Direct measurements of the distributions of the thermal positive ions H+ and He+ in the magnetosphere reveal a distinct variability in the position and structure of the plasmapause. Such variability is observed to be most pronounced in the afternoon-dusk local time sector and is indicative of magnetospheric irregularities in the same region. As the OGO 3 satellite made progressive duskside (1500–1900 LT) and nightside (2200–0100 LT) passes during June–July 1966, the duskside plasmasphere was observed to exhibit an outward expansion or bulge, accompanied in some cases by considerable fine structure. In particular, the plasmapause was observed at L positions as distant as L = 7–8 in the afternoon-dusk sector, in contrast to positions near L = 5–6 observed near midnight on the same day and at comparable levels of moderate magnetic activity (Kp≤3). Within the bulge and just above the initial plasmapause, structured plasma recoveries are observed, wherein n(H+) returns to concentrations of the order of 50–100 ions/cm³ over intervals of 0.5–1.5 L. Both the duskside bulge and fine structure are observed to persist during periods of enhanced magnetic activity (Kp = 4–6). The above variability is superimposed on an average diurnal distribution of the plasmapause that is similar in shape to that deduced from whistler data during 1963, although the 1966–1967 results place the plasmapause at a position generally more distant by about 1.5–2 L.
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