This paper describes Super Dual Auroral Radar Network observations of an unusual mesoscale (scale size L≈300 km) structure in the auroral convection pattern. This structure is characterized by an expansion motion of the plasma in a plane perpendicular to the magnetic field and then by an anomalously high velocity divergence. This study describes those observations and eliminates possible artifacts in the data analysis. Because the radar data strongly suggest that the structure is located in the F region, the observations are thus in opposition with the well-known divergence-free motion hypothesis. They are interpreted in terms of an ion demagnetization process, in which the collisionless ions become locally collisional in the F region. ¿ 2000 American Geophysical Union