The middle and upper atmosphere (MU) radar of Japan is a 46.5-MHz, pulse-modulated, monostatic, Doppler radar with an active phased-array antenna which consists of 475 crossed yagis. This system has been used primarily, since its initial observations with a partial system in 1983, to observe the coherent backscatter from irregularities in the troposphere, stratosphere, and mesosphere (MST radar). However, this system was also designed to be able to observe the weak incoherent scatter (IS) from the free electrons of the ionosphere. We report here the MU radar IS observations made during the strong geomagnetic storm of 6--8, February 1986. During this period the MU radar observed the echo power (an approximate measure of electron density) simultaneously in four antenna beam positions. Coincident with the rise in the Kp index from 3+ to 6- near 1800 LT (0900 UT) on February 7, the MU radar detected the beginning of several hours of wave activity having a period of 100 min. By correlating the density observations in the different beams and at different altitudes, we were able to compute the phase speed (410 m/s), direction of travel (9¿ west of south), and horizontal (2500 km) and vertical 290--490 km) wavelengths of the wave. The F layer peak density and height oscillated during this nighttime period, and we consider several mechanisms that might cause these effects. We find that the density oscillation was too large to be caused solely by wave dynamics. We suggest that an influx of ionization from the plasmasphere, and a wave-induced modulation of the assimilation of this plasma flux into the ambient density, may have been the cause of the observed density increases. During the following morning (February 8) the F layer density was observed to become increasingly large while the layer peak height dropped, and we also consider the cause for this effect. ¿ American Geophysical Union 1988