The Communication/Navigation Outage Forecasting System (C/NOFS) satellite was launched in 2008, during solar minimum conditions. An unexpected feature in the C/NOFS plasma density data is the presence of deep plasma depletions observed at sunrise at all satellite altitudes. Ionospheric irregularities are often embedded within these dawn depletions. Their frequencies strongly depend on longitude and season. Dawn depletions are also observed in coincident satellite passes such as DMSP and CHAMP. In one example the depletion extended 50° × 14° in the N-S and E-W directions, respectively. These depletions are caused by upward plasma drifts observed in C/NOFS and ground-based measurements. The reason for these upward drifts is still unresolved. We discuss the roles of dynamo electric fields, over-shielding, and tidal effects as sources for the reported depletions.