[1] 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.Citation: Remenets, G. F., and M. I. Beloglazov (2013), Ultrarelativistic electrons in the near cosmos and X-ray aurora in the middle polar atmosphere,

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The effect of geomagnetic cutoff with respect to the primary ionizing flux, which caused anoma lous VLF disturbances on a completely auroral radio path with a moderate length (~900 km), has been proved. Abnormality manifested itself in a qualitative similarity of amplitude and phase variations at all three working frequencies of this radio path. This should not be observed if the contribution of the second iono spheric ray to the received field is substantial. This similarity is still the only signature of ultrarelativistic elec tron precipitation. The VLF data for the auroral and long partially auroral radio paths with a common receiver point (10-16 kHz) in the auroral zone have been qualitatively compared in order to prove the geo magnetic cutoff effect. The experimental database on disturbances occupies six years in the vicinity of the solar activity minimum (1982)(1983)(1984)(1985)(1986)(1987). The manifestation of the magnetic cutoff in the above experimental data indicates that the primary ionizing flux is of the corpuscular nature. This flux generates X ray bremsstrahlung, which produces the ionized sporadic D S layer in the middle auroral atmosphere.