Bibliography

Analysis of the annual variation of the E-layer critical frequency median foE in the nighttime (22-02 LT) auroral zone by the data of several stations of the Northern Hemisphere has shown the median maximum in winter and minimum in summer, even though the summer contribution of solar radiation to foE is greater. Thus, a new phenomenon was discovered\textemdashan foE median winter anomaly in the nighttime auroral zone. Its amplitude (ratio of winter to summer foE figures) can reach 10\textendash15\%; however, this anomaly was weakly expressed and statistically insignificant at particular stations located in the auroral zone. The winter anomaly is more distinct for foE _avr, the median of the E-layer critical frequency foE caused by the auroral source of atmospheric ionization, i.e., excluding the solar radiation contribution to foE. For foE _avr, the amplitude of the winter anomaly can reach 15\textendash20\%. Based on the qualitative analysis, it has been found that foE winter anomaly is stipulated by the winter/summer asymmetry of energy flow of accelerated electrons, which induce discrete aurorae in the nighttime auroral zone.