Plasma and electric field boundaries at high and low altitudes on July 29, 1977

The IMS event of July 29, 1977, was the topic of a coordinated data analysis workshop in October 1979. During this workshop the hot plasma observations obtained by several spacecraft (GEOS 1, ATS 6, S3‐3, and AE‐C) both at high and low altitudes were compared. The high‐altitude data show that the magnetopause was compressed to between 6.6 and 7.0 R E near 1300 MLT. The plasma ion composition measured, at high altitudes outside the plasmasphere, by GEOS 1 was seen to be primarily O + . These heavy ions were also observed at low altitudes outside the plasmasphere. Both GEOS 1 and S3‐3 observations show that at times these ions were found well below the plasmapause inside the plasmasphere. Comparisons of the low‐altitude (S3‐3) plasma and DC electric field data show that the outer limits of the plasmasphere was not always corotating at the low L shells ( L ≲ 3) on this disturbed day. The corotation boundary, the estimated plasmapause boundary, and the boundary of the inner edge of plasma sheet ions were usually found to be at the same position. The inner edge of the plasma sheet electrons near dawn and dusk, plus the polar cap boundaries, were determined from the low‐altitude data. The inner edge of plasma sheet electrons was observed at higher latitudes ( L shells) than the plasmasphere boundary during disturbed times. This inner edge of the plasma sheet showed a strong dawn to dusk asymmetry. These inner edges of the plasma sheet ions and electrons were found to be in relatively good agreement with the predictions of the Rice University convection model (Wolf et al., this issue). The inner edge of the plasma sheet electrons was found to be in good agreement with inner edge of the >80‐keV ring current ion precipitation. The S3‐3 data showed that late on July 29 the auroral precipitation extended above Λ ∼ 85.7° on the nightside. At the same time the inner edge of the ring current and plasma sheet also moved to high latitudes (Λ ∼ 72°), reflecting an apparent inflation of the magnetosphere.