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

Abstract

[1] 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.

Generation and characteristics of equatorial plasma bubbles detected by the C/NOFS satellite near the sunset terminator

“…[25] We now compare the current study at high solar activity with the results of Huang et al [2011Huang et al [ , 2012 at low solar activity. There are a number of significant differences in the characteristics of plasma bubbles.…”

“…In contrast, broad plasma depletions are defined as a region over a large longitudinal range (>1000 km) within which the plasma density is greatly decreased and multiple bubbles exist within the depleted region [Huang et al, 2011[Huang et al, , 2012. Previous studies show that broad plasma depletions occurred near dawn under solar minimum conditions [de La Beaujardière et al, 2009].…”

“…An example of the findings of the C/NOFS mission is the simultaneous existence of a series of plasma bubbles over a longitudinal range of 3800 km during solar minimum. These bubbles are generated in the evening sector and evolve into broad plasma depletions near dawn [Huang et al, 2011[Huang et al, , 2012. The studies of Huang et al [2011Huang et al [ , 2012 also show that plasma bubbles during solar minimum can last for 7 h and that the vertical plasma flow inside the bubbles is always strongly upward.…”

“…These bubbles are generated in the evening sector and evolve into broad plasma depletions near dawn [Huang et al, 2011[Huang et al, , 2012. The studies of Huang et al [2011Huang et al [ , 2012 also show that plasma bubbles during solar minimum can last for 7 h and that the vertical plasma flow inside the bubbles is always strongly upward. The solar activity now approaches maximum, and the prereversal enhancement is, in general, quite large and moves the F region to high altitudes.…”

“…C/NOFS can fly across the sunset terminator along the magnetic equator over many orbits, so it can detect whether plasma bubbles can be continuously generated near the sunset terminator, how the occurrence of plasma bubbles depends on longitude, what upward plasma drift is required for the generation of plasma bubbles, and how long the lifetime of the postsunset plasma bubbles is. The broad plasma depletions reported by Huang et al [2011Huang et al [ , 2012 occur as a result of merging of multiple plasma bubbles and form near dawn during solar minimum. In this study, we present new observations of wide regions with very low plasma density and multiple bubbles in the evening sector and examine the mechanism responsible for the generation of these structures.…”