We analyze the temporal distribution of 4,130 Terrestrial Gamma-ray Flashes (TGFs)detected by the Fermi Gamma-ray Burst Monitor (2008–2018) as a function of LocalSolar Time (LST). The distribution is highly non-uniform (χ² = 579.6, df = 23, p ≈0). TGF occurrence peaks at 16–17h LST (pre-dusk), with a rate of 256.7 events/hourat dusk (17–20h) versus 87.2 events/hour during midday (10–14h) — a ratio of 2.9×.A secondary enhancement occurs at dawn (4–7h, 174.3 events/hour). This temporalpattern correlates with the passage of the solar terminator, the boundary betweenthe sunlit and shadowed hemispheres. We examine concurrent data from ionosphericsoundings (Juliusruh foF2, 2023), geomagnetic indices (Kp, 94-year record), and tran-sient luminous events (Eurosprite catalog, 2009–2013). The ionospheric electron den-sity gradient (dfoF2/dt) peaks at dawn and dusk transitions. We propose that theenhanced TGF rate at the solar terminator reflects a systematic variation in upper-atmosphere electromagnetic permeability at the day-night boundary, and discuss im-plications for future measurements of local spacetime curvature oscillations.
Blanc et al. (Sun,) studied this question.