Formation of the Ionospheric G-Condition Following the 2017 Great American Eclipse

A total solar eclipse (TSE) traversed the continental US (CONUS) from west to east on 21 August 2017. Ionosondes located under the eclipse totality at Lusk (Wyoming) and Boulder (Colorado) observed the ionospheric G-condition 20 min after totality. The Millstone Hill mid-latitude incoherent scatter radar recorded an anomalous low altitude F₂ peak during the recovery phase of the eclipse, which can be attributed to an ionospheric G-condition. We perform WACCM-X simulations to investigate the physical processes that drive the ionospheric G-condition. Specifically, we conducted a diagnostic analysis of the simulated atomic oxygen ion continuity equation to examine the source of the G-condition. We defined two metrics describing the G-condition: (a) the duration and (b) the maximum difference between $N_m{F}_1$ and $N_m{F}_2$. Results indicate that E- and F₁ plasma density reduction closely follow the eclipse obscuration, whereas the F₂-layer density depletion lags the obscuration by $\sim$20 min. This delay increases with altitude and is caused by slower radiative recombination and transport processes in the diffusion-dominated F₂-region. The delay creates a period during the eclipse recovery when the F₁-plasma density exceeds that of the F₂-peak, which manifests as the ionospheric G-condition. The simulation study illuminates the space-time behavior of the G-condition indicating that this state can last for more than 50 min during the recovery phase of the eclipse.