The ionosphere contains many small-scale electron density variations that are under represented in smooth physics-based or climatological models. This can negatively impact the results of Observation System Simulation Experiments, which use a truth model to simulate data. This paper addresses this problem by using ionosonde data to study ionospheric variability and build a new truth model with empirically-driven variations. The variations are studied for their amplitude, horizontal and vertical size, and temporal extent. Results are presented for different local times, seasons, and at two different points in the solar cycle. We find that these departures from a smooth background are often as large as 25\% and are most prevalent near 250 km in altitude. They have horizontal spatial extents that vary from a few hundred to a few thousand kilometers, and typically have the largest horizontal extent at high altitudes. Their vertical extents follow the same pattern of being larger at high altitudes, but they only vary from 10s of km up to 200 km in vertical size. Temporally, these variations can last for a few hours. The procedure for using these spatial and temporal distributions to add empirically-driven variance to a smooth truth model is outlined. This process is used to make a truth model with representative variations, which is compared to ionosonde data as well as GPS Total Electron Content (TEC) data that was not used to inform the model. The new model resembles the data much better than the smooth models traditionally used.

It is now well established that waves generated in the lower atmosphere can propagate upward and significantly impact the dynamics and mean state of the ionosphere-thermosphere (IT, 100-600 km) system. Given the geometry of magnetic field lines near the equator, a significant fraction of this IT coupling occurs at low latitudes and is driven by global-scale waves of tropical tropospheric origin, such as the diurnal eastward-propagating tide with zonal wavenumber 3 (DE3) and the ultra-fast Kelvin wave (UFKW). Despite recent progress, lack of coincident global observations has thus far precluded full characterization of the sources of day-today variability of these waves, including nonlinear interactions, and impacts on the low-latitude IT. In this work, in-situ ion densities from Ionospheric Connection Explorer (ICON) and Constellation Observing System for Meteorology, Ionosphere and Climate 2 (COSMIC-2) Ion Velocity Meter (IVM) along with remotely-sensed zonal winds from ICON Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) are used to reveal a rich spectrum of waves coupling the lower (∼90-105 km) and middle (∼200-270 km) thermosphere with the upper F-region (∼540 and ∼590 km) ionosphere. Spectral analyses for a 40-day period of similar local time demonstrate prominent IT coupling via DE3, a 3-day UFKW, and the two ∼1.43-day and ∼0.77-day secondary waves from their nonlinear interactions. While all these waves are found to dominate the F-region spectra, only the UFKW and the 1.43-day secondary wave can propagate to ∼270 km suggesting E-region wind dynamo processes as major contributors to their observed ionospheric signatures.