Conditions for intense ionospheric storms expanding to lower midlatitudes

[1] For studying conditions of intense ionospheric storms at lower midlatitudes, several storms that occurred in March and April 2001 were compared. In the two largest magnetic disturbances that occurred during this period, the disturbance (Dst) index reached -387 and -271 nT. The two largest ionospheric storms occurred in response to other weaker magnetic disturbances, in which Dst reached -149 and -102 nT. For the largest ionospheric storms on 21 March 2001 and 22 April 2001, a sequence of ionospheric and thermospheric disturbances was analyzed. We found that a nighttime disturbance dynamo plays an important role in expanding ionospheric storms to lower latitudes. When an eastward disturbance dynamo electric field forms after midnight, the ionospheric layer is uplifted, and the ion drag is reduced. The storm-time equatorward surge effectively reaches lower-latitude regions without decaying under the condition of reduced ion drag. The equatorward surge pushes up the ionosphere, in turn, and the ion drags might be further reduced. The large event-to-event variation of ionospheric storms might be partly owing to such a positive feedback process in addition to the well-known local time effect.