Statistical occurrence and dynamics of the Harang discontinuity during steady magnetospheric convection

The Harang discontinuity is a longitudinally extended ionospheric signature near midnight of flow reversal from westward to eastward with decreasing latitude. Its occurrence indicates enhanced convection in the magnetotail that requires an upward field-aligned current from the ionosphere due to diamagnetic ion drift. Previous reports using event studies have been conflicting as to the occurrence of the Harang discontinuity during a mode of enhanced magnetotail response called steady magnetospheric convection (SMC). With a comprehensive list of SMC events from 1997 through 2007, we utilize data from the Super Dual Auroral Radar Network and a novel spatial superposition technique to statistically examine the occurrence of the Harang discontinuity during SMC events. We find that the statistical signature of the Harang discontinuity begins before SMC starts and strengthens as the SMC events progress. We also detail the typical size and strength of the Harang discontinuity and find that it is more pronounced during interplanetary magnetic field +B-y conditions.