Are plasma bubbles a prerequisite for the formation of broad plasma depletions in the equatorial <i>F</i> region?

Formation of broad plasma depletions (BPDs) at night in the equatorial F region is understood in association with plasma bubbles. However, we report BPDs that do not show a connection with bubbles. The characteristics of BPDs are investigated using the observations of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite on 31 December 2008, 28 July 2010, and 1 February 2011. On those days, BPDs are detected in the longitude regions where C/NOFS did not detect bubbles prior to the detection of BPDs. The coincident C/NOFS and radar observations over Jicamarca in Peru show the occurrence of BPDs at the time when backscatter echoes are absent and at the height below backscatter echoes. These observations indicate that bubbles are not a prerequisite for those BPDs. The detections of those BPDs can be explained by the uplift of the equatorial F region peak height above the satellite orbit.