Ionization of the lower ionosphere by γ-rays from a Magnetar: Detection of a low energy (3-10 keV) component

A gigantic periodic flare from the soft γ repeater SGR 1900+14 produced enhanced ionization at ionospheric altitudes of 30 to 90 km, which was observed as unusually large amplitude and phase changes of very low frequency (VLF) signals propagating in the Earth-ionosphere waveguide. The VLF signals remained perturbed for ∼5 min and exhibited the 5.16 s periodicity of the giant flare detected on the Ulysses spacecraft [Hurley et al., 1999]. Quantitative analysis indicates the presence of an intense initial low energy (3-10 keV) photon component that was not detectable by the Ulysses instrument.