Characteristics of theEsregion at Brisbane

Abstract

The Es region as recorded on routine h'f records taken at Brisbane (latitude 27°.5 south, longitude 153°.0 east) between June 1943 and December 1949 has been studied. At all seasons the critical frequency is lowest at dawn. In summer months, the critical frequency reaches a maximum at about 10h and then declines gradually, remaining high until after midnight. In winter, the rise is slower and the maximum critical frequency occurs around 14h, dropping markedly by sunset. In general, a high Es critical frequency is accompanied by blanketing of F echoes up to a comparatively high frequency; but, whereas the occurrence of high critical frequencies shows a summer maximum and a winter sub-maximum, the occurrence of high blanketing frequencies is least frequent in winter. The observations suggest that there are two distinct types of Es common at Brisbane—one formed at greater heights and descending to its final position, the other formed in situ. The first, the predominant type in summer, blankets strongly and has probably a uniform ionization density; but the second, the winter type, blankets little and probably has pronounced lateral irregularities. No evidence of correlation could be found with sunspot numbers, ionospheric storms, or meteor occurrence frequency, and the conclusion is reached that the Brisbane Es is not predominantly of meteor origin. There is some slight evidence of correlation between the constant-height type of Es and F region diffuseness.