Horizontal Electron Density Gradients and Wave Propagation at Mid-latitudes 

D. H. Chua, S. A. Budzien, K. F. Dymond, and C. Coker, Space Science Division, 
Naval Research Laboratory, Washington, DC 20375 
 
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 weat 
 her. 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.