On the determination of lower ionospheric perturbations detected as fast amplitude variations of VLF signals

Abstract

We analyze the occurrence of very low frequency (VLF) events and their association with geomagnetic activity. VLF events are detected as fast amplitude variations of VLF signals from Hawaii (NPM Navy transmitter) and received at Comandante Ferraz Brazilian Antarctic Station (EACF) in the period from September through November 2003. The results show VLF events occurring predominantly during the recovering phase of geomagnetic storms produced by high speed solar wind streams (HSSWSs), and by coronal mass ejections (CMEs) which were accompanied by HSSWSs impacts before returning to quiet geomagnetic level. Based on these results, and upon the good association of VLF events with enhancements in the energetic electron flux of the radiation belts, we suggest they might be the ionospheric response to the energetic electrons precipitated from the radiation belts which were predominantly scattered by whistler-mode waves generated by the electrons injected in the magnetosphere during geomagnetic activity produced by HSSWSs.