Abstract
The rare phenomenon of ultrarelativistic electron precipitation into the middle polar atmosphere, prevalent under calm geophysical conditions, was established from ground‐based radio wave measurements during the period of 1982–1992. Precipitating electrons with energy ∼ 100 MeV and sufficient density to generate X‐ and gamma‐ray bremsstrahlung create a sporadic layer of ionization in the atmosphere under the regular D layer of the ionosphere. Very low frequency radio waves reflect from this sporadic layer with abnormal weakening and with an unusually low height of reflection. The layer has a horizontal linear scale of about several thousand kilometers, with a thickness in altitude of about 20–30 km, and persists for several hours. Due to this layer of electric conductivity, the effective height of this “ground‐ionized atmosphere” waveguide diminishes in exceptional cases by 2–2.5 times. The auroras of X‐ray bremsstrahlung have been detected by the reflection of radio waves with wavelengths of 30–20 km. This phenomenon may be termed “a polar cap absorption effect of the second kind” as an electron analog of proton precipitation.
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