Contribution of Solar and Auroral Ionization of the Atmosphere to the E-Layer Critical Frequency of the Nighttime Auroral Region

Abstract

The contribution of the solar f_sol and auroral f_avr components of foE to the critical frequency of the E layer foE are estimated based on an analysis of the medians of the E-layer critical frequency foE in the nighttime (2200–0200 LT) auroral region with allowance for the noncoincidence in the heights of maxima of these components. This noncoincidence is taken into account with a correction coefficient that manifests the relation of f_sol to the critical frequency of the E layer due to solar radiation foE_sol. The analytical dependence of this coefficient on the solar zenith angle for χ < 85° is derived. It is shown that one can usually disregard the contribution of f_sol in foE in the nighttime auroral region for χ > 85° and approximately foE = foE_avr for these conditions in which foE_avr is the critical frequency of the E layer due to precipitation of auroral electrons. It is obtained that the allowance for the noncoincidence of the heights of maxima in the solar and auroral components of foE with estimation of the annual changes in foE_avr based on the foE measurements leads to an increase in foE_avr in summer and thus provides a decrease in the amplitude of the winter anomaly in foE_avr. Nevertheless, even in this case, the amplitude of the winter anomaly in foE_avr is higher than the winter anomaly in foE, and that demonstrates the important role of accelerated auroral electrons in formation of this anomaly.