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

Abstract 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