Latitudinal transitions from low to high Doppler spectral width in backscatter measured by the Super Dual Auroral Radar Network (SuperDARN) are now routinely used as proxies for the polar cap boundary (PCB) in the cusp-region ionosphere. In this paper we perform a statistical study of the nature of similar spectral width transitions at other magnetic local times (MLTs). This analysis illustrates that these latitudinal spectral width transitions exist at all magnetic local times, and that the latitude, gradient, and amplitude of the transitions vary systematically with MLT. In particular, the probability of a transition occurring at any latitude, identified independently in each MLT sector, is continuous with MLT from the cusp, through the morning sector, to the nightside. This suggests that the transition represents the PCB, as this is known to be what it represents in the cusp region. However, the picture in the afternoon sector (12:00-18:00 MLT) is more complex with no clearly preferred transition latitudes.

Key words. Ionosphere (ionosphere-magnetosphere interactions; instruments and techniques). Magnetospheric physics (magnetopause, cusp, and boundary layers.)