Plasma sheet ion injections into the auroral bulge: Correlative study of spacecraft and ground observations

Multiple and sporadic time-of-flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy-dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection-induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy-dispersed ion beams.

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