Massive disturbance of the daytime lower ionosphere by the giant γ-ray flare from magnetar SGR 1806-20

The giant γ-ray flare from SGR 1806-20 created a massive disturbance in the daytime lower ionosphere, as evidenced by unusually large changes in amplitude/phase of subionospherically propagating VLF signals. The perturbations of the 21.4 kHz NPM (Lualualei, Hawaii) signal observed at PA (Palmer Station, Antarctica) correspond to electron densities increasing by a factor of ~100 to ~10³ cm^-3 at ~60 km and >~1000 to ~10 cm^-3 at ~30 km altitude. Enhanced conductivity produced by flare onset endured for >1 hour, the time scale determined by mutual neutralization. A brief (~100 ms) low frequency (~3 to 6 kHz) emission is also observed during the flare onset.