Radar and Optical Measurements of Equatorial Plasma Depletions

The primary focus of the recently launched Air Force Communication/Navigation Outage Forecasting System (C/NOFS) satellite is to quantify and forecast ionospheric irregularities responsible for the development of equatorial spread F that can severely affect communication and navigation systems. In support of this mission goal, we present new measurements and analyses using previously obtained ground-based radar and optical measurements from two important equatorial sites. Using available data from the Jicamarca JULIA and incoherent scatter radar observations from 1996 to 2006, we have studied the initial development of equatorial spread F from Peru (11.95°S, 76.87°W) over a full 11 year solar cycle. Detailed analysis show that onset heights and peak heights of radar plumes increase with increase in solar activity, as previously suggested from case studies. In contrast, investigations of spread F onset times show a little variation with solar activity, while onset times of radar plumes decrease from solar minimum to moderate conditions and then remain nearly constant during solar maximum. In addition to this study, we have also made novel investigations of spread F optical signatures (termed depletions) from Christmas Island (2°N, 157.4°W) in central Pacific region using CCD image measurements of the thermospheric OI 630 nm airglow emission. The measurements were obtained using a USU all-sky imager from Sep.14 to Oct 2, 1995, under solar minimum conditions similar to current levels that C/NOFS is measuring. We have analyzed the zonal velocities of the plasma depletions and their horizontal scale sizes. Large variations in the day-to-day spatial characteristics and the zonal velocities were found. We have proposed new measurements from equatorial Brazil in coordination with C/NOFS to investigate bubble dynamics and associated atmospheric conditions.