Plasma acceleration at the earth's magnetopause - Evidence for reconnection

Observations of high-speed plasma at the magnetopause in agreement with theoretical predictions of magnetic field reconnection are reported. Plasma ion and electron distributions measured by the quadrispherical analyzers on board the ISEE 1 and 2 spacecraft were obtained during the outbound traversal of the subsolar magnetopause. Plasma flow speeds of up to 450 km/sec were observed in the magnetopause layer, in contrast to speeds of 50 to 100 km/sec in the adjacent magnetosheath. The observations agree with the predictions of the reconnection model of the dayside magnetopause, in which the magnetopause is described as a rotational discontinuity, or a large-amplitude Alfven wave. It is noted that the lack of observations of plasma acceleration in most other cases of favorable magnetic field orientation could be a product of the rarity of magnetic recombination, or its small scale and nonstationarity.