THE FINE-SCALE STRUCTURE OF THE OUTER PLASMASPHERE

The density and temperature structure of the low-energy ion population in the outer plasmasphere is examined by using data from Los Alamos N ational 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 t o distinguish between the main plasmasphere or detached plasma regions existing beyond the plasmapause. We find that for moderate to high le vels of geomagnetic activity the outer plasmasphere typically has a fi ne-scale density structure. The amount of variability in the density g enerally increases with increasing geomagnetic activity, as indicated by Kp, and the most variable intervals are associated with substorm ac tivity. The dense (>10 ions cm(-3)), cold (approximate to 1 eV), plasm aspherelike plasma is adjacent to, and often interspersed with, low-de nsity (1-10 ions cm(-3)), warm (2-10 eV), ''troughlike'' plasma region s. We find that occasionally these two temperature populations can coe xist, but that most often they do not. The scale size of the fine-scal e structure in the dense, cold plasma regions often is of the order of 1000 km or less, These observations suggest that fine-scale density s tructure in the dusk sector of the outer plasmasphere is imposed by pe netrating substorm electric fields.