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During a large geomagnetic storm, the electric field from the polar ionospherecan expand far enough to affect the mid-latitude and equatorial electricfields. These changes in the equatorial zonal electric field, called thepenetration field, will cause changes in the meridional ion flows that can beobserved by radars and spacecraft. In general this E × Bion flow near the equator caused by the penetration field duringundershielding conditions will be upward on the dayside and downward on thenightside of the Earth. Previous analysis of the equatorial meridional flowsobserved by CINDI instrument on the C/NOFS spacecraft during the26 September 2011 storm showed that all of the response flows on the daysidewere excess downward flows instead of the expected upward flows. Theseobserved storm-time responses are compared to a prediction from aphysics-based coupled model of thermosphere–ionosphere–inner-magnetosphere inan effort to explain these observations. The model results suggest that theequatorial downward flow could be attributed to a combined effect of theovershielding and disturbance dynamo processes. However, some discrepancybetween the model and observation indicates a need for improving ourunderstanding of how sensitive the equatorial electric field is to variousmodel input parameters that describe the magnetosphere–ionosphere couplingprocesses

Storm-time meridional flows: a comparison of CINDI observations and model results