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

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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.