Ground and satellite observations of substorm onset arcs

Abstract

Auroral features and associated particles and fields are investigated for a sequence of substorm onsets observed at ∼1900 MLT on 24 October 2000. During a 30‐min interval at 1020–1050 UT, we identified three Pi 2 wave packets with positive H enhancements at midlatitudes and associated auroral brightenings in auroral images obtained by a ground all‐sky camera and the IMAGE FUV imager. The DMSP F15 satellite crossed brightening arcs during the third Pi 2 pulsation in the field‐of‐view of the ground camera at Tixie (66.0° MLAT), Russia. The crossing was 1–2 hours duskside of the main onset local time. The brightening arcs were located in the region 1 upward field‐aligned current system. The most equatorward arc, which brightened just after the second Pi 2 pulsation, was at the equatorward boundary of the region 1 current and in the sunward convection region. The arcs corresponded to inverted‐V accelerated electrons with energies of ∼10 keV. On the basis of these simultaneous ground‐satellite measurements, we discuss substorm onset models of reconnection/flow braking and near‐Earth plasma sheet instabilities.