Using the published plots of 15-min interval values of h'F2, the minimum virtual height of the ionospheric F2 region, at five Japanese stations spread over 26¿--45 ¿N (15¿--35 ¿N geomagnetic) and 128¿--142 ¿E, it is shown that there are frequent rapid increases and less frequent decreases of h'F2 by 100--200 km in about 2 hours with near-simultaneous start and end at all five stations in the postmidnight hours and at three to five stations in the premidnight hours. In the months of September and December 1988 and March 1989, about 80 h'F2 perturbations of the above type are identified. The near simultaneity of the h'F2 disturbances at the five stations suggests that an Ē¿B¿ plasma drift due to an east-west electric field is the cause of the h'F2 disturbances. This interpretation is validated by showing that there is a simultaneous presence of bay disturbances in auroral latitude magnetograms over a wide longitude range during almost all the h'F2 increases. It is shown that neutral wave disturbances propagating equatorward even with a speed of 750 m/s cannot account for the h'F2 disturbances.

The study reveals two categories of h'F2 perturbations: (1) h'F2 increases starting near the beginning of the geomagnetic negative bays at auroral latitudes and (2) rapid h'F2 increases starting near the maximum intensity phase of the auroral bays and preceded by h'F2 decreases starting at the onset time of the bay disturbance or a little earlier. Using the vast amount of h'F2 data collected all over the globe, it is possible to quantify and characterize the substorm-related electric fields and their variabilities. Such characterization is very useful in understanding the magnetospheric and the magnetosphere-ionosphere interaction processes involved in the generation of magnetospheric substorms. It is also pointed out that the understanding of F2 region changes during magnetic storms may improve greatly if the large F2 region changes caused by electric fields on the substorm time scale are taken into account. The h'F2 disturbances reported in this study are similar in time scale and appearance to the h'F2 disturbances caused by long-period atmospheric waves; and this suggests the possibility of a mixup of the two effects and the consequent wrong identification on some occasions. ¿ American Geophysical Union 1992