Auroral dynamics in relation to plasma sheet particle injections during substorm expansions

We present multipoint observations of two substorms, focussing on the multi-stage latitudinal expansions of the aurora with intensifications of its poleward boundary, and relating these to plasma sheet dynamics. The observations were made by meridian scanning photometers and an all-sky camera at Ny Alesund, Svalbard, situated at 76^o magnetic latitude, magnetometers of the IMAGE array in Svalbard-Scandinavia, the HYDRA instrument on the Polar spacecraft, crossing the inner edge of the equatorial plasma sheet, particle detectors on the DMSP F13 and F14 spacecraft traversing the ionospheric projection of the plasma sheet, and the CUTLASS Finland HF radar. The high-latitude branch of the aurora (at ~75^o--78^o MLAT) was subject to short-lived ( ~1-2 min) intensifications, so-called ``poleward boundary intensifications" (PBIs). Subsequent to each of these brightenings auroral forms travelled equatorward at an estimated speed of 1.0--1.5 km s^-1. The BPIs are related on a one-to-one basis with injections of electrons in the 5-20 keV energy range at the inner edge of the equatorial plasma sheet, delayed by ~5 min.