Peculiarities of the ionosphere in the Far East: A report on IGY observations of sporadic<i>E</i>and<i>F</i>-region scatter

Abstract

This paper considers the results for the period October 1, 1957, to October 1, 1958, from the IGY ‘VHF oblique-incidence sporadic-E measurements’ program which operated circuits at 50 Mc/s in the Far East and the Caribbean. Sporadic E is found to be three to five times more frequent in the Far East than in the Caribbean for reflection coefficients of −20 to −80 db relative to inverse distance. Negligible dependence of magnetic activity is observed in either area, but diurnal and seasonal variations are more regular in the Far East. It is suggested that this longitudinal difference may be due either to the influence of the East Asiatic monsoon, perhaps through the mechanism proposed by Martyn, or to the difference in the relationship of magnetic dip to geographic latitude in these two areas. A peculiar evening signal enhancement, referred to as the ‘Far Eastern anomaly’ or the ‘evening signal anomaly’ appeared quite regularly in the Far East, and pulse-delay measurements indicate the probable source of the reflection to be the F region. The corresponding effect in the Caribbean is about 100 times less frequent, if it exists at all. The nF (layer tilt) reflection mechanism proposed by workers at Stanford does not appear too promising in this case, owing to the pulse broadening of the order of 1 millisecond which is normally encountered in the evening signal anomaly. A mechanism that would explain the structure of the observed signal involves reflection from field-aligned ionization similar to the mechanism invoked to explain the ‘low-latitude auroral echoes’ observed at Stanford.