Hard X-ray impact on the ionosphere D-layer: new results from VLF measurements

The ionospheric electron density reacts to a change of ionization condition by a time delay Δt. Appleton (1953) demonstrated that this time delay is inversely proportional to the product of the electron density Ne and recombination rate coefficient α. Thus, the evaluation of the time difference between the peak time of VLF emission, which is supposed to represent the instant of maximum ionization, and the ionization source's peak time provides an easy way to estimate α Ne. First used to evaluate the increase of electron density at noon from H α peak emission, this technic was also employed to estimate the recombination rate during solar flares. The GOES Soft X-ray emissions (i.e. in the range 1.5-12keV) are then considered to determine the ionising source peak time.

Based on VLF measurements obtained from the SUPERSID antenna installed at the Meudon site of the Paris Observatory (France), we computed each flare's time delay from January 2017. We benefit from the events of September 2017, the strongest from the last 10 years. We thus demonstrate the prominent role of Hard X-Rays in ionizing the D-layer of the ionosphere.