Electron densities and temperatures as well as omnidirectional fluxes of hyperthermal positive ions (E>28 eV) were measured by means of spherical Langmuir probes aboard Injun 5 at altitudes of >2000 km during a major geomagnetic storm. The electron density in the mid-latitude trough decreased and the temperatures increased during the early phases of the storm. As the strom progressed, the trough's position moved equatorward, plasma depletions being observed to an invariant latitude of 40¿ during the early recovery phase. Hyperthermal ions detected in the vicinity of the trough were ring current particles reaching to the satellite's altitude. The minimum thermal electron densities in the trough were found to be within 1¿ of latitude of the maximum ring current flux. Maximum electron temperatures were measured several degrees equatorward of this position. Injun 5 electron temperatures are compared with those measured by Alouette 2 over the reported positions of stable auroral red arcs. The observations are also used to evaluate further a method dveloped Cole (1970) for calculating the positions of the ring current by using ground magnetic fluctuations.