Abstract On 9 October 2022, a powerful gamma ray burst (GRB), GRB221009A, caused significant changes in the electron density of the lower ionosphere, as evidenced by VLF (3–30 kHz) radio wave observations. However, GRB221009A did not yield any observable signatures at the Schumann resonances (∼8, ∼14 Hz), which are also sensitive to the lower ionosphere. We show that the effects of GRB221009A are observable in a decrease in propagation velocity of ELF (3–1,000 Hz) lightning impulses. Analytical and numerical analysis points to an ionospheric perturbation in which the GRB increased the daytime electron density uniformly over a wide altitude range from 50 to 90 km. The GRB perturbation is markedly different from perturbations from solar flares, which significantly increase the vertical gradient of the electron density. ELF propagation velocity is shown to be a technique that can identify the altitude range of ionospheric perturbations.