We investigate the spatial variability of electron densities in the nightside ionosphere and its effects on very-low frequency (VLF) wave propagation using a suite of instruments from the FORMOSAT-3/Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) spacecraft.We use observations from the Tiny Ionospheric Photometer (TIP) instruments to infer the horizontal electron density gradients along each satellite track. We demonstrate that the OI 1356 Å radiance measured by the TIP instruments tracks 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 observations can be used to measure the horizontal gradient of the refractive index of whistler-mode VLF waves propagating in a cold, collisionless plasma. These measurements are critical for understanding how gradients in electron density associated with ionospheric structure such as depletions and the Appleton anomalies affect VLF wave propagation through the equatorial and mid-latitude ionosphere.