Origination of Ionospheric G-condition following a Total solar eclipse

Total solar eclipses (TSEs) offer a unique opportunity to study ionospheric plasma processes under unusual but controlled circumstances. Among various geophysical phenomena that can be probed during a TSE, the ionospheric G-condition is relatively unexplored. A G-condition in the ionospheric F-region occurs when the critical frequency of the F1 layer is greater than or equal to that of the F2 layer, i.e., foF1≥foF2. The sources of ionospheric G-condition have long been studied using observations and numerical modeling. Observational studies using ionosonde and incoherent scatter radar (ISR) found that the occurrence probability of the G-condition increases with geomagnetic activity, latitude, and decreasing zenith angle and solar activity. Additionally, numerical studies suggested that the escape of atomic oxygen ions and associated electrons (O+/e-) from the F2 layer keep the F1 layer relatively unperturbed during the above-mentioned geophysical conditions, creating the flip in the density profile that leads to the G-condition. A recent observational study found the G-condition following the 2017 TSE using ionosonde observations. This study explores the physical mechanisms that cause the G-condition following the TSE. We conducted the Whole Atmosphere Community Climate Model with a Thermosphere and Ionosphere eXtension (WACCM-X) simulations to investigate the physical processes. First, we validated the WACMM-X simulation results against the observations; then, we conducted a diagnostic analysis of the atomic oxygen ion continuity equation. We found: (a) the ion density (and electron density) in the E-, F1, and F2- layers all decrease in response to the eclipse, with the peak depletion and recovery in the E- and F1- layers closely follow the peak occultation and the recovery phase of the eclipse; (b) however, the peak depletion in the F2 layer is delayed almost 30 minutes, as is the recovery, which increases with altitude (>140 km); (c) the delay creates a time period when the ion density in the F1 layer is recovering while the density in the F2 layer is still decreasing, and the ion density in the F1 layer becomes greater than that in the F2 layer, which manifest as the G-condition; (d) the delay is due to plasma transport effects, which are important for the dynamics of the F2 layer. We found that ambipolar diffusion plays a primary role in manifesting the G-condition among various transport factors.