Solar Hard X-ray impact on ionospheric D-layer 

Srivani Inturi - Observatoire de Paris; Carine Briand - Observatoire de Paris, France; Baptiste Cecconi - Observatoire de Paris, France 

Session: Thermosphere-Ionosphere Interactions and Effects

Abstract

This work presents a study of the response of the ionospheric D-region (50-90 km) during periods of solar flare occurrence. The increase of X and UV fluxes during flares alters the degree of ionisation down to the lowest layers. From a space weather point of view, an increase of the electron density of the D-layer induces an increase of absorption of the high-frequency radio-emission, which used by the civil aviation for instance. An efficient way to study these ionospheric variations is to measure the temporal evolutions of VLF/LF waves that propagate from man-made, ground-based VLF transmitters in the waveguide formed by the earth and the ionosphere. In our case, we follow VLF/LF signals from six stations simultaneously, with a SUPERSID instrument located near Paris (France), covering a large latitudinal and longitudinal extension over Europe. We analysed 30 flares, from C3.0 to X9.3 event occurring in 2017. In particular, we focus on the flares that occurred on September which are the strongest flares since 2005. It is well known that Soft X-rays (SXR, 0.1-0.8nm) strongly affects the D-layer. We demonstrate that Hard X-rays (HXR, 0.05-0.001nm) play a major role in ionising the lowest layers during the strongest X-class flare.