Multistage substorm expansion: Auroral dynamics in relation to plasma sheet particle injection, precipitation, and plasma convection

[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 (76degrees magnetic latitude (MLAT)), an ASC in Lovozero, Russia (64degrees 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 70degrees and 80degrees MLAT consisted of two branches separated by similar to5degrees in MLAT. The higher-latitude branch (at similar to75degrees-78degreesMLAT) was subject to a sequence of short-lived (similar to1-2 min) intensifications, so-called poleward boundary intensifications (PBIs), recurring at similar to3-min intervals. Subsequent to each brightening, auroral forms traveled equatorward at a speed of similar to1.0-1.5 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 similar to5 min. Electron precipitation within 60degrees-77degrees 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 63degrees-64degrees MLAT near the equatorward edge of plasma sheet precipitation, which was followed by (2) two successive brightenings/auroral expansions appearing within 72degrees-74degrees MLAT/similar to2100 MLT, separated by 14 min, (3) a 20-min-long brightening sequence in the poleward auroral branch (75degrees-78degrees 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.