Ion densities Ni and electron temperatures Te measured on board Cosmos 900 satellite (launched at March 30, 1977, into near-polar circular orbit: i=83¿, h=500 km, orbital period=94 min) are used to study the behavior of the ionosphere during the prestorm, initial, and main phases of the magnetic storm of December 1--2, 1977. The spatial resolution of the measurement was ≤0.3¿ latitude for Te and ≤0.1¿ latitude for Ni. During this period, 27 orbits were recorded in the onboard large-storage memory. This enabled detailed study of the positions and latitude profiles of midlatitude ionospheric trough and subauroral electron temperature enhancement (SETE) in the midnight local time sector. Simultaneous proton density data measured on the high-altitude satellites Prognoz 5 and Prognoz 6 confirm that the poleward boundary of the Te peak corresponds to the ionospheric magnetic field projection of the high-altitude plasmapause density knee. During the main phase of a geomagnetic storm the polar edge of the SETE becomes very steep. It coincides with an equally steep poleward edge of the midlatitude ionospheric trough and with the position of the newly forming density gradient of the high-altitude plasmapause. The sequence of Cosmos 900 observations clearly shows how the nighttime midlatitude ionospheric trough fills up as well as how the subauroral electron temperature enhancement and the outer layers of the plasmasphere are eroded during a geomagnetic storm. These results shed new light on the formation of the plasmapause in the postmidnight sector and on the time-dependent electric field distribution in the nightside sector before and during a geomagnetic storm.¿ 1997 American Geophysical Union