Massive disturbance of the daytime lower ionosphere by the giant <i>γ</i>‐ray flare from magnetar SGR 1806–20

Abstract

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 ∼10 3 cm −3 at ∼60 km and ≳1000 to ∼10 cm −3 at ∼30 km altitude. Enhanced conductivity produced by flare onset endured for &gt;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.