A statistical comparison of zonal mean and tidal signatures in FORMOSAT-3/COSMIC and ground-based GPS TECs

Atmospheric tidal components in the ionosphere can reflect either the in-situ generated quiet-time variation in the ionosphere, or vertically propagating tidal components generated through coupling to lower or middle atmosphere phenomena. Frequency-wavenumber tidal decomposition is a valuable tool for isolating the primary tidal components that drive the dynamics of the middle and upper atmosphere, allowing temporal and spatial variability to be quantified in a systematic manner, provided sufficient local time sampling. To date, two commonly used data sources for such tidal studies in the ionosphere are the FORMOSAT-3/COSMIC (F3/C) satellite constellation and ground-based GPS-derived Global Ionosphere Maps (GIMs). In this study, the migrating diurnal and semidiurnal tidal components, the nonmigrating diurnal eastward 3 (DE3) component, as well as the zonal mean component that dominate quiet-time ionospheric variability are extracted from 2008 F3/C and GIM Total Electron Content (TEC) data, using integration times of 20 days. We find that the zonal mean and tidal TEC components in F3/C and ground-based GIM data show qualitatively similar seasonal variability and spatial structure. However, the maximum amplitudes of the zonal mean and migrating tidal components computed from F3/C are consistently smaller than those from the ground-based GIMs, revealing a systematic difference between the two datasets. Conversely, the DE3 amplitudes are generally larger in F3/C compared to GIM, potentially due to the higher zonal wavenumber of that component.