A post sunset bubble manifested by total electron content depletion was observed at midlatitudes (~30¿--34¿N, ~130¿--134¿E) during the main phase of a storm on 12 February 2000. With loss of lock and the rate of the total electron content index maps, the bubble was seen to bifurcate at its early growth phase. The upward drift speed was observed ~300 m/s at ~2150 km, and decreasing with increasing altitude and time. The bubble had unusually large latitudinal extension reaching midlatitude of 36.5¿N (31.5¿N magnetic latitude), indicating an apex height of ~2500 km. In process of the evolution, the bubble drifted eastward at a speed of ~50 m/s. The F region peak height and density obtained by a meridional ionosonde chain suggested a prompt penetrating magnetospheric electric field helped to trigger the super bubble.