| Two event studies are presented in this paper where intense
convergent electric fields, with mapped intensities up to 1350 mV/m,
are measured in the auroral upward current region by the
Cluster spacecraft, at altitudes between 3 and 5 Earth radii. Both
events are from May 2003, Southern Hemisphere, with equatorward
crossings by the Cluster spacecraft of the pre-midnight auroral
oval.
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) |