Explorer 12 observations of the magnetospheric boundary and the associated solar plasma on September 13, 1961

Phenomena at and near the boundary of the magnetosphere during the prolonged initial phase of a mild magnetic storm on September 13, 1961, were observed with a variety of magnetic field and particle detectors in Explorer 12. There was an abrupt termination of the regular geomagnetic field and a coincident discontinuity in the intensity of trapped particles at a geocentric radial distance of 8.2 earth radii. A mean directional energy flux of the order of tens of ergs (cm/sup 2/ sec ster)/sup -1/ was observed outside the boundary. This energy flux is interpreted as being due to an omnidirectional intensity of the order of 10/sup 10/ (cm/sup 2/ sec)/sup -1/ of electrons having energies of a few kev. From the Chapman- Ferraro point of view, these electrons are believed to be the observable component of a piston' of quasi-thermalized plasma, of the order of 10,000 km thick, which acted as the pressuretransmitting agent between the directional solar wind and the geomagnetic field. Possible relationships between these observations and the third radiation belt' of Gringauz are considered. 29 references. (auth)