The density and temperature structure of the low-energy ion population in the outer plasmasphere is examined by using data from Los Alamos National Laboratory's three-dimensional magnetospheric plasma analyzer at synchronous orbit. We define the ''outer plasmasphere'' as regions of dense, cold plasma observed at synchronous orbit, with no attempt to distinguish between the main plasmasphere or detached plasma regions existing beyond the plasmapause. We find that for moderate to high levels of geomagnetic activity the outer plasmasphere typically has a fine-scale density structure. The amount of variability in the density generally increases with increasing geomagnetic activity, as indicated by Kp, and the most variable intervals are associated with substorm activity. The dense (>10 ions cm-3), cold (≈1 eV), plasmaspherelike plasma is adjacent to, and often interspersed with, low-density (1--10 ions cm-3), warm (2--10 eV), ''troughlike'' plasma regions. We find that occasionally these two temperature populations can coexist, but that most often they do not. The scale size of the fine-scale structure in the dense cold plasma regions often is of the order of 1000 km or less. These observations suggest that fine-scale density structure in the dusk sector of the outer plasmasphere is imposed by penetrating substorm electric fields. ¿American Geophysical Union 1995