Observations from five separate experiments on the Isis 2 spacecraft have permitted a study of the atmosphere/ionosphere during the December 16--20, 1971, magnetic storm. The data are most complete in the mid-latitude region, thus permitting a study of the SAR arc (subauroral red arc) which developed during the night of December 17--18. Ion composition and temperature, electron temperature, electron density height profiles from the spacecraft to the F region, and the intensity of the 6300-¿ oxygen emission are all presented for the region of interest. It is found that the H+ concentration has sharp gradients near the SAR arc and that the plasma temperature was significantly enhanced over typical nighttime values, reaching nearly 7000¿K at 1400 km on the field line which intersected the arc. To facilitate interpretation of the data presented, we have also solved a system of time dependent equations for atmospheric/ionospheric composition and temperature by using boundary conditions which were selected such that the solutions were in agreement with the observations. From these solutions we are able to assess the influence of (1) the efflux of plasma from the ionosphere to the magnetosphere, (2) the decrease in [O>/[N> at the turbopause, (3) the increase in the loss coefficient as a result of an increase in the vibrational temperature of N2, and (4) the conduction of thermal energy into the ionosphere.