The ionospheric footprint of antiparallel merging regions on the dayside magnetopause

The antiparallel merging hypothesis states that reconnection takes place on the dayside magnetopause where the solar and geomagnetic fields are opposi tely directed. With this criterion, we have mapped the predicted merging re gions to the ionosphere using the Tsyganenko 96 magnetic field model, disti nguishing between regions of sub-Alfvenic and super-Alfevnic magnetosheath flow, and identifying the day-night terminator. We present the resulting sh ape, width and latitude of the ionospheric dayside merging regions in both hemispheres, showing their dependence on the Earth's dipole tilt. The resul ting seasonal variation of the longitudinal width is consistent with the co njugate electric fields in the northern and southern cusps, as measured by the SuperDARN HF radars, for example. We also find a seasonal shift in lati tude similar to that observed in satellite cusp data.