[1] A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all-sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north-south (N-S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N-S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre-existing open-closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward-transport of the new plasma leads to a near-Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere-ionosphere electrodynamic coupling, which may play an important role in the motion of pre-onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre-onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well-developed region 2 current system within the duskside convection cell, and also a high probability of diffuse-appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

This paper presents observations of postsunset super plasma bubbles over China and adjacent areas during the second main phase of a storm on 8 September 2017. The signatures of the plasma bubbles can be seen or deduced from (1) deep field‐aligned total electron content depletions embedded in regional ionospheric maps derived from dense Global Navigation Satellite System networks, (2) significant equatorial and midlatitudinal plasma bite‐outs in electron density measurements on board Swarm satellites, and (3) enhancements of ionosonde virtual height and scintillation in local evening associated with strong southward interplanetary magnetic field. The bubbles/depletions covered a broad area mainly within 20°–45°N and 80°–110°E with bifurcated structures and persisted for nearly 5 hr (∼13–18 UT). One prominent feature is that the bubbles extended remarkably along the magnetic field lines in the form of depleted flux tubes, reaching up to midlatitude of around 50°N (magnetic latitude: 45.5°N) that maps to an altitude of 6,600 km over the magnetic equator. The maximum upward drift speed of the bubbles over the magnetic equator was about 700 m/s and gradually decreased with altitude and time. The possible triggering mechanism of the plasma bubbles was estimated to be storm time eastward prompt penetration electric field, while the traveling ionospheric disturbance could play a role in facilitating the latitudinal extension of the depletions.

[1] The Harang reversal is a prominent feature frequently observed in the electric and magnetic field patterns in the high-latitude auroral zone and plays an important role in substorm dynamics. A comprehensive set of instruments, including Super Dual Auroral Radar Network (SuperDARN), Defense Meteorological Satellite Program (DMSP), and Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), is used to investigate the relationship between the Harang reversal and substorms. On the basis of nine events that have been analyzed, we find that the Harang reversal forms and becomes well defined during the growth phase. Azimuthal flows equatorward of the Harang reversal, a majority of which are in the subauroral region, enhance during the growth phase. The observations indicate that subauroral polarization streams (SAPS) and the Harang reversal evolve together as part of the growth phase development of the region 2 current system. Furthermore, the substorm auroral onset is seen to occur quite near the center of the Harang flow shear, which suggests that the substorm upward field-aligned current develops there. After onset, the evolution of convection flows in the vicinity of the Harang region depends strongly on their location relative to that of the onset. SAPS flows equatorward of the Harang reversal suddenly increase at the substorm onset; flow shear east of the auroral onset relaxes after the onset; and poleward flows, part of a clockwise vortex, are observed west of the auroral onset after the onset. These observations demonstrate the strong coupling between the Harang reversal evolution and substorm dynamics and suggest that the nightside region 2 physics is closely related to substorm dynamics.

Storm-enhanced density (SED) plumes are prominent ionospheric electron density increases at the dayside middle and high latitudes. The generation and decay mechanisms of the plumes are still not clear. We present observations of SED plumes during six storms between 2010 and 2013 and comprehensively analyze the associated ionospheric parameters within the plumes, including vertical ion flow, field-aligned ion flow and flux, plasma temperature, and field-aligned currents, obtained from multiple instruments, including GPS total electron content (TEC), Poker Flat Incoherent Scatter Radar (PFISR), Super Dual Auroral Radar Network, and Active Magnetosphere and Planetary Electrodynamics Response Experiment. The TEC increase within the SED plumes at the PFISR site can be 1.4-5.5 times their quiet time value. The plumes are usually associated with northwestward E × B flows ranging from a couple of hundred m s À1 to > 1 km s À1. Upward vertical flows due to the projection of these E × B drifts are mainly responsible for lifting the plasma in sunlit regions to higher altitude and thus leading to plume density enhancement. The upward vertical flows near the poleward part of the plumes are more persistent, while those near the equatorward part are more patchy. In addition, the plumes can be collocated with either upward or downward field-aligned currents (FACs) but are usually observed equatorward of the peak of the Region 1 upward FAC, suggesting that the northwestward flows collocated with plumes can be either subauroral or auroral flows. Furthermore, during the decay phase of the plume, large downward ion flows, as large as~200 m s À1, and downward fluxes, as large as 10 14 m À2 s À1, are often observed within the plumes. In our study of six storms, enhanced ambipolar diffusion due to an elevated pressure gradient is able to explain two of the four large downward flow/flux cases, but this mechanism is not sufficient for the other two cases where the flows are of larger magnitude. For the latter two cases, enhanced poleward thermospheric wind is suggested to be another mechanism for pushing the plasma downward along the field line. These downward flows should be an important mechanism for the decay of the SED plumes. IntroductionDuring enhanced geomagnetic activity periods, in particular storms, significant electron density enhancements are often observed in the midlatitude and subauroral region, which are named storm-enhanced densities (SEDs) [Foster, 1993]. Often, SEDs extend northwestward to higher latitudes and form a plume. This plume can occasionally be entrained into the cusp region and then enter the polar cap, where it is termed the tongueof-ionization (TOI) or, if it is not continuous, a polar cap patch, depending on the shape and spatial coverage of the high-density region there [e.g., Foster et al., 2005;Moen et al., 2013]. The SED plume is suggested to be the ionospheric projection of a plasmaspheric plume in the magnetospheric equatorial plane . Within the magnetosphere, plasmaspheric plumes can play...