Ionospheric flows associated with a transpolar arc

A theta aurora is observed in the northern hemisphere on January 21, 1982, using the auroral imaging instrumentation on board the DE 1 spacecraft. The period of observations is from 1807 to 2121 UT. The transpolar arc is observed to advance toward the dusk sector, in the direction of the interplanetary magnetic field By component. Coincidentally, the intersection between the arc and the auroral oval near local midnight passes through the field of view of the STARE coherent radar system, thereby providing the opportunity for measurements of the spatial pattern of ionospheric electron drift velocities. The convection flow in the arc is directed toward the nightside auroral oval, where it divides into westward and eastward directed flows (eastward and westward electrojets). Sunward flows are not observed along the transpolar arc near midnight. The overall flow pattern is identical to that expected at the Harang discontinuity. Two traversals of the auroral oval and polar cap with DE 2 demonstrate that, as expected, transitions from antisunward to sunward flow are associated with the day side of the transpolar arc. The evolution of the arc and its convective features observed simultaneously on both the dayside and the nightside allow a more complete description of its global configuration than has been made previously.