Large-scale quasiperiodic plasma bubbles: C/NOFS observations and causal mechanism

Large-scale periodic plasma bubbles are often observed by ionospheric radars and satellites. The seeding effect of atmospheric gravity waves has been widely used to explain the generation of periodic plasma bubbles. However, it has not been well understood where the seeding process occurs and how a series of plasma bubbles is triggered. In this study, we present the observations of equatorial plasma bubbles by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. We show examples of quasiperiodic plasma bubbles in the post-midnight sector, with nearly equal distance of 800-1000km between adjacent bubbles, in 2008 under deep solar minimum conditions. The bubble chain covered a longitudinal range of similar to 7000km between 00:00 and 04:00 LT. Quasiperiodic plasma bubbles were also measured by C/NOFS in the evening sector in 2011 during the ascending phase of the solar activity, and the longitudinal distance between adjacent bubbles was similar to 500km. We propose a causal mechanism to explain the generation of quasiperiodic plasma bubbles. In this scenario, atmospheric gravity waves are generated near the sunset terminator and initiate the Rayleigh-Taylor instability there. The spatial (longitudinal) periodicity of plasma bubbles is determined by the temporal periodicity of the seeding gravity waves. A period of 15-30min of the seeding gravity waves corresponds to a longitudinal separation of 500-1000km between adjacent bubbles. This mechanism provides a reasonable explanation of the observed quasiperiodic plasma bubbles.