Artificial and Natural Disturbances in the Equatorial Ionosphere
The low-latitude ionosphere is characterized by large-scale instabilities in the post-sunset hours due to the distinct geometry of the earth’s magnetic field lines at the equator. The magnetic field lines are horizontal at the equator contributing to the high vertical drift velocity of the plasma bubbles growing from the bottomside of the ionospheric F-region. The phenomenon, commonly known as equatorial spread F, is an important problem in aeronomy as it can cause radio wave scintillation effects representing the most critical impacts of space weather on man-made technologies, such as satellite communications and global navigation satellite systems (GNSS). Here, we present results from an artificial ionospheric modification experiment as well as from naturally occurring instabilities in the equatorial ionosphere. An artificial plasma cloud was created in the bottomside of the ionospheric F-layer during the Metal Oxide Space Cloud (MOSC) experiment in May 2013 to study the interactions of artificial ionization with the background plasma under the hypothesis that the artificial plasma might suppress the occurrence of natural instabilities. While the suppression hypothesis remains open to debate, the propagation results confirm that the injection of artificial ionization in the lower F–region causes dramatic changes to the ambient HF propagation environment. We also calculate various parameters needed to evaluate the growth rate of Rayleigh- Taylor instability created in the F-region bottomside of the ionosphere from the thirteen days of High-Frequency (HF) radar data during the MOSC campaign. These parameters have been used to calculate the growth rate to predict the diurnal variability of the spread F occurrence. The growth rate has also been calculated from model ionospheric profiles optimized by ray-tracing techniques to match actual delays as observed in the oblique HF links. The calculated growth rate provides a close prediction of spread F development as seen in its correlation with the ground scintillation observations. With regard to natural processes, data from the Air Force Research Laboratory (AFRL) / the National Aeronautics and Space Administration (NASA) Communications/Navigations Outage Forecasting System (C/NOFS) satellite mission has been analyzed to investigate the characteristics of equatorial ionospheric irregularities from in situ observations. We present a comprehensive investigation on the variation of apex-altitude distribution of equatorial ionospheric irregularities with solar activity supported by modeling, simulation and comparisons with ground- and space-based in situ density observations. We also analyze Physics Based Model (PBMOD) ionospheric model results to determine if a physics-based model can reproduce the observed dependence of bubble height on solar activity.