Preliminary Explorer 12 data on protons below 20 kev

We attempted by means of Explorer 12 to obtain the energy spectrum of protons along the trajectory in the energy range 100 ev to 20 kev. The data for the period September 27 through October 4, 1961, were examined in some detail because of the concurrent solar and geomagnetic activities. The proton fluxes in the directions sampled were generally below the threshold of detectability of the instrument. This is an extremely significant result in view of the fact that the magnetometer records clearly demonstrate that the apogee of the satellite was beyond the normal, compressed geomagnetic field. It is concluded that neither a proton ring current nor a sharp Chapman-Ferraro boundary was present, but that a broad, diffuse transition region existed between the geomagnetic field termination and the interplanetary medium.