Transient reconnection in the cusp during strongly negative IMF By -: art. no. A04204

[1] We investigate transient magnetic reconnection ( flux transfer events, FTEs) in the cusp and its ionospheric signatures. We present in situ observations by the Polar spacecraft of multiple impulsive plasma injections and related magnetopause crossings in the outer stagnant cusp of the northern hemisphere. At this time the interplanetary magnetic field ( IMF) was northward, dominated by strong By ( - 11 nT). Plasma injections are observed in the direction of the magnetic tension force due to the IMF By and resemble the "time of flight'' energy-latitude dispersion. Associated magnetopause crossings exhibit large magnetic field rotation (> 150 degrees) and strong depression of the field magnitude ( down to several percent of the value in the magnetosheath). We use conjugate observations from the Greenland magnetometer network and Sondrestrom incoherent scatter radar together with the Polar UV imager to show that the transient reconnection causes a direct response of cusp auroral brightness and ionospheric convection in the direction consistent with the IMF By polarity. The observations, in space and on the ground, are consistent with a sequence of bulges launched by transient reconnection. We present a qualitative model of the bulge structure that is consistent with most of the Polar observations during the studied interval. Conjugate ground-based observations provide additional evidence for transient reconnection and are consistent with the ground signature of FTEs.