FORMATION AND ERUPTION OF SUN-ALIGNED ARCS AT THE POLAR-CAP AURORAL OVAL BOUNDARY

This paper reports on results from a study of the poleward edge of the auroral oval in the morning sector using a comprehensive blend of in situ and ground-based measurements. Three rockets, equipped to measure electric and magnetic fields, energetic particles, and plasma density flew into an auroral display whose dynamical features were recorded w ith a digital image intensified all-sky camera as well as with an inco herent scatter radar. In addition, a number of DMSP satellite measurem ents bracketed the launch time. Evidence is presented here that in a c ondition of declining magnetic activity Sun-aligned arcs are injected into the polar cap at velocities similar to 7 km/s from locations of p eriodic brightening along the morningside of the auroral oval. The mul tipoint in situ measurements allow some separation of temporal and spa tial effects and strongly suggest a poleward contraction of the convec tion pattern of about 0.25 degrees INVL in 70 s. The most equatorward of the two brightest arcs studied erupted into a region which already was characterized by strong sunward convection. The most poleward, how ever, pushed into a region that had been convecting in an antisunward direction at velocities exceeding 1 km/s less than 2 min earlier, and it is likely that sunward convection subsequently pertained poleward o f that are as well. We believe that these events mark the reconfigurat ion of the magnetosphere into a system characterized by a smaller pola r cap.