Riometer absorption and spectral index for precipitating electrons with exponential spectra

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The altitude profiles computed by Berger et al. (1974) for energy deposition in the atmosphere by electrons and electron‐produced X rays are transformed to electron density profiles in order to calculate the absorption of cosmic radio noise at 20, 30 and 51.4 MHz. The electrons are precipitated isotropic over the downward hemisphere with exponential spectra having e‐folding energies of 10, 50 and 200 keV. It is shown that the absorption of a wide angle (± 34°) riometer antenna is about 1.4 times larger than zenithal absorption, that a relationship A(ƒ) = k(ƒ)N0.45 exists between precipitation electron flux N and riometer absorption, with k(ƒ) a frequency dependent constant, and that the spectral index n, defined by A(ƒ) ∝ ƒ−n, is ≃2.0 for all three the e‐folding energies if the precipitation is uniform over the field of view of the antenna, but attains values well below 2.0 for the higher e‐folding energies only if the effective electron collision frequency is enhanced by hyperthermal electrons. The spectral index for wide beam riometer absorptions decreases also to values much less than 2.0 if a narrow strip of the upper atmosphere across the field of view is strongly ionized. It appears then that the frequency dependence of the absorption may not be related uniquely by A(ƒ) ∝ ƒ−n. The absorption event recorded at Sanae, Antarctica, on June 27, 1982, at 0122 not only illustrates that the spectral index n is not an unique parameter when electron precipitation is not uniform in the field of view of a riometer antenna but suggests also highly wavelength selective scattering of cosmic radio noise from the more luminous parts of the radio sky by the Bragg condition as suggested by D'Angelo (1976).