Horizontal Electron Density Gradients and Wave Propagation at Mid-latitudes

We investigate the spatial variability in the large-scale electron density gradients associated with the Appleton anomalies in the low-latitude, nightside ionosphere using a suite of instruments from the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) spacecraft. We use measurements from the Tiny Ionospheric Photometer (TIP) instruments to infer the horizontal electron density gradients along each satellite track. We demonstrate that the OI 135.6 nm emission intensities measured by the TIP instruments track the horizontal electron density structure well with high spatial resolution and unprecedented sensitivity. Accurate measurements of the horizontal electron density gradients are important for improving retrieved electron density profiles from GPS occultation and other tomographic remote sensing techniques. The processes underlying the variability in the large-scale, nightside electron density gradients are the main drivers of ionospheric weather. TIP observations reveal significant variability in both the small and large scale structure of the nightside ionosphere. The relative intensities, relative widths, and latitudinal separation of the northern and southern ionization crests of the Appleton anomalies show a high degree of longitudinal variation. We demonstrate how the TIP measurements can be used to understand how the propagation of VLF whistler-mode waves is affected by gradients in electron density associated with equatorial ionospheric structure such as depletions and the Appleton anomalies.