We present the results of a series of incoherent-scatter radar experiments conducted at Sondrestrom, a high-latitude station (invariant latitude, 73.9¿) over the period August 1994 through August 1998. A novel radar mode was employed that allowed simultaneous observation of the structure of thin ion layers along the geomagnetic meridian and of the accompanying ionospheric electric field; however, only layers with peak densities exceeding ~1¿105 cm-3 could be detected. Layers were observed during 31 out of 85 experiments for a total of 43 distinct events. The overall objective of this investigation was to examine the limits of the current theory of layer formation. Several features are evident in the data: (1) individual layers tend to be patchy in both a spatial and a temporal sense; (2) the direction of the ionospheric electric field when layers were observed is consistent with theory, but for northwesterly fields the layers form ~10 km lower than expected; (3) variation in a layer's altitude along the geomagnetic meridian can often be correlated to corresponding variation in the direction of the electric field; (4) layers tend to appear when the electric field suddenly changes in direction, magnitude, or both and then remain steady; and (5) layers often disappear from the field of view despite the continued existence of a favorable electric field. We suggest that the altitude discrepancy alluded to in item 2 may result from a systematic error in the model for ion-neutral collision frequency widely employed in studies of thin metallic-ion layers. We also suggest that horizontal drift of layer structures may play an important role in explaining items 4 and 5, which are otherwise difficult to reconcile with the prevailing theory. ¿ 2001 American Geophysical Union