitudinal expansion agree with instability due to longitudinal expansion of intruding bubble Westward traveling surge associated with separate flow channel bringing new plasma to field lines head of expansion phase auroral bulge

About 40 years after the discovery of the substorm, auroral observations were used to propose that substorm onset was triggered in the inner plasmas sheet (equatorward portion of the auroral oval) by an intrusion of the low entropy plasma by plasma sheet flow channels. Longitudinal localization makes such flow channels difficult to observe with sparse spacecraft, but they can be seen in the ionosphere via the broader, two‐dimensional coverage by ground‐based radars. We have analyzed all eight substorm auroral onset events with appropriate auroral and Poker Flat radar viewing during a radar campaign. These, together with five previously analyzed events provide evidence that intruding flow channels play an important role in substorm onsets. That the flux tube integrated entropy of this new plasma must be lower than that of the surrounding plasma implies that the change in the entropy distribution within the plasma sheet gives rises to the substorm onset instability that is seen via the growing auroral and electromagnetic waves seen with substorm auroral onset. Our analysis also adds to previous evidence that the flow channels leading to onset move into the plasma sheet/auroral oval from the open polar cap region of the magnetotail lobes/polar caps, that these flow channels move across the polar cap following their origin near the dayside boundary of the polar cap, and that substantial ground magnetic depressions associated with substorm do not occur with substorm onset but instead occur a few to several minutes later in association with expansion phase streamers. Substorm onsets with appropriate auroral and radar viewing provide further evidence that intruding flow channels lead to substorm onsetsThe reduced entropy of intruding plasma indicates change in plasma sheet entropy distribution gives rises to the substorm onset instabilityFurther evidence that flow channels leading to onset move into the plasma sheet/auroral oval from the polar cap, open‐field‐line region Substorm onsets with appropriate auroral and radar viewing provide further evidence that intruding flow channels lead to substorm onsets The reduced entropy of intruding plasma indicates change in plasma sheet entropy distribution gives rises to the substorm onset instability Further evidence that flow channels leading to onset move into the plasma sheet/auroral oval from the polar cap, open‐field‐line region