Effect of solar flares on ionospheric VLF radio wave propagation, modeling with GEANT4 and LWPC and determination of effective reflection height

Solar flares are among the most prominent transient events affecting the radio wave propagation in the ionosphere. In this paper we model the ionizing effect on VLF wave, reflected from D-layer of the ionosphere during different classes of flares and compare with observed VLF modulation. A well known detector simulation software GEANT4 is used to calculate the ionization produced in different heights (~ 60-80 km) of the D layer. The chemical balancing between the ions and neutrals in the region is taken into account with a simplified chemical model, the GPI model. The VLF modulation is reproduced with LWPC. We have seen that the peaks of electron density arise at different times at different heights. Matching with the time delay of the VLF signals we have proposed an alternative effective reflection height during such events.