The bidirectional particle event of October 12, 1977, possibly associated with a magnetic loop

A fortuitous set of interplanetary and solar events which occured on October 12, 1977, allowed the observations by several interplanetary spacecraft, IMP 7 and 8, of features in charged particle angular distributions which suggest the presence of a closed magnetic loop. This loop must have extended beyond the orbit of earth and have been connected to particle sources and/or mirrors at both ends. This magnetic topology is similar to a suggestion made by Gold (1960). The salient feature of the anisotropies which calls attention to the possible presence of a loop is the occurrence of a trapping‐type electron distribution peaked at 90° to the magnetic field, while the ions are strongly field aligned and bidirectional.