| Based on Astrid-2 satellite data, results are presented from a
statistical study on subauroral ion drift (SAID) occurrence. SAID
is a subauroral phenomenon characterized by a westward ionospheric
ion drift with velocity greater than 1000m/s, or equivalently, by
a poleward-directed electric field with intensity greater than
30mV/m. SAID events occur predominantly in the premidnight sector,
with a maximum probability located within the 20:00 to 23:00 MLT
sector, where the most rapid SAID events are also found. They are
substorm related, and show first an increase in intensity and a
decrease in latitudinal width during the expansion phase, followed
by a weakening and widening of the SAID structures during the
recovery phase. The potential drop across a SAID structure is seen
to remain roughly constant during the recovery phase.
The field-aligned current density and the height-integrated
Pedersen conductivity distribution associated with the SAID events
were calculated. The results reveal that the strongest SAID
electric field peaks are associated with the lowest Pedersen
conductivity minimum values. Clear modifications are seen in the
ionospheric Pedersen conductivity distribution associated with the
SAID structure as time evolves: the SAID peak is located on the
poleward side of the corresponding region of reduced Pedersen
conductivity; the shape of the regions of reduced conductivity is
asymmetric, with a steeper poleward edge and a more rounded
equatorward edge; the SAID structure becomes less intense and
widens with evolution of the substorm recovery phase. From the
analysis of the SAID occurrence relative to the mid-latitude trough
position, SAID peaks are seen to occur relatively close to the
corresponding mid-latitude trough minimum. Both these features show
a similar response to magnetospheric disturbances, but on
different time scales - with increasing magnetic activity, the
SAID structure shows a faster movement towards lower latitudes
than that of the mid-latitude trough.
From the combined analysis of these results, we conclude that the
SAID generation mechanism cannot be regarded either as a pure
voltage generator or as a pure current generator, applied to the
ionosphere. While the anti-correlation between the width and the
peak intensity of the SAID structures with substorm evolution
indicates a magnetospheric source acting as a constant voltage
generator, the ionospheric modifications and, in particular the
reduction in the conductivity for intense SAID structures, are
indicative of a constant current system closing through the
ionosphere. The ionospheric feedback mechanisms are seen to be of
major importance for sustaining and regulating the SAID structures.
Key words. Ionosphere (mid-latitude ionosphere; electric
fields and currents; ionosphere-magnetosphere interactions) |