Analyses of solar activity effects on the low-latitude ionosphere

Long-term measurement datasets have recently become available from several satellite missions. The TOPEX and JASON satellites obtain the total electron content (TEC) from ~1200 km orbit and the DMSP satellites measure the in-situ ion densities at ~800 km, while the GUVI instrument onboard the TIMED satellite retrieves the electron density profile below ~550 km altitude. TOPEX, JASON, and TIMED are slowly precessing satellites that are able to obtain complete local time coverage approximately every two months. The DMSP satellites are sun-synchronous and in situ sample the ionosphere at 4 or more discrete local times. In addition, the SSUSI instrument onboard DMSP F16 obtains profiles of electron density below ~840 km. This rich dataset allows us to examine the solar activity effects on the low-latitude ionosphere on different timescales including solar flare, rotational, and 11-year solar cycle effects. In this paper we describe the effects of solar activity on the low-latitude ionosphere as observed by three different sets of measurements, including the maximum electron number density, the peak height, the latitudinal separation of the equatorial arcs, and the asymmetry of the peak densities. We also compare these results with those from the IRI model and a first-principles general circulation model, the TIE-GCM. The characterization and analysis of the observed effects are essential for understanding low-latitude electrodynamics.