Periodic structures in the equatorial ionosphere

The plasma densities detected by the Planar Langmuir Probe (PLP) on board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite have been analyzed for the June 2008 period. This interval, which corresponds to one of the quietest periods in the space era, exhibited broad plasma decreases (BPDs) which we have reported previously. In order to treat the data quantitatively, we have detrended the PLP data by using the International Reference Ionosphere model to remove variations in density due to changes in spacecraft altitude and latitude along the orbit. In this paper we present results of a statistical analysis of C/NOFS detrended plasma densities during June 2008 as well as neutral densities from the Gravity Recovery and Climate Experiment measured during the same period. The results show periodic structures in both charged and neutral species, most evident in limited local times. These structures resemble wave-4 nonmigrating tides. BPDs persist in the detrended data and appear as one of the minima in the periodic signatures, most strikingly on the nightside.