Whistler evidence of the dynamic behavior of the duskside bulge in the plasmasphere


  • D. L. Carpenter


Approximately 180 days of whistler data for 1963 and 1965 from Eights and Byrd, Antarctica, were reviewed for information on the bulge in the plasmasphere, the region of large plasmapause radius that is frequently found in the 18–21 LT sector. Previous findings on the essential permanance of the bulge and the abruptness of its westward end (involving an increase in plasmapause radius of 0.5–2.5 RE within 15–20 degrees longitude) were verified. The position in local time of the westward end of the bulge depends on the preceding history of substorm activity as evidenced in the auroral electrojet (AE) index. The bulge position was observed in the afternoon during particularly high levels of substorm activity, near 18 LT during more or less steady activity, and near 19–20 LT during periods preceded by 6–8 hours of quieting. The bulge is evidently released during periods of deep prolonged quieting and moves in the direction of the earth's rotation. The observations are interpreted in terms of sunward surges of the bulge plasma during substorms, followed by increased influence of the earth's corotation field during quieting. Several types of whistler evidence indicate shielding of the dayside plasmasphere from substorm convection fields. The plasmapause is not generally coincident with an equipotential of the combined magnetospheric flow, but it may approach an equipotential during relatively steady planetary conditions. Under such conditions the duskside radius of the plasmapause may provide a crude time-averaged measure of the stagnation distance and hence the intensity of the convection electric field in the dusk sector. Values of convection EYSM within the duskside plasmasphere, inferred from the stagnation distance and from motions of the bulge, are ∼1–4 mv/m during substorms, about 0.4–0.6 mv/m between substorms (but during moderate magnetic storms), and of order 0.1 mv/m during prolonged quiet periods. These values are larger by a factor of 3–4 than values of EYSM inferred from tracking cross-L drifts of whistler ducts in the plasmasphere near midnight.