An unusually comprehensive set of ionospheric, particle, optical and magnetic measurements were made during simultaneous Isis 1 and 2 passes on December 14, 1971, at about 0520 and 0720 UT, when the northern magnetic polar ionosphere was in darkness to almost the maximum possible extent. During the first part of the day, magnetic activity was very low, but substorm activity began between the two sets of observations, and a westward surge is seen in the optical data of the second set. The results show that while particle precipitation plays a role in controlling the density of the F layer and the topside ionosphere, the details of the horizontal drift associated with magnetospheric convection must also be important. Before the substorm activity there was a region of low F layer density in the central polar cap which must have been in a stagnant part of the convection pattern. This region was subsequently filled in with ionization, evidently by convection that penetrated more poleward than before. In the topside the height of transition between O+ and light ions was found to be much lower in winter than in summer, and it was found to be lower equatorward of the polar cap boundary than in the polar cap. A detailed correspondence between electron precipitation and optical emissions was found.