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[1] We present observations of the auroral expansions during two substorms, focusing on multistage intensifications and the morphology of the poleward boundary, and relate these auroral observations to the local plasma convection and plasma sheet dynamics. The observations are made by meridian scanning photometers and an all-sky camera (ASC) at Ny Alesund, Svalbard (76° magnetic latitude (MLAT)), an ASC in Lovozero, Russia (64° MLAT), the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain in Svalbard and Scandinavia, the HYDRA instrument on Polar located at the inner edge of the plasma sheet, particle detectors on DMSP F13 and DMSP F14 traversing the ionospheric projection of the plasma sheet, and the CUTLASS Finland HF radar. In each substorm the aurora between 70° and 80° MLAT consisted of two branches separated by ∼5° in MLAT. The higher-latitude branch (at ∼75°–78°MLAT) was subject to a sequence of short-lived (∼1–2 min) intensifications, so-called “poleward boundary intensifications” (PBIs), recurring at ∼3-min intervals. Subsequent to each brightening, auroral forms traveled equatorward at a speed of ∼1.0–1.5 km s−1. On Polar the PBIs are related on a one-to-one basis with injections of electrons in the 5- to 20-keV energy range at the inner edge of the equatorial plasma sheet with predominantly a trapped distribution, delayed by ∼5 min. Electron precipitation within 60°–77° MLAT, corresponding to a large radial extent of the plasma sheet, is documented by DMSP flights in the 1800–2000 magnetic local time (MLT) sector. In discussing the branches of the high-latitude aurora within the context of current understanding of the relation of bursty bulk flows to substorm expansion phase dynamics, we note the following: (1) the initial auroral breakup located at 63°–64° MLAT near the equatorward edge of plasma sheet precipitation, which was followed by (2) two successive brightenings/auroral expansions appearing within 72°–74° MLAT/∼2100 MLT, separated by 14 min, (3) a 20-min-long brightening sequence in the poleward auroral branch (75°–78° MLAT), consisting of six discrete events (PBIs) within the boundary plasma sheet precipitation, and (4) the presence of auroral vortex motion/strong field-aligned current sheets in some of these PBIs, which were accompanied by (5) electron injections at the inner edge of the plasma sheet, (6) brightenings of the lower-latitude auroral branch when equatorward moving auroral forms (EMAFs/streamers) arrive there, and (7) localized bursts of equatorward ionospheric convection at speeds of 0.5–1 km s−1 in the latitude range of the EMAFs/streamers. The documented associations between PBIs/EMAFs, plasma sheet injections, and the local convection events are explained in terms of a substorm scenario involving bursty bulk flows in the late expansion phase.

We document the correspondence between different categories of auroral forms/ activities and particle precipitation/plasma convection regimes in the 1100-1300 MLT/ 70°-77° MLAT sector during strongly negative interplanetary magnetic field B y (-7 nT) and smaller (2 to -4 nT) B z conditions. Ground observations of the aurora are combined with data from two overflights by the spacecraft DMSP F 11 and F 12 on 8 January 1999. We describe the dynamics of the aurorae in the midday sector corresponding to the particle precipitation regimes which in the literature are designated (1) the dayside extension of the central plasma sheet, (2) void, (3) the electron edge, (4) the low-latitude boundary layer (LLBL), and (5) the plasma mantle. The aurorae observed in the corresponding latitude regimes are the following: (1) the pulsating, diffuse, green line-dominated plasma sheet aurora (type 3), (2) a ∼100 km wide latitudinal gap in auroral emission, (3) and (4) the type 1 cusp aurora characterized by recurrent (T = 2-3 min) equatorward boundary intensifications (EBIs), and (5) poleward moving auroral forms. Special focus is placed on (1) the association between EBIs and the electron edge/ion cutoff observed at the equatorward boundary of the LLBL precipitation and (2) the gap in the auroral emission profile located on its equatorward side. The latter corresponds to the observed strongly depleted field-aligned electron fluxes (void), which in our view is due to the loss of plasma sheet particles along newly opened field lines. Thus, in this case study we document the auroral fine structure corresponding to the open LLBL, with its electron edge, as well as the signature of the escape of magnetospheric electrons along the adjacent field lines on its equatorward/inward side.