Sudden changes in the amplitude of long-distance subionospheric VLF transmissions have been found at night in association with whistlers. Both increases and decreases in signal strength have been observed, depending on signal frequency and orientation of the receiving antenna. Sample observations at Eights Station in Antarctica of station NSS (Annapolis, Maryland) on 22.3 kHz showed increases in signal strength that averaged 3 db, with rise times of about 2 sec and durations of about 30 sec. Coincident with every rise was a midlatitude (L ≈ 2.5) whistler originating in the northern hemisphere. To explain this association, it is suggested that the whistler dumps energetic (30–300 kev) electrons into the D region. The resulting ionization then alters the properties of the earth-ionosphere wave guide. The mechanism of precipitation is thought to be pitch angle scattering of trapped electrons that resonate with the magnetic field of the whistler wave near the magnetic equator.
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