The application of GPS observations to equatorial aeronomy

Routine observations of the ionospherically imposed propagation effects upon GPS satellite signals are available online from the International GPS Service for Geodynamics. With over 200 ground-based stations now reporting data, ionospheric studies ranging from the analysis of single-site observations to the full global network have demonstrated the geophysical science yield possible from this remarkable resource. In this paper we expand on the use of GPS data for comprehensive "regional studies" at low latitudes. Using the GPS observing sites in South America, we show how routine observations are processed to form reliable total electron content(TEC) values capable of describing the latitudinal, longitudinal, local time, and storm time behavior over the full span of the continent. To study the dominant F region structure at low latitudes, the Appleton anomaly, two indices are developed to assess its strength (I-s) and asymmetry (I,). TEC data at 30 s intervals are used to form phase fluctuation indices that capture plasma irregularity patterns at 15-min (f(p)) and hourly (Fp) time resolution. Tests of Fp at Atlantic and Pacific sector locations show them to reproduce accurately all known occurrence patterns for equatorial spread F (ESF). The use of the three indices (I,, I,, and Fp) to formulate predictive capabilities for ESF on the basis of the enhancement or suppression of growth rate indicators was not particularly successful.