Substorm current wedge driven by plasma flow vortices: THEMIS observations

A multipoint analysis of conjugate magnetospheric and ionospheric flow vortices during the formation of the substorm current wedge (SCW) on 19 February 2008 is presented. During the substorm, four Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft were located close to the neutral sheet in the premidnight region between 9 and 12 R-E geocentric distance, of which three closely (similar to 1-2 R-E) clustered at similar to 23 MLT and one was farther west at similar to 21 MLT. The closely clustered spacecraft were engulfed by a counterclockwise plasma flow vortex, while the single spacecraft recorded a clockwise plasma flow vortex. Simultaneously, a pair of conjugate flow vortices with clockwise and counterclockwise rotation appeared in the ionosphere, as inferred from equivalent ionospheric currents. The counterclockwise space vortex, which corresponded to a downward field-aligned current, was at least similar to 1-2 R-E in diameter and had rotational flow speeds of up to 900 km/s. Current density estimates associated with the formation of the space vortex in the first 30 s yielded 2.8 nA/m(2) (14 mu A/m(2) mapped to the ionosphere), or a total current of 1.1 x 10(5) A. Model calculations based on midlatitude ground magnetometer data show a gradual increase of the field-aligned current, with 1-2 x 10(5) A within the first minute and a peak value of 7 x 10(5) A after 10 min, associated with the SCW, and a matching meridian of the downward current of the SCW and the downward current (counterclockwise) space vortex. The combined ground and space observations, together with the model results, present a scenario in which the space vortices generated the field-aligned current of the SCW at the beginning of the substorm expansion phase and coupled to the ionosphere, causing the ionospheric vortices.