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Based on Astrid-2 satellite data, results are presented from astatistical study on subauroral ion drift (SAID) occurrence. SAIDis a subauroral phenomenon characterized by a westward ionosphericion drift with velocity greater than 1000m/s, or equivalently, bya poleward-directed electric field with intensity greater than30mV/m. SAID events occur predominantly in the premidnight sector,with a maximum probability located within the 20:00 to 23:00 MLTsector, where the most rapid SAID events are also found. They aresubstorm related, and show first an increase in intensity and adecrease in latitudinal width during the expansion phase, followedby a weakening and widening of the SAID structures during therecovery phase. The potential drop across a SAID structure is seento remain roughly constant during the recovery phase.The field-aligned current density and the height-integratedPedersen conductivity distribution associated with the SAID eventswere calculated. The results reveal that the strongest SAIDelectric field peaks are associated with the lowest Pedersenconductivity minimum values. Clear modifications are seen in theionospheric Pedersen conductivity distribution associated with theSAID structure as time evolves: the SAID peak is located on thepoleward side of the corresponding region of reduced Pedersenconductivity; the shape of the regions of reduced conductivity isasymmetric, with a steeper poleward edge and a more roundedequatorward edge; the SAID structure becomes less intense andwidens with evolution of the substorm recovery phase. From theanalysis of the SAID occurrence relative to the mid-latitude troughposition, SAID peaks are seen to occur relatively close to thecorresponding mid-latitude trough minimum. Both these features showa similar response to magnetospheric disturbances, but ondifferent time scales - with increasing magnetic activity, theSAID structure shows a faster movement towards lower latitudesthan that of the mid-latitude trough.From the combined analysis of these results, we conclude that theSAID generation mechanism cannot be regarded either as a purevoltage generator or as a pure current generator, applied to theionosphere. While the anti-correlation between the width and thepeak intensity of the SAID structures with substorm evolutionindicates a magnetospheric source acting as a constant voltagegenerator, the ionospheric modifications and, in particular thereduction in the conductivity for intense SAID structures, areindicative of a constant current system closing through theionosphere. The ionospheric feedback mechanisms are seen to be ofmajor importance for sustaining and regulating the SAID structures. Key words. Ionosphere (mid-latitude ionosphere; electricfields and currents; ionosphere-magnetosphere interactions

Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution