Enhancements in relativistic electron fluxes in the outer radiation belt often occur following magnetic storms and have been suggested to result from resonant interactions with enhanced whistler‐mode chorus emissions observed on the dawnside. Using observations during a period of persistent high‐speed, corotating, solar wind streams, we investigate the aspects of solar wind‐magnetosphere coupling that lead to these enhanced chorus emissions. We find that relativistic electron energization occurs in association with large‐amplitude Alfvén waves within the high‐speed streams. These waves last for multiday periods and cause multiday intervals having intermittent periods of significantly enhanced convection. The enhanced convection periods are followed by repetitive substorm onsets caused by the Alfvén wave related repetitive reductions in convection. We use these substorm onsets, identified using geosynchronous particles and midlatitude H components, as indicators of preceding periods of enhanced convection and of reductions in convection. We use ground‐based chorus observations from the Halley station VLF/ELF Logger Experiment (VELOX) instrument to indicate magnetospheric chorus intensities. These data give evidence that the periods of enhanced convection that precede substorm expansions lead to the enhanced dawnside chorus wave. We also see that the enhanced solar wind densities n_sw ahead of high‐speed streams are associated with significant energetic electron loss at geosynchronous orbit and that the subsequent flux increases appear to not begin until n_sw drops below ∼5 cm⁻³ even if the solar wind speed increases earlier. The sequence of loss during the leading interval of high n_sw, followed by energization during high‐speed streams, occurs whether or not the high n_sw interval leads to a magnetic storm.

We performed a comprehensive statistical study of electromagnetic ion cyclotron (EMIC) waves observed by the Van Allen Probes and Exploration of energization and Radiation in Geospace satellite (ERG/Arase). From 2017 to 2018, we identified and categorized EMIC wave events with respect to wavebands (H⁺ and He⁺ EMIC waves) and relative locations from the plasmasphere (inside and outside the plasmasphere). We found that H⁺ EMIC waves in the morning sector at L > 8 are predominantly observed with a mixture of linear and right‐handed polarity and higher wave normal angles during quiet geomagnetic conditions. Both H⁺ and He⁺ EMIC waves observed in the noon sector at L ∼ 4–6 have left‐handed polarity and lower wave normal angles at |MLAT| < 20° during the recovery phase of a storm with moderate solar wind pressure. In the afternoon sector (12–18 MLT), He⁺ EMIC waves are dominantly observed with strongly enhanced wave power at L ∼ 6–8 during the storm main phase, while in the dusk sector (17–21 MLT) they have lower wave normal angles with linear polarity at L > 8 during geomagnetic quiet conditions. Based on distinct characteristics at different EMIC wave occurrence regions, we suggest that EMIC waves in the magnetosphere can be generated by different free energy sources. Possible sources include the freshly injected particles from the plasma sheet, adiabatic heating by dayside magnetospheric compressions, suprathermal proton heating by magnetosonic waves, and off‐equatorial sources.

Recent THEMIS spacecraft observations have shown longitudinally narrow, earthward moving, plasma sheet flow channels with large, abrupt magnetic field dipolarizations that have been referred to as dipolarization fronts. We undertake a detailed study of the subset of these events that had good quality auroral imaging. We find that many occurred during the substorm expansion phase after onset (determined by auroral intensification) and were related to auroral streamers. Both the fronts' relation to streamers and their plasma sheet changes are similar to what has been previously shown for other plasma sheet flow channels and to other flow channels seen during the expansion phases of the events considered here. Surprisingly, the dipolarization fronts examined made major contributions to substorm onset ground and space magnetic signatures. These include substorm near‐Earth dipolarization, which indicates that the current wedge may develop via a series of well‐defined, narrow, post‐onset wedge‐like structures. These also include the auroral zone ground magnetic field, which showed only modest responses to the onsets but abrupt, large responses to post‐onset dipolarization‐front‐related streamers, and midlatitude positive bays, which started near the time of streamer formation. Our observations suggest that traditional magnetic signatures of substorm onset, such as near‐Earth dipolarization, auroral‐zone negative bays, and midlatitude positive bays, may misidentify the expansion phase onset determined by auroral intensification by up to 10s of minutes for some events. It should be interesting in the future to determine the generality of the above relations between dipolarization front signatures and onset signatures.

Observational properties of throat aurora are investigated in detail by using 7 year continuous auroral observations obtained at Yellow River Station (magnetic latitude 76.24°N). From our inspection, throat aurora is often observed under the condition of stripy diffuse aurora contacting with the persistent discrete auroral oval, and the long‐period throat aurora observations generally consist of intermittent subsequences of throat aurora brightening followed by poleward moving auroral form and throat aurora dimming. We also noticed that the orientation of throat aurora is aligned along the ionospheric convection flow, and its local time distribution shows clear dependence on the interplanetary magnetic field (IMF) B_y component. These observational results indicate that factors inside the magnetosphere may play important role on occurrence of throat aurora. We thus suggest that throat aurora may present the ionospheric signature of redistribution of reconnection rate on the magnetopause by cold magnetospheric plasma flowing into the reconnection site. In addition, we also found that the occurrence rate of throat aurora clearly decreases with increase of the IMF cone angle (arccos(|B_x|/B)), which is very similar with the occurrence rate of high‐speed jet (HSJ) observed in magnetosheath depending on the IMF cone angle. This is suggested as that the HSJs occurred outside the magnetosphere may also play important role for generation of throat aurora by triggering magnetopause reconnection or by direct impacting. Although further studies are needed to clarify how the throat auroras are generated in detail, the relevant observations about throat aurora have presented important implications on a variety open questions, such as distribution and generation of cold plasma structures in the outer magnetosphere, magnetopause deformation, and possible relation between HSJ and reconnection.

Magnetosheath high‐speed jets (HSJs) are dayside upstream transient disturbances with enhanced flow velocity and dynamic pressure. They are associated with significant magnetopause perturbations, ultralow frequency waves in the dayside magnetosphere, and localized flow enhancements in the ionosphere. However, whether HSJs have corresponding dayside aurora signatures is still an open question. If auroral signatures are found, 2‐D structure and evolution of HSJ effects on the magnetosphere can be imaged in a much higher precision than possible by other means. In this study, eight HSJ events are identified by the THEMIS satellites located within ±1 MLT of the center of the field‐of‐view of the South Pole station all‐sky imager. In all of those cases, the HSJs are observed to have a nearly one‐to‐one relationship with individual localized discrete/diffuse auroral brightenings. The azimuthal size of HSJ‐related diffuse aurora signatures is ~800 km at 230‐km altitude in the ionosphere and ~3.7 Re in the magnetosphere, which is slightly larger but of the order of the cross‐sectional diameter of HSJs (~1 Re). Furthermore, most of those aurora signatures have azimuthal motion, whose magnitude and direction agree with magnetosheath background flows. This study for the first time shows high‐resolution, two‐dimensional observations of localized structure and fast propagation of precipitation due to magnetosheath HSJs. We conclude that magnetosheath HSJs can have substantial impacts on the coupled magnetosphere‐ionosphere system, causing localized magnetospheric compression and aurora brightening, in a similar manner to responses during interplanetary shocks except with a smaller scale size.