Statistical and simulation study on the separation in Junction Frequencies between ordinary (O) and extraordinary(X) wave in oblique ionograms

Abstract The most important aim in interpreting an oblique ionogram is to obtain the accurate Junction Frequencies (JFs) of the ordinary (O) and extraordinary(X) mode. This requires the correct identification of O- and X-mode traces, so it is very helpful and worthy to grasp the relative position between the two modes. In this paper, a statistical and simulation study of the separation in JFs between O- and X-wave is carried out based on observed oblique ionograms over three mid-latitude paths within China and a 3-D ray tracing program. The dependences on local time, season, geomagnetic activity, solar activity, direction and length of propagation, O-wave JF and group path are investigated. The main conclusions are as follows : (a) the separation on east-west path is susceptible on ionospheric variability, while the separation on north-south path does not show a significant correlation with local time and season;(b) a general diurnal tendency and a summer anomaly on east-west propagation are first proposed and discussed, the latter may be related to the effect of the lower layers; (c)the separation varies approximately as the cosine function of the propagation direction owning two maximums at north-south direction and two minimums at east-west direction in mid-latitude region of China; (d) the variation patterns of the separation with the propagation length are obviously not the same in different directions. For the east-west propagation path, the separation decreases to a minimum near ground range of 2000km and then increases very slowly with increasing ground range, while it monotonically increases for north-south propagation path.