Evolution of equatorial ionospheric plasma bubbles and formation of broad plasma depletions measured by the C/NOFS satellite during deep solar minimum

Abstract

An unexpected feature revealed by the measurements of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite is the presence of broad plasma depletions in the midnight–dawn sector during deep solar minimum. It has not been well understood what causes the broad plasma depletions and how equatorial plasma bubbles are related to the broad depletions. In this paper we present the C/NOFS measurements of equatorial plasma bubbles and broad depletions in a few cases. The ion density perturbations and enhanced ion vertical velocity are first identified in the topside F region at ∼2200 LT, suggesting that the plasma bubbles start to form earlier at lower altitudes. The observations show that the plasma bubbles observed in the midnight–dawn sector may originate in the evening sector. The plasma bubbles continue growing for more than 3.3 h, and the decay time of the bubbles is also longer than 3.3 h. The continuous growth of the plasma bubbles in the evening sector and the slow decay after midnight determine that most plasma bubbles become fully developed and are easily detected in the midnight–dawn sector. The plasma flow inside the bubbles remains strongly upward throughout the entire nighttime. We propose the following mechanism for the generation of wide plasma bubbles and broad depletions. A series of plasma bubbles is generated through the Rayleigh‐Taylor instability process over a large longitudinal range. These plasma bubbles grow and merge to form a wide bubble (width of ∼700 km as observed), and multiple regular and/or wide bubbles can further merge to form broad plasma depletions (thousands of kilometers in longitude). The ion vertical drift inside each plasma bubble is driven by the polarization electric field and remains large after the bubbles have merged. This mechanism provides a reasonable interpretation of the large upward ion drift velocity inside the broad depletion region.