Sudden Ionospheric Disturbances Generated by Solar Flares—Not so Sudden?

It is known that solar flares generate instantaneous ionization enhancement, referred to as sudden ionospheric disturbance (SID), that occurs simultaneously across the entire dayside. In this work, we use high-rate 1-s data of total electron content (TEC) to analyse ionospheric response to 13 solar flares that occurred in 2003–2023. For the first time, we demonstrate that the SID first appears at the subsolar point, and, within the next 20–25 s, expands toward dawn and dusk regions with an apparent horizontal expansion rate of 250–500 km/s, depending on the spectrum of the flare. The expansion is due to the solar zenith angle impacting the magnitude of the ionospheric response and, consequently, causing the time delay between the TEC increase detected at the subsolar point and in dawn/dusk regions. We performed high-rate 10-s Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations that reproduced SID morphology and confirmed the flare enhancement expansion.