During the night of January 20--21, 1989, ionospheric incoherent scatter measurements were made with the MU (middle and upper atmosphere) radar at Shigaraki (geographic latitude 34.85¿N, longitude 136.10¿E; geomagnetic latitude 24.9¿, longitude 204.3¿), Japan, and the electron density profiles in the 180- to 1000-km height range were derived at 8-min intervals. The observations showed the presence of three F region disturbances during the night. During the very large first disturbance, which lasted from 2300 to 0240 LT approximately, the height of maximum electron density Nm increased by 220 km in 2 hours to reach an altitude of 600 km. The other two, smaller disturbances occurred during 0300--0500 LT and 0530--0700 LT approximately. A detailed interpretation of the above F region disturbances is given. Examination of some high- and middle-latitude magnetograms showed the beginning of an intense geomagnetic substorm at auroral latitudes at the start of the first F region disturbance, and a less intense substorm around the starting time of the second F region disturbance. On the basis of this evidence, the first two F region distrubances are interpreted as the result of large vertical drifts of F region ionization due to the substorm-generated east-west electric fields appearing at mid-latitudes. The patterns of h'F variations during this night at five ionospheric stations in Japan support the above interpretation.

Additionally, during the second disturbance the possible presence of a wind perturbation due to the equatorward propagation of a wave disturbance, generated probably by the first major substorm, is indicated by the MU radar data and the ionosonde data. The third F region disturbance is attributed to a neutral wind perturbation associated with a gravity wave traveling equatorward, the wave being generated most plausibly by the Joule heating during the first major substorm. The brief, unusual distortions of the F layer shape, as revealed by MU radar observations, are also interpreted as the consequence of an unusually large convergence of ionization in the bottomside of the F layer due to the unusually large electric field and wind perturbations. Theoretical calculations are made using the MU radar data and F region continuity equation, and the east-west electric field values during the disturbances are quantified. During the first major rise of the F layer during 2300--0115 LT the inferred eastward electric field in the ionosphere at Shigaraki is found to be as large as 6¿1 mV/m; and after this disturbance a westward electric field and/or a poleward meridional wind is found to occur for some time. An eastward electric field of about 3.0 mV/m is found to occur during the second F region disturbance, along with a possible meridional wind perturbation. The large eastward electric field around local midnight is most unusual in both magnitude and direction, because the theoretical models of disturbance electric fields predict a strong westward electric field in this LT sector; this result has important implications for understanding the physics of disturbance electric fields at middle and low latitudes during substorms. ¿American Geophysical Union 1990