| Volumes and Issues Contents of Issue 7 |
Event 1 occurs during the end of the recovery phase of a strong substorm. A system of auroral arcs associated with convergent electric field structures, with a maximum perpendicular potential drop of about ~10 kV, and upflowing field-aligned currents with densities of 3 µA/m2 (mapped to the ionosphere), was detected at the boundary between the Plasma Sheet Boundary Layer (PSBL) and the Plasma Sheet (PS). The auroral arc structures evolve in shape and in magnitude on a timescale of tens of minutes, merging, broadening and intensifying, until finally fading away after about 50 min. Throughout this time, both the PS region and the auroral arc structure in its poleward part remain relatively fixed in space, reflecting the rather quiet auroral conditions during the end of the substorm. The auroral upward acceleration region is shown for this event to extend beyond 3.9 Earth radii altitude.
Event 2 occurs during a more active period associated with the
expansion phase of a moderate substorm. Images from the Defense
Meteorological Satellite Program (DMSP) F13 spacecraft show that
the Cluster spacecraft crossed the horn region of a surge-type
aurora. Conjugated with the Cluster spacecraft crossing above the
surge horn, the South Pole All Sky Imager recorded the motion and
the temporal evolution of an east-west aligned auroral arc, 30 to
50 km wide. Intense electric field variations are measured by the
Cluster spacecraft when crossing above the auroral arc structure,
collocated with the density gradient at the PS poleward boundary,
and coupled to intense upflowing field-aligned currents with
mapped densities of up to 20 µA/m2. The surge horn consists
of multiple arc structures which later merge into one structure
and intensify at the PS poleward boundary. The surge horn and the
associated PS region moved poleward with a velocity at the
ionospheric level of 0.5 km/s, following the large-scale poleward
expansion of the auroral oval associated with the substorm
expansion phase.
Keywords. Ionosphere (Ionosphere-magnetosphere interacctions;
Electric fields and currents; Particle acceleration)