A statistical characterization of plasmasphere density structure and boundary locations

A statistical study of plasmaspheric density profiles and their boundaries is performed, using measurements of core (<50 eV) ions by the retarding ion mass spectrometer (RIMS) on Dynamics Explorer 1. The plasmasphere density profiles are classified into essentially six different categories, indicative of density structure out to the outer boundary of observable cold, isotropic plasma. The most common profiles observed are those which are relatively featureless out to this outer boundary and those which exhibit multiple plateaus and plasmapauses. The multiple plateau profiles occur predominantly in the afternoon and evening sectors, while the “featureless” profiles are most common in the midnight through morning sectors. In the multiple plateau profiles, the average MLT (magnetic local time)‐L locus of the inner “plasmapause” gradients is nearly circular at L = 3–4. Profiles with significant density troughs occur most often in the evening sector, and the troughs are widest in this sector as well. The MLT‐L local time shape of the outer boundary of RIMS‐observed cold ions (termed here low‐energy ion transition, or LEFT) is similar to previously reported average plasmapause shapes, though its bulge region occurs in the afternoon sector. This MLT‐L shape also tends to become more circular with increasing geomagnetic activity. Using simultaneous plasma density measurements from the plasma wave instrument, it is found that the LEIT typically is located between densities of 10 and 100 cm −3 , with an average plasma density there of about 60 cm −3 .