Abstract A sub-auroral region of ionospheric irregularities with a well-defined equatorward boundary produces sharp transitions in the scintillation observed on VHF signals from the beacon satellite BE-B recorded at three New Zealand stations. Triangulation shows that the irregularities are located at heights between 240 km and 600 km, with about half of the values between 300 km and 400 km; the boundary tends to be aligned along geomagnetic latitude and lies in the range 50°S–55°S geomagnetic latitude. Sharp boundaries are observed mainly between ground sunset and local midnight; their occurrence shows a minimum in winter and correlates weakly with magnetic activity.